Mis-wiring a 120-volt RV outlet with 240-volts

Apr 1st, 2013 | By | Category: RV Safety
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Mis-wiring a 120-volt RV outlet with 240-volts

I’ve been answering a lot of forum questions lately from RV owners who paid an electrician to install a 30-amp/120-volt TT-30 RV outlet for powering their RV in the driveway. But the electrician somehow gets the wiring wrong and connects 240-volts to their 30-amp RV outlet rather than 120-volts. Of course, plugging your 120-volt RV into an outlet mis-wired with 240-volts will destroy just about every electrical appliance, converter, inverter, and electronic gadget in your RV in a matter of seconds.

RV-Dryer_OutletsSo why does this happen? Don’t electricians know better? Well, they should be reading the markings on outlet itself for the proper voltage, but it’s typically in very small black-on-black writing. I think the real cause of this costly mistake is that a 30-amp/120-volt RV outlet closely resembles a 30-amp/240-volt Dryer outlet. If you look closely at the pictures you’ll see that the 120-volt RV outlet has a U-shaped ground contact, while the 240-volt Dryer outlet has an L-shaped ground/neutral contact. That’s right… 30-amp Dryer outlets often have a missing safety ground, depending on the Neutral wire to properly ground your appliance. And yes, that’s still code compliant in older installations.

So unless you specifically hand your electrician a wiring diagram and check his work after completion, you could be taking a big risk. I’ve created a graphic of just how similar these two very different electrical outlets look, and the proper wiring hookups for each one.  Please print out this diagram and hand it to your electrician. If he or she doesn’t seem to understand the difference, then you need to get a new electrician.

Finally, test the outlet yourself for proper voltage before plugging in. Here’s how your meter should read on a correctly wired 20 or 30 amp / 120-volt outlet.

20-30_Amp_120V metered

 

 

 

 

 

 

 

 

 

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70 Comments to “Mis-wiring a 120-volt RV outlet with 240-volts”

  1. Ronnie B says:

    I am not an electrician, but I just cannot believe a person can plug even in error the two completely different female plugs. They are made to prevent accidental misuse, I mean look at the Y shape, the 120 is wider, the ground is so different. If someone can somehow force a connection and survive the shock, I’d love to read about it.

    What I do see is being really POed after getting to a campground and cannot plug up.

    • Mike Sokol says:

      Ronnie,
      I’m not talking about a campground or electrician installing a dryer outlet and an RVer jamming the plug into the wrong socket. I’m talking about an unknowing electrician installing the proper RV outlet in your garage, but mistakenly wiring it to 240-volts rather than 120-volts. Since it appears to be a proper RV outlet to the unsuspecting home owner, they just plug their RV into it to check it out, and that’s when the fireworks begins.

      The reason electricians and DIY homeowners can make this mistake is that a 30-amp/120-volt RV outlet “looks like” a 30-amp/240-volt Dryer outlet. And since that electrician has probably wired in dozens of Dryer outlets while this could be his first RV outlet, that’s why the mistake occurs. It’s always best to test every unknown outlet for proper voltage and polarity BEFORE plugging in your expensive RV.

      • Ian Scott says:

        I am 37 years old and I have been working as an electrician for the past 6 or 7 years, before that I was a helper for8 years. I just cleaned my mother in laws trailer so she can sell it. I have done a ton of work on Mobile Homes and I like a real jerk jammed her 120 volt 30 amp plug into the 240 volt dryer outlet. When nothing was working, I went for the whole trying trying to figure the problem. I thought some idiot miswired the darn thing because I was getting 120 volt on my neutral. Well inside the trailer I was in 120 volt mode but every time I went to outside I was in 240 volt mode. I am just an idiot, I realized my error last night.

      • visionq2 says:

        I just had an electrician do this exact thing…wiring a 120 V outlet in a 240 V mode. I had told him in a text to be careful, but he just wired it up wrong. Dummy me, did not check it out with my voltmeter. Luckily, I had a surge protector on, which likely saved my appliances. However, I ran it over to the RV shop and they said that my converter is blown. The electrician is paying for the new converter. My question is – Should I be concerned about any wiring damage that may cause a fire, etc. from this event? Thanks for the advice!!

        • visionq2 says:

          PS – I read later posts that a surge protector will not help….What good are they then? Also. anything I should be watching for once the converter is replaced to determine if further damage has been done?

          • Mike Sokol says:

            Yeah, surge protectors will do nothing to protect you from a over-voltage situation like a 120-volt outlet mis-wired with 240-volts. The only thing they’re really good for is protecting you from a nearby lightning strike to the ground. But any kind of direct lightning hit to the electrical wiring is going through them anyways. So don’t confuse a simple surge protector with something like a Progressive Industries EMS, which has relay contacts to disconnect your RV from over-voltage or under-voltage conditions. And yes, they also include a surge protector circuit for nearby lightning protection and other spikes on the power line from things like pump motors and such.

