I looked at a large Motor Home a customer wants power to. It has a 6-3 w/grd 50a 240v cord on it. I checked the continuity from neutral to ground and no connection. He said he plugs it in to a camp ground with a 3 prong adaptor and it works (2 prong with ground). How would this work if there is no connection to neutral at the motor home panel. And the adaptor has no connection between nuetral to ground on the plug itself. Is there something different about how a motor home is wired?

The neutral and ground are isolated in the panel.

But then why would it work when he plugged it in at the camp park? Theoreticly there would be no connection for the neutral.

The 3-wire to 4- wire adapter plugs into a 125 volt 3-wire receptacle. It connects L1 and L2 of the motor home together so all 120 volt electrical items will function but you will be limited on how many items that can be used at the same time.

Curt

Then how would his 240v A/C work? Something doesn't make sense.

There are no 240V loads in the motorhome* the adapter jumpers both hots so it can be used for 30A 120V service.

* At least in any that I have seen.

Yes, that makes sense, I do remember campers being about the only 120v 30a service. The customer said it was 240v, but that is wrong.

Another thought:
The motor home has a 4 prong 240v 50a cord & plug. Without the adaptor, someone could could plug that into a 240v outlet and smoke the MH.

Why would that be a problem? Isn't there 2 hots, neutral and ground at the plug?

-Hal

hbiss, No there isn't, as I said early there is an adaptor changing it to a 3 prong.

It will not smoke anything.

iwire, what do you mean it will not smoke anything. If the MH is all 120v and you plug into a 240v 4 prong (on the Motor Home cord & plug). ??

Because it is wired for 120/240 although there are no 240 volt loads.

-Hal

I know an electrician that is getting ready to do a job like this , he talked to the camper dealer and they said that some of the bigger motor homes & campers had 2 A/c units , so they had to go to the 50 amp 240 volt ,, 1 phase runs the motor home and 1 A/C unit the other phase just operates the second A/C unit

Lane, exactly what I thought, but will do some further checking.

4 wire 50 amp service is common to motorhomes. It is designed for standard residential configuration 120/240. There are typically no 240 volt loads so the system can be used with one of the many adapters on almost any 30 or 15 amp recepticle (with the adapters in place, both hots are tied together so there is 120v for all the loads and 0 across L1 and L2). The panel in the motorhome is wired like a standard subpanel with neutral and ground isolated. I'm not sure when the trend started but I have a '77 model with the 50 amp service and that manufacturer was using it in the early 70's.
Just think residential subpanel and you have a picture of most of it.

So on the 3 prong adaptor, the 2 hots coming from the MH are tied together on one side, and the other side must be tied to the neutral and center ground.
So the 2 A/C's have to be 120v with one using one leg and the other using the other leg. Correct?
That could put a large load on the neutral when using the adaptor (plugged into 120v) if everything was turned on in the MH.

Right but with the adapter plug the receptacle is OCP'd at 30 amps. Turn everything on and the breaker trips.

-Hal