120volt to 240 volt conversion

April 8, 2005 4:58 PM

I have an older travel trailer that I am turning into a guest house. The trailer runs on 110/120 volts and the main powercord has an oversized end plug with angled blades and cylindrical grounding plug. Kind of like the powerplug on an oven. How do I go about converting this to work with the existing 220/240 breaker? Is it complicated / unwise to do this myself? My electrical experience is limited to installing new light fixtures and electrical outlets.
posted by mollykiely to technology (8 comments total)

Because the 220-volt breaker is enough voltage to kill you if you screw up, I'd call a pro.
posted by Miko at 8:22 PM on April 8, 2005

It's 110V, you just need an adaptor to plug it in to a regular outlet. It has a funny shape because it is rated for 30 amps, whereas most 110V outlets are only rated for 10 or 15 amps. If you are going to permanently have it hooked up, and are going to be drawing alot of power (like an air conditioner, microwave and coffemaker all at the same time) you could get a 30amp breaker installed and the proper receptacle wired up. Other wise it may be possible to overheat your extension cord or trip your breaker when running alot of stuff. If you are going to just run lights and stuff, then just go to an RV place get the adaptor.
posted by 445supermag at 8:40 PM on April 8, 2005

The reason for the large plug is that it's built to handle much more current than the typical plug/socket combo.

The problem. House power comes in like this..

-120V----------------N---------------+120V

To get a 120V circuit, we tap it like so...

-120V----------------N---------------+120V
  |------------------|

...or like this.

-120V----------------N---------------+120V
                     |-----------------|

To get 240V, we tap like this.

-120V----------------N---------------+120V
  |------------------------------------|

Thus, the problem. Your 240V breaker is wired from one phase to the other. A 120V breaker is wired from one phase to the neutral bar. So, to hang your trailer, you can't use the 240V breaker.

The rub: Most one bar 120V breakers won't handle the current. You need large ones. The usual answer is to install a subpanel for just that circuit, with a larger breaker (and often an external cutoff switch.)

As you might have guessed, this is complicated, and given the energy involved (120V @ 50A = 6000W), all I can say is "Call a pro."
posted by eriko at 8:41 PM on April 8, 2005

Ahh, 445Supermag provides me with a bit of info I didn't know -- if it's just 30A, then you can get a single bar breaker and wire it into the normal panel. I was thinking that you'd be dealing with 50A or so.
posted by eriko at 8:44 PM on April 8, 2005

I'll be running lights and small appliances, and a water pump / water heater. The range and icebox are propane. But, Miko had me at "Because...". Sounds like the least damage I could do is blow out the trailer itself, and that killing stuff (me!) would be a real drag. I'll go with a pro. Thanks!
posted by mollykiely at 6:54 AM on April 9, 2005

Also, that is the best idea for a guest house I ever heard. Good luck.
posted by Miko at 7:09 AM on April 9, 2005

Ahh, 445Supermag provides me with a bit of info I didn't know -- if it's just 30A
Actually, you remind me that I was assuming that it was 30 amp (because it was older, and that's what mine has), but there is also a 50 amp plug used on the "Ultimate Behemoth" sized RVs with two deep fat fryers and a pizza oven (I never miss a chance to misquote "The Simpsons"). You can google around and find pics of both, or just visit an RV place. Or email me and I can send a pic of mine (and adaptor). If you hook it to 110V the only thing you can do is trip the breaker in your house. 220V does not enter into this at all, the trailer is not 220V, they don't make an adaptor for 220V, you would have to make an adaptor out of two mismatched plugs to ever get 220V.
The water heater is probably propane also and more than likely you don't need to run the water pump, just run a hose from an outside faucet (hose should be rated for drinking water, not a cheap garden hose).
posted by 445supermag at 12:53 PM on April 9, 2005

Here is a chart of many different NEMA receptacles, the plug on your trailer is bound to be one of them.

The safety problem is pretty minor really. If you get an adapter like 445supermag suggests and plug the trailer in, the breaker will protect you from overloading the circuit.

It may be a long term problem however, because it sounds like you could be pulling close to 15amps for extended periods (like when the water heater is going). Breakers are meant to be reset only very rarely and they take a long time to switch when run near rated current.

Another issue is whether your outlet is weather proof when in use. In the electrical wiring FAQ:

NEC: There's a new rule on outdoor outlets. If exposed to the weather, and if used for unattended equipment (pool filters, outdoor lighting, etc.), the outlet must still be weatherproof even when the device is plugged in.

You probably don't have an outdoor outlet that can meet this standard. It might not be required, because your house met the standard when built. You want to have a fully weather proof outlet regardless.

With a weatherproof outlet and a breaker there shouldn't be any safety issues, but you should still investigate the electrical code issues. This must come up regularly around the RV community. Try asking questions on RV forums or at RV supply businesses.
posted by Chuckles at 9:45 PM on April 9, 2005