            As far as what can be damaged, basically anything with electronics is at risk if it was turned on at the time of the over-voltage condition. While the general wiring in the walls of the RV and the circuit breakers won’t care if they had 120 or 240 volts, you should check your refrigerator, air conditioner, television, inverter, converter, tank monitoring system, etc… And the sad thing is that some of these electronic component failures many not show up for a few months. So be sure to get your RV powered up (once the converter has been replaced) and run all the appliances for a few days, carefully monitoring for anything acting strange. And tell your electrician to keep the insurance claim open for a full year since you could have something electronic fail this fall that was a result of his mis-wiring in the spring. If his insurance questions this, they can call me and I’ll charge my normal $250/hr consulting rate to educate them about over-voltage failure mechanisms in electronics.

          • visionq2 says:

            Mike – Thanks…this is very helpful. I don’t quite understand how an over-voltage can wreak havoc 6 months later. Please explain. If this is truly the case, then shouldn’t I be really worried about a potential fire situation. This would be my biggest concern…

          • Mike Sokol says:

            What often happens in electronic gear is the filter capacitors that have been exposed to over-voltage begin to out-gas and the case they’re enclosed in begins to swell up. Sort of like a miniature cola can that’s been overheated and tries to burst, but not quite. Eventually that capacitor will fail, and when it does the circuit will die.

            Also, transistors don’t do well with over-voltage and their PN junctions will be stresses to the point of failure at some future point. There’s only so many over-voltage spikes they can withstand, because each voltage spike punches away a little more of the insulating layer.

            But a lot of very modern gear won’t be affected by 240-volts because they have universal switching power supplies designed to run on anything from 90 to 250 volts. Your situation just depends on the type of power supplies in your electronic gear and just how long you exposed everything to 240 volts.

        • Keith says:

          I just purchased a used 2003 Keystone RV 5th wheel from an indiviual. The TV and stereo was taken out. The microwave didn’t work. New frig. All 110 volts electrical, converter, battery, slid outs worked, using an extension cord plugged into an 20amp outlet in my garage, to my 30 amp / 120 volt TT cord. I purchased a RV Power Outlet 120 volt 30 amp [ provides an electrical receptacle in a single enclosure ] and a 30 amp breaker for my breaker box in my garage. I wired both black & white wires to the breaker & ground to the ground bar. [ Not thinking that gave 220 volts ] Bad mistake $$$$$$$. My ac still works, all battery devices [ lights, slide outs, power jacks ] works. My frig light works, but says no electrically. My GFI outlets work, but all 110 lights don’t, maybe more. Need all advice .

          • Mike Sokol says:

            That is exactly why I wrote this article. TT-30 RV outlets look a lot like a 30-amp/240-volt dryer plug, so it’s far to easy to wire one up incorrectly. Basically, anything with a circuit board, filament, or motor could be burned out. YOu have to divide and conquer, testing each appliance or device individually. This can be so costly that I’ve seen entire RVs “totaled” since the entire electrical system was toasted. Most of the time the circuit breakers and actual wires are OF, but the fridge, HVAC, Inverter, Converter, Hi-Fi, Television, and monitoring systems can be taken out. The more modern and expensive the RV, the more electrical appliances and controllers to destroy.

            Since TT-30 receptacle are so easily miswired, I highly recommend that everyone add an electrical monitoring system to their shore power connection. It’s just too expensive to fix all the fails from an over-voltage condition.

          • Keith says:

            Mike, what should I do now?

          • Mike Sokol says:

            Test everything for proper operation, one piece at a time. You’ll have to take everything apart and inspect the circuit boards for burnt marks and bulging capacitors. Also, use your nose since burnt electronics has a sweet smell that’s hard to miss. Anything with an auto-switching power supply (90 to 250 volts) is likely OK, but anything with old-school single-voltage supplies could be damaged. Things connected to only 12-volts DC are probably OK. But realize that many components have been stressed and might fail in a few weeks or months. So don’t just turn things on for a quick check. You need to run things for a few hours at least and monitor for a later failure.

      • Donald says:

        Can i take a 5250 watt generator with a 240 outlet and change that outlet to a 30 amp three prong for a rv?

        • Mike Sokol says:

          Yes, but you must be careful to use only one of the hot legs. If you use both legs of the generator that will create a 240-volt TT-30 condition and burn up your RV’s electrical system.

    • Patty Moskowitz says:

      Not true anymore that plugs in different countries or different voltages are always different. Our 220 volt outlets here in Thailand look exactly like 120 outlets in the US.
      So, being less than attentive, I just ruined my ancient scanner by plugging it into the 220 outlet instead of the transformer which sits in front of the plug. Dang.

