Originally Posted by rssnape
At my storage building I have a 30 amp 120 volt receptacle. I have blown the 30 amp fuse on the buss that it is connected to a few times, so I want to add more power. The easiest option is to pull in one more wire and connect another leg to get 240 and change the receptacle to a 50 amp. The wire will only be rated for 30 amps so it will only have 30 amp fuses installed. This basically doubles my current capacity and should eliminate any blown fuses.
Changing the wire and disconnect box to get a full 50 amps is much more involved.
Your idea has some merit, and should increase the amount of available power. If you have an energy management system that detects the type of power, and automatically sheds loads to keep it under the available limit, it will be confused by this arrangement. There will be nothing that prevents it from allowing more than 30 amps per leg (other than the breaker feeding the outlet.) Not likely to be a serious issue, but something to be aware of, since you will have to do your own load management and rely on your management system.
So it should work, but is it legal? You say you want to pull an extra wire. Are the existing wires in conduit, and you are adding an extra single conductor? If so, is the conduit large enough to handle the extra power from the added conductor? Is there enough volume in the junction boxes to legally allow the addition of another conductor?
If the existing circuit is not in conduit, and is using jacketed non-metallic cable (Romex), then is it legal to run another cable in parallel to it? I would doubt that it would be legal to run a single non-jacket extra conductor outside of conduit. And does the junction box for the socket have enough volume to support the extra conductor?
Finally, while it's obviously bad to put a 30 amp socket on a 50 amp circuit, is it legal to put a larger socket on a smaller circuit? When you get right down to it, the breaker doesn't instantly trip when it goes over 30 amps, it may be possible to draw 35 or 40 amps on that wire for a while - will that be enough to heat things up and cause problems before the breaker trips?
And it doesn't hurt to say that the second hot line must be on the opposite supply phase from the existing hot line, because you will overload the existing neutral line if it isn't. But I think you know this since you're mentioning 240 volts.
I don't have the answers to all of these questions, but they should be answered before you do anything. It's so tough to give definitive suggestions without seeing the actual conditions, so it's always a good idea to get the a professional opinion from someone who can actually see the job conditions, even if you are planning on doing the work yourself. Often times, your local building inspector is willing to talk to a homeowner about planned work and give advice about what is an isn't safe and legal.
Theoretically, your idea will give more power to the RV, and your RV will be happier. But will it be legal, and more importantly safe? It would be better to find that out before doing the work rather than finding out the hard way.
Now, with all of this said, if you're going to be pulling new wire(s), why not just do it right and pull a new 50 amp 240 volt circuit and not have any issues? It's a little more up-front money and work, but in the end it's the right thing to do.
Originally Posted by Melmoses
I guess you are aware that your MH is 120v so a 50 amp box should be wired as such. If you connect a 50 amp 240 dryer type box you will fry everything inside.
As long as that 50 amp 240 volt dryer socket is a standard four prong socket, there will not be an issue. A 50 amp RV uses the exact same plug and socket as the modern 50 amp 120/240 volt 3 pole 4 pin dryer socket. The four pins consist of two hots, one neutral, and a ground.
If it's an older socket with only three pins (no neutral) then it would be a very bad thing to connect to, whether it be 30 amps or 50 amps.