Hello Mike,
If you have a 50-amp trailer and 30-amp service is the only service available at the campground post, can you show me a wiring schematic of the adapter pigtail I would use to adapt from 30-amp service to a 50-amp trailer. When we run across this situation I am always curious how this adapter can supply power to both 120-volt sides of our 50-amp trailer connections. Thank you for your comments on this subject. —Gary Reed
Hi Gary,
Here’s how it works. First of all, you need to understand that the 240-volt plug on your USA RV really doesn’t supply any 240-volt appliances (except in very rare circumstances). It’s really supplying two separate 120-volt services. If you take a look at the diagram below, you’ll see how a service panel in the USA is split down the middle to make 120 volts.
So to make a 30-amp to 50-amp adapter, you just need to go backwards. The TT-30 plug is wired to the 50-amp outlet with the Green ground to ground, and the White neutral to neutral. But the Black hot wire from the 30-amp plug jumps to both hot sides of the 50-amp outlet. Now, this doesn’t really generate any more power than a 30-amp outlet can provide; it’s just jumping the 120-volt power to both sides of the 50-amp outlet.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
I’d like to add a note. This will NOT give you 240 volts, although both sides of the service will have 120 volts. They are supplied by the same phase, so the voltage will not add.
That’s correct. But as I noted there’s generally no 240-volt appliances in a standard RV. In your home the stove, hotwater heater and other big appliances usually use both hot legs for 240 volts. RVs are 120 volts except for some special things like the Cheap Heat products.
nor will the 240 volt apliances work wired this way.
Correct, but 240-volt appliances in an RV are pretty rare, except for full sized coaches or in smaller RVs with a 120/240-volt Cheap Heat system installed. And IIRC the Cheap Heat system knows how to deal with 120-volts instead of 240-volts and will automatically reduce its wattage output without any damage. But I’ll let Larry McGaugh give the definitive answer on that, since he designed the Cheap Heat system.
Mike, if the two legs are different phases (they have to be to get 240V) wouldn’t you be creating a dead short by jumping between them? I always thought the 30 amp adapter just connected to one leg of the 50 amp plug.
No it doesn’t create a short circuit going in this direction. Of course, if you jumped between the two hot legs on an incoming 240-volt outlet there would be a dead short with spectacular results. But we’re going the other way by jumping a single hot line to both hot legs on an outlet.
I don’t see the difference – AC current goes both ways, and you show a direct connection between the two hot legs. Why is it any different if the direct connection is connected to another wire? If the post is wired with both hot legs on the same phase, then it wouldn’t matter, but then it wouldn’t be a 240 volt outlet.
Don’t confuse polarity reversal from AC (60 Hz) with an actual current reversal. Yes, technically the electron flow does reverse itself 120 times a second, but the power is flowing in one direction only. In this case, it’s flowing from the circuit breaker panel (power source) to the load (the RV). So you can jumper a single power source to multiple loads, but you can’t combine power sources together.
Mike, if you put a jumper between the red and black wire on a 50 amp service at the pedestal you will have a dead short. The BLACK wire and the RED wire are 180 degrees out of phase. Granted you can put both hot legs together but they MUST be the same phase i.e.. Black to Black or Red to Red, never Black to Red.
I tried to attach a diagram but this system won’t allow me to do that sorry.
Mr. Larry you are 100% correct. Attempting to do otherwise will be an extremely dangerous situation with spectacular results & will certainly fry everything electrical as it would likely jump the breakers prior to burning out.
Other than violating NEC code for proper use of the 4 prong 240 receptacle, the 30 amp 120 vac source will supply the same 120 v phase to both 120v hot legs in your rv. However the source pedestal will be current limited to 30 amps while the rv is designed to pull up to 50 amps. So it is quite possible and maybe expected that this adapter hack can trip the source 30 amp breaker. Probably after a few minutes to several hours depending how much more than 30 amps the rv is pulling. Not the greatest solution for reliability in the middle of the night.