    • James says:

      Hi Ronnie–what I believe happened is I live in a old converted house that still has nob and tubeing old wire -the lady upstairs blew a breaker one day with her microwave same as me. and I had my microwave on -I checked breaker box and it was in the half position-so I flipped back up –I believe since the old wire was aluminum it soddered together to make 220 volts in a junction box. So in time I believe my microwave blew and now my heater

      • Mike Sokol says:

        Sounds like an open neutral to me. That will create a condition where the incoming 240 volt isn’t split evenly in half to make 120-120 volts. So it can shift around depending on the load imbalance creating 180-60 volts or whatever. If your microwave is on the high side that will blow it up for sure.

  2. Ray says:

    I installed an Electrical Monitoring System into my RV
    http://loveyourrv.com/progressive-industries-model-ems-hw30c/
    The device goes in line on the AC Power cord to the RV and will not let power into the rig if it’s too high or too low, wrong frequency, etc. Also protects against surges. Nice unit and gives me piece of mind when plugging into campground power pedestals.

    • Mike Sokol says:

      Yes, a voltage monitor such as you linked to would protect your RV from 240-volts in this instance. But a simple surge suppressor will do nothing to stop 240-volts from entering your RV’s 120-volt electrical system. However, even the voltage monitor you mention will NOT protect you from a hot-skin situation caused by an RPBG (Reverse Polarity Bootleg Ground) outlet. More on that subject later this week.

      Mike Sokol

      • Ray says:

        Interesting, I’ll stay tuned.

        • Mike Sokol says:

          I have confirmation from the engineering departments of two major Surge/Voltage protector manufacturers that their products will NOT identify or protect your RV from a hot-skin condition caused by an RPBG outlet. My research indicates there a NO currently manufactured Surge or Voltage protectors that will identify or protect you from an RPBG.

          More on this topic next week.

          Mike Sokol

  3. Karen and Steve says:

    Yup… happened to our parents. They built a new house and handed the electrician the proper outlet they bought at the RV store.. .and told him it was for an RV and where to install it alongside of their garage. The Idiot wired it up 220 instead of 110 and blew out all the electronics and even melted some of the wiring sealed in the walls. It was a VERY expensive mistake– and time consuming for the RV repair place to try to correct. Even then, some strange electric things still happened within the RV aftewards. All the control boards for every unit went haywire, including the fridge, water heater and furnace too. Not to mention the TV’s, stereo, DVD player, micro/convection oven, and light fixtures. Every bulb was burned BLACK. The electrician had the audacity to ask them to turn a claim on their homeowners insurance instead of him rectifying the situation. Can you imagine that???

    • Mike Sokol says:

      It’s an easy mistake for an electrician who’s never hooked up an RV outlet. Now, I’m not excusing his mistake, but commenting that I understand how he could get confused. And as you noted, I believe the electrical damage caused by plugging a 120-volt RV into an outlet mis-wired with 240-volts can create all sorts of “random” electronic failures months after the incident. If it happened to me, I would ask for the RV to be “totaled” and expect full replacement cost from the electrician’s insurance company.

      Modern RVs are essentially wired like a smart house, with so many interconnected electronic gadgets that it’s difficult to find and replace EVERYTHING that’s damaged by this type of mistake. Yes every electronic and electrical device in your RV can be fried by a few seconds of over-voltage. See why I recommend that all RV owners test pedestal outlets for correct voltage, polarity, and grounding BEFORE plugging in…

      Mike Sokol

  4. DON CONDON says:

    SORRY ELINOR. HE SHOULD PAY FOR IT. THAT WAS NOT A HARD JOB. IF HE WAS AN ELECTRITION HE SHOULD HAVE KNOWN. YOUR BUDDY DON C..

  5. and the fact that a “50” amp cord CAN be plugged into a properly wired 240 source tends to confuse novice electtricians even farther if they are installing a 30 amp recepticle to be used with a reducer/adapter on the end of the big cord.

    • Mike Sokol says:

      Don’t get me started on reducer/adapters. I get emails every week from RV owners that develop hot-skin conditions, and they’re normally caused by a ground break inside a cheap, imported dogbone adapter. Or someone will adapt a 30-amp outlet to a long 15-amp extension cord, then run it into an RV that doesn’t have a master circuit breaker. They’re then trying to draw 30-amps through an extension cord rated for only 15 amps of current. Here’s a video I made about overloading extension cords: http://www.youtube.com/watch?v=WZznobYGF_c

      Mike Sokol

  6. steve says:

    Mike,
    I also have a Progressive protection system I installed in the 5th wheel. I’m convinced it has saved us from several brown-outs and surges. I still check the podium at RV parks before plugging in. I just feel better having added that check to my procedure.

    Steve

    • Mike Sokol says:

      I think that installing a good voltage/surge protection system is the best money you’ll ever spend on your RV. The Progressive Industries products are well engineered and solidly built. Remember, there’s a BIG difference between a “surge suppressor” and a “voltage protector”. The surge suppressor is only good for reducing voltage spikes from nearby lightning strikes, but a voltage protector will identify low or high voltage conditions and disconnect your RV’s electrical system from the incoming power line.

  7. Paul says:

    As an electrician/construction administrator for an engineering firm, who just made this very mistake on my father in laws RV, I can tell you the problem does not lie with the electrician. This information should be clearly marked on the dogbone, at the plug on the RV and the receptacle. These items should be clearly marked DO NOT CONNECT TO A 240 VOLT, 30 AMP CIRCUIT!!! CONNECT TO A 120 VOLT, 30 AMP CIRCUIT!!! With so much dollar value riding on the correct and incredibly unusual type of circuit, THEN CLEARLY MARK IT!!! A normal single pole receptacle has different colored screws that indicate connection to designated wires. This receptacle does not. Neither of the two different dogbones in my possession have any markings indicating the voltage change whatsoever. The original 50 amp, 2 pole service leads one to believe that two hots are acceptable. And for those who are not aware, some relatively simple electronics and a small transformer will create a neutral from a two hots. This is common knowledge in my industry.

    So yeah, I screwed up and cost myself a ton of money, but don’t try to act like the electrician should have known. Obviously, this is a very common mistake that could be prevented with some $5 stickers. And yes, I’m pretty p’od right now. I can’t believe that the RV industry is so nonchalant about this occurrence (I called an RV store and they were quite aware of this common mistake). But hey, they get more money out of people having to fix this.

    Now that my rant is over…
    Can these appliances be fixed? Or do they need to be completely replaced? The inverter/charger is simple enough to replace, but all of the others systems are what concerns me. Fridge, microwave, monitoring systems, water heater, the AC unit still works.

    • Mike Sokol says:

      Sorry to hear about your problem. I do agree that there’s not enough markings or obvious differences on a 120-volt TT-30 RV plug to prevent miswiring one as a 240-volt dryer outlet. That’s exactly why I wrote this article after receiving numerous emails from readers about this same sort of mistake. How often this happens is a mystery, but according to my discussions with Gary Bunzer and Mark Polk, it seems to happen a lot.

      Now, I don’t work for the RV industry so I don’t know the history of this design. But since the TT-30 receptacles are manufactured by a variety of companies, many of them foreign, marking all of them with big stickers would be problematic. If you look very closely at your TT-30 receptacle you should see a very small marking that states 125-volt max as per NEC specs. But that’s in really small black-on-black writing that nearly impossible to notice.

      As for repair or replacement of the RV appliances, that’s a roll of the dice. Anything that was turned off during the overvoltage event could probably survive. But many appliances NEVER really turn off, even when you hit the off switch. I would contact each of the manufacturers to see if they can suggest what components might be damaged by 240-volts, and then inspect each appliance for blown fuses and obvious power supply component damage such as blown capacitors and diodes. I think that less expensive appliances such as a microwave may be too expensive to fix compared to replacement, but certainly you should be able to get a swapped control board for a built-in refrigerator. And you might find out if their homeowners insurance will cover the cost of replacement due to overvoltage. Of course there will be a deductible to pay but that’s certainly less than the full bill.

      • Paul says:

        Thanks for the reply and the website. Being an electrician/gigging musician/field engineer (Construction Administrator is just a fancy title), I find this site to be very informative and useful.

        I already have a new converter and new battery in my possession. The plan is to install them after work today. Hopefully, it won’t all be bad news from there. I already know the microwave is toast, probably the TV and DVD player too. I do hold out some hope for the equipment powered by 12 VDC, since I unplugged the trailer immediately upon discovery of the failed smoke test.

        Either way, thanks again.

        • Paul says:

          Just an update that might be useful for anyone that unknowingly follows in my footsteps of stupidity. I installed the new converter and battery.

          The only losses, were the electronics plugged directly into the 120 Volt outlets in the trailer: the microwave, TV and DVD player. All others systems have been fully tested and are operational. This may be due to the fact that the converter was directly in line with all a other systems and therefore protected them. Wiring insulation is typically rated for 600 Volts, which was the case with this trailer, so I would be very leery of anyone who says the wiring is bad within the trailer in this type of over voltage situation. Seems like a way to take advantage of uninformed customers.

          Mike,
          Thanks again for the website, keep up the good work!

          • Bruce H says:

            Paul – When you say “converter”, do you mean “inverter” — the electrical component that is tied to the 120V system? It normally takes input power from the shore cord and “through supplies” this power to the 120V circuits in the RV and also re-directs some of that input current to charge the house batteries; when the shore power (or on-board generator) is disconnected, the inverter draws DC power from the batteries, inverts it to 120V and supplies it to the 120V circuits in the RV.

            The inverter is a major first-step component in the supply of current into an RV from shore power. (We will ignore the fact – for the moment – that
            1) some RV’s are wired like a house and will supply 240V from a 50Amp/240 shore supply to electric cooking ranges, clothes dryers, etc, while splitting legs to supply 120V to ordinary 120 V circuits;
            2) some RV’s are set up to receive 240V from the shore cord but immediately “split” the legs so that the only power circuits on the RV are 120V supply;
            3) other RV’s are set up to only receive 120V from their shore cord. These various systems must be considered when designing or troubleshooting an RV electrical system but aren’t really relevant to our discussion here.)

            Unfortunately for simple description, there are devices which are called “converters” that are sometimes used on RV’s. These are components that take one voltage of DC and provide output at another DC voltage; for instance, if the house storage batteries on the RV are 12V and it’s necessary to supply 24V to a component on the bus, a 12V DC – to – 24V DC converter can be used. But “converters” are very different from “inverters”, so to keep discussion clear, it’s necessary to separate the terms as we used them and be very accurate as to what components we’re talking about.

            I’m sorry to hear of your troubles caused by the poor labeling on the 30Amp connector. I especially regret the cost of the components ruined and I hope that you’ve worked through all the damage done.

            Best wishes, Bruce H NC USA

          • Paul says:

            Bruce,
            It is an RV Converter/battery charger. See the link to the manual. It converts AC to DC.

            http://www.appliedgmc.com/images/PD9200_Manual.pdf

            You are correct in how an inverter works, but this converter coverts 240 Volts to DC. I realize that there are different types of converters. The trailer is set up for 240 Volt, 50 Amp shore power and my mistake was assuming and rushing to hook up a 30 Amp receptacle. It would have been painless, if I just used a 240 Volt, 50 Amp receptacle fed by the existing 240 Volt, 30 Amp circuit breaker that I was trying to use. That is more available Volt-Amps than the 120 Volt, 30 Amp breakdown that the dog bone would provide. So simple and yet such a pain in the…

            This is the first time in 22 years of working in the electrical industry that I failed a smoke test. Embarrassing!!!

            Moral of the story…
            Know exactly what you’re getting into before you plug something up.

            Thanks a bunch for the reply,
            Paul

          • Paul says:

            Sorry, I meant to say, this converter converts 120 volts to DC.

            This trailer does not have an inverter.

            Your point is still valid. Know what you have and use the correct terms.

            Wish there was an edit feature.

  8. Bruce H says:

    Oh, yes, thank you for that clarification. So all the loads on the RV are DC loads @ 12V (nominal)? That certainly simplifies a lot of things (of course, for the same wattage, you’re pulling 10 times the amps but that can be accommodated too).
    Again, I’m sorry for your damages caused by the poor labeling of the sockets. BH

    • Mike Sokol says:

      On a side note I should write about, mixing 12-volt DC and 120-volt AC in an RV can become confusing simply because a black-colored wire in a 12-volt DC system is typically negative or frame ground, while in a 120-volt AC system it’s the hot wire. So you can have a spot in an RV where white wires and black wires are electrically tied together. Yikes!!!

      Mike Sokol

    • Paul says:

      The trailer is a mixture of 120 volt circuits and 12 VDC as Mike mentions (that would be a great article, by the way). Like I mentioned earlier, with all of the different possiblilties regarding voltages, things should be clearly marked throughout the trailer, extension chords, dog ones and receptacles. There is just too much money and safety riding on this potential mistake.

      One thing we do in our construction specifications, is require contractors to clearly mark everything. The NEC is going in this direction too, regarding increased requirements for Arc-Flash Warning Stickers. Stickers are cheap, easy to install and could potentially save a life.

      • Bruce H says:

        Ok, so I understand this.

        There is a 12V system with a battery and when the RV is not plugged into “shore power”, there is 12V available from the battery; when it is plugged in, the “converter” provides 12V into the system and charges the battery (..ies). There is also a 120V system and when the RV is plugged into shore power there is 120V available from that system and when it’s unplugged, there is no power available from this system.

        Is this correct? No 120V unless it’s “plugged in”? Thanks, BH

        • Paul says:

          Yep, you got it. Certain circuits do not run without the shore power. The converter runs off of one circuit breaker from the shore power supplied electrical panel. The converter, battery and truck power can all provide power to the fuse block that supplies the 12 VDC circuits.

          I just went back to read all of my posts and it’s pretty obvious to me that I have learned a lot in the last few weeks. I really appreciate your response and Mike’s page. This was clearly the most informative site regarding this subject.

          Thanks,
          Paul

          • Mike Sokol says:

            Paul,

            I’m glad that NoShockZone was a help to you. RV’s can be complicated electrical beasts, so education is the key to avoiding expensive mistakes.

            Mike Sokol

          • Bruce H says:

            Thanks for that info, Paul. I assume that you know that if an inverter were put into the system replacing the converter, it would act as the converter does but also does more.
            When the RV is plugged in, the inverter would provide 12V to the 12V system and charge the battery; it would also pass through 120V to the 120V wiring system. When the RV is no longer plugged in, the 12V system would be powered direct from the battery and the inverter would pull power from the battery and provide 120V to the 120V system. When the vehicle is going down the road, either the engine alternator or an auxiliary alternator would provide 12V to the battery, keeping it charged and providing power to both circuits (the 12V directly and the 120V through the inverter). A generator could be installed in the RV but it would just provide 120V and operate like a shore cord.
            Using a system like this, you always have power to 12V and 120V systems; but it should be added that pulling 120V from the battery(ies) can run the batteries down pretty quickly. Since this is pretty flexible and provides power to all the circuits in many situation, it’s more useful than the converter alone. (But, an inverter is a lot more expensive than a converter, and there is the issue of the batteries being run down.)

  9. Bill Johnson says:

    Mike,

    I am about to purchase a small motorhome….the previous owner of my house had one and installed an outside plug at the parking area to, I assume, plug in their RV…the plug looks just like the one on the wall in the house…110V… not sure about the size of the breaker or the Romex….how would you deal with this if you wanted to plug in your RV to this?

    Bill

    • Mike Sokol says:

      Bill,
      I’m guessing that your motorhome uses a 30-amp/120-volt shore power plug, technically called a TT-30. If that’s the case I would recommend that you have your home outlet (Edison outlet) changed out for a small RV receptacle box. Something like this would do the trick: http://rvpoweroutlet.com/p-4-home-rv-electrical-box-30-amp-no-breaker.aspx Also, you’ll need to check to see what size Romex wire is already run there, but it’s probably 12-gauge which is rated for a max of 20 amperes. If so, you can’t upsize the circuit breaker to 30 amps, but it’s OK to feed a 30-amp outlet with a 20-amp circuit breaker and 12-gauge Romex. However, I would suggest you label the TT-30 outlet as “20 amps” so that no future inspector has an issue with it. As long as you don’t have everything running at once in your motorhome, you’ll probably be fine with 20 amps of power. This is WAY better than using some sort of 20 to 30 amp dogbone adapter plugged into your existing outlet, since they’re notorious for failing and allowing a hot skin condition to occur on your RV.

  10. wayne ballou says:

    So- I actually did this all myself— I personally wired the connection a 220 (240v) and fried my trailer-

    even though i made an idiots mistake I am (at the moment) feeling confident i can go through and replace what is needed and sort it all out…

    so far i diagnosed that the
    A/C doesn’t turn on
    Fridge Doesn’t turn on
    No outlets work

    Microwave does work! 😀

    …. that said, i am assuming the converter (inverter?) is fried…

    I volt tested my main breaker panel and I am getting no voltage on it… I am assuming this is because of the converter being fried in the connection before hand?

    so before i go replacing everything, is there a chance that when i replace the converter the other non working items may click on (main breakers and such?)

    just curious

    • Mike Sokol says:

      Perhaps, but don’t count on it. I would start by replacing the converter, than powering up one thing at a time and evaluate. How long was this running on 240 volts? If only a few seconds, then probably just electronics was fried. However, if it was left on for a long time (minutes) it’s possible that some wring was melted. But you really do have to start as far upstream as you can and replace each fried item, then check for operation.

  11. Rich Line says:

    I know stuff happens but I have wired a few 30 amp receptacles and all were clearly marked.
    Yes I knew they were for 120 v so I was looking for the hot screw. If I was wiring a 30 amp 240 volt besides the
    proper receptacle I would be using a 4 conductor cable.

  12. Dummy in Cape Coral says:

    I am the idiot everyone talks about. I made a “converter” out of a dryer cord and a 30 amp receptacle and plugged it in. Fried my 2500 watt inverter and microwave. Cost me $2000.

    I never thought it was 240, just never entered my mind. (small as it seems now). But I was also under the stupid impression that my surge system would suppress the error. Nope. Heard a loud “bang” and everything went dead. Needed a new inverter for $1600 and a new microwave/convection oven for $400 (used).

    I have been kicking myself every day since. I am a cheap sob and wasting money on stupidity makes me nuts.

    • Bruce H says:

      If we disqualified anyone who has done something “not smart”, we wouldn’t have any readers or any followers.

      Sorry it happened that way for you. One question, if the only outlet available for an RV hookup is a 240V and the vehicle is wired for a 120V panel, is it possible (or if possible, good practice) to wire one “hot leg”, neutral, and ground and use feed that 120V power into the RV? I know that it would be “out of balance” and the neutral would be carrying all the “negative load” but would that be a bad thing? Is the gauge of a neutral wire usually large enough to carry the full return path of one leg of a 240V circuit?

      And the reverse, if you have only a 120V and the RV is wired for 240V, can you supply the 120V feed to both pins of the 240V plug (neutral and ground connecting straight, of course)? You wouldn’t have “phase” and I guess that the total power use would again be “return path” down the neutral but 120V does that anyway.

      Implicit in this is the inverter being set to limit power use to approx. 14A on a 20A/120V circuit.

      The details are tricky. Thanks, BH NC USA

  13. josey miller says:

    I plugged a 120 volts technics receiver to a 220 volts. Would it be still repaired or not? It turned on and died quickly. No bang or whatever, it just died. when I remember that i was not able to plugged it in the transformer with 120 volts, I realized my mistake. Please help if this can still be fixed. I was thinking , it might only be a fuse .

    • Mike Sokol says:

      Maybe it’s only a blown fuse, but most of the time there will be transistors and capacitors blown out. Still, it’s worth a try to identify and replace the fuse. Let us know how you make out with the fix.

  14. braf says:

    I wired a plug to use a generator the way I do 220 and as of now all works except a TV.
    What is the chance I might get lucky and all other things are OK?

    • Mike Sokol says:

      Maybe, but a lot of electronics will suffer stressed components such as bulged capacitors and transistor junctions that have been damaged. So over-voltaged appliances might work for a few days or even a few month, and then just die. If there’s an insurance claim you should ask for them to keep the ticket open for a year in case your refrigerator dies. However, it sounds like you did this yourself, so the best you can do is hope that nothing else dies. So I would use everything you can as much as you can for the next few weeks, and most of the damaged gear will probably show up soon.

  15. tom kuhn says:

    I have a 50 amp 220 outlet I used for a previous RV. I want to use a 50 amp to 30 amp adapter to power a new RV which used 120v 30 amp. I tested to output of the 30 amp end of the adapter and it has 120 from leg 1 to ground, 120 between legs 1 and 2 and 0 from ground to leg 2

    will that keep my new RV safe?

    • Mike Sokol says:

      Yes, that’s a safe way to do this. That’s because a 240-volt receptacle in the USA is really two separate 120=volt legs that are 180 degrees out of phase with each other. So a 50-amp/240-volt shore power plus uses both 120-volt legs, while a 30-amp adapter only uses one of the incoming power lets which results in 120-volts being supplied to the RV. Where electricians and DIY guys get into trouble is when they improperly use both legs of the incoming 240-volt wiring to connect to a 30-amp/120-volt TT-30 plug. That’s what will destroy an RV’s electrical system in a few seconds.

  16. Stanley Fink says:

    I have a situation that’s a little different, we had the panel at our church replaced which is a 3-phase delta system (the third leg is 240v to ground but still only 240v to the remaining two legs. So the 50 amp breaker for the rv was put across the high leg because it’s “240” so any 240 appliance will work fine. What the electrician didn’t realize is that an rv is not a true 240 it’s actually two 120’s. So technically I can’t blame the electrician since it would of worked on any other appliance.
    Will I have all the same problems or will it be less or worse? The ac blower and compresser run but does not cool, the display on the microwave doesn’t work but the light comes on when you open the door, the display on the fridge works but not sure if it cools. The slides all work and the tv, all the 12volt lighting works but it could of still been powered by the battery. I didn’t get a chance to check everything since I just packed it back up and came home since I couldn’t stay in it.

    • Mike Sokol says:

      Stanley,

      Well, you’ll have exactly the same type of over-voltage damage as a mis-wired TT-30 outlet. While High-Leg Delta panels never exist in a residential home, they should be well known to industrial and commercial electricians since they were very popular in the 60’s and 70’s in commercial buildings that used mostly 3-phase motors. I’ve personally been in dozens of churches with this type of panel and also know of at least one campground that miswired a whole bunch of pedestals from the “High-Leg” bus in the panel. Lots of damage with that one. I just don’t understand why electricians don’t use a meter to check new circuits. It only takes a few seconds and will save thousands of dollars in damage. Likely everything with a circuit board that was running from the 120-volt circuit is damaged. Do you or the electrician have insurance to cover this?

      • Stanley Fink says:

        I’m not going after the electrician on this since a 240 volt appliance would of worked fine on that breaker and since he didn’t know it was a rv when he wired. The worst part was I “checked” the location by counting down but miscounted the high leg. I’m going to see what all it fried before I consider using the insurance for the repair. Will probably end up paying out of pocket since was partly my mistake.

        • Mike Sokol says:

          Stanley, he miswired this outlet for EVERYTHING that fits into it, not just an RV. So if you had plugged a stove into this receptacle (its normal function) you probably would have burned up the stove electronics, but not the heating element. Nope, he needs to own this mistake since he didn’t meter it. If he’s licensed then he missed that part of the code exam.

          • Stanley Fink says:

            I’ve been told by several electricians that any 240v appliance would of been fine. Stove, dryer, water heater, single phase AC etc would of been fine since voltage between any two of the three is 240/208, it’s only to ground that it’s the higher voltage.
            http://www.bigassfans.jp.php53-6.dfw1-1.websitetestlink.com/onlineguides/Powerfoil8/Delta_secondary.png

          • Mike Sokol says:

            Not true….. With all due respect, they don’t know what they’re talking about. A lot of 240-volt appliances use the neutral and one hot leg to drive a 120 volt motor (in a clothes dryer) or 120-volt control circuitry (in a stove). So there’s a 50/50 chance that this mis-wiring condition would have blown up an appliance. Your electrician created a 14-50 receptacle with a serious code violation that should never have passed inspection. The next person who plugged into that outlet would also have blown up their appliance or RV. Don’t ask an electrician about this, you need to talk to an electrical inspector. Electricians who wire outlets without metering them are asking for trouble.

  17. Eddie B says:

    Mike

    I had a friend wire my 2005 Dutchman trailer at our hunting camp today. He zapped it with 240.
    There was a loud pop and lots of smoke that came from the power box inside the trailer with in seconds. No breakers were on except the main breaker which I turned on. When it blew. The coverter is toast. Is there a chance sense my other breakers were off that I saved the AC and other things. When were took the convert out my 12 volt lights worked off the battery. Thanks

  18. Bruce H says:

    Again, a slightly different issue. I’ve recently moved to a new storage site. They have three-pin 240V outlets; there are two female “blade” terminals and one female round terminal in each outlet/socket. I’ve checked with a meter, they show 120V from each blade terminal to the round terminal and 240V between the two blade terminals. My RV is wired with a 14-50 plug with four rectangular blade connectors. It splits to two 120V legs at the main breaker box; there are no 240V devices in my RV.
    What do I do? I’m assuming that I’ll have to ask the storage owner to convert to a 14-50 socket. What will that involve, such as running new wiring, etc.?
    Many thanks, I’ve learned a lot from your site and forum. Bruce H, North Carolina, USA

    • Mike Sokol says:

      Well, you’re most likely looking at a 30-amp/240-volt dryer outlet that became popular (and was code compliant) in the 1960’s as a way to save copper. You can still find them in a lot of US homes, and they’re grandfathered in so homeowners don’t need to replace them. But as far as using them with an adapter to power an RV that seems like a bad idea. They don’t have a dedicated ground wire, hence the 3 wires vs. the 4 wires required by your RV’s shore power plug. I think you should ask the storage facility to install a proper receptacle with correct wiring. Yes, they’ll need to run new wiring back to the service panel to do this correctly. They can’t just put in a 14-50 receptacle and bootleg the ground to the neutral wire. However, since this is a storage facility and doesn’t need full 50-amps, they can likely run even a 20-amp breaker with 12-gauge wire to the 50 amp receptacle, marking it for reduced amperage. That will save some money on copper and maybe get the job done.

      • BH says:

        Thank you for this information. I was pretty sure that an adaptor would not be acceptable (or if *just acceptable*, I wasn’t going to go that route). I’m pretty sure that the storage place is amenable to rewiring — they’re pretty good at staying up to date with things. This information is very helpful.

  19. Craig says:

    Mike,

    I have a 50 amp plug I had put in, and from both hots to the neutral, it’s 120, but both hots together are 240. Is this wired correctly?

    • Mike Sokol says:

      Yes, that’s correct. Most consumers don’t realize that we have 240-volts coming into our homes in the USA, but it’s split in half with the neutral. So Neutral to either Hot should measure 120 volts, and Hot1 to Hot2 should measure either 240 volts or 208 volts (depending if it’s wired into a single-phase panel or a 3-phase panel). Ground to Neutral should measure very close to zero volts if there’s no load on the panel, but can go as high as 3 volts during high current draw in the panel, and still be in code compliance.

  20. Dan says:

    Mike

    Thank you for bringing this to light. I had been a licensed journeyman electrician for over ten years. Wired a plug for 30A, 240V. Costly mistake; more folk need to understand that RV electricity does not follow the norm for other wiring.

    • Mike Sokol says:

      I get emails every week from an electrician or homeowner who’s made this same mistake. The first time I saw one of these TT-30 plugs on an RV I assumbed assumed it was 240 volts, until I looked at the plug closely and saw it was rated for 125 volts. That was the hint I needed to avoid making an expensive mistake as well.

    • Keith says:

      Same happened to me, 240 v instead of 110 v,
      It only blew a fuse in my panel, didn’t harm anything else, year later still no problems.

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