No power on one leg.

MEXICOWANDERER wrote:
Hokay!

NORMAL when plugging in a 50-amp plug into a 30-amp receptacle is to lose one leg.

WHY? Think about it. Powering 2 50-amp rated circuits with 30-amps worth of power in the supply line "Leaves a 70-amp 120 volts shortfall. Very similar to like plugging an eleven hundred watt microwave using an 18-gauge power wire.

An Ambulance chasing tort mongerer would sweat and get giddy at the thought of suing once the fire is put out. Another way of putting it is to have a long length of 1/2" tow chain with one 3/16ths link.

This is why you cannot find such a coupling adapter off the shelf. You can wire one yourself and I know the recipe but I will not share it. Literally and fugurativelly such a device is a fire waiting to happen.

Yes the connection can be made up but so can a few quarts of nitroglycerin be juggled for sport.

Pretty much everything here is not quite correct, unfortunately (or fortunately, depending...)

Most if not all 30A male to 50A female adapters will connect the 30A hot to both 50A hots, and this is perfectly safe. The reason it's safe is very simple: the 30A outlet will be protected by a 30A (or smaller) overcurrent device of some sort, generally a circuit breaker. If more than 30A flows through it, the breaker will trip before the wiring gets too hot to be damaged. That's the whole reason one has circuit breakers.

Similarly, a microwave could be safely plugged into an 18 gauge cord if there was a suitably sized fuse or whatever protecting the cord. The microwave wouldn't be able to cook without the fuse blowing, but there wouldn't be any fire or other safety hazard, and you could for example use the timer on the oven. Likewise, a 50A RV plugged into a 30A circuit won't be able to run as many electrical appliances at once as usual, but there's nothing inherently unsafe about the setup.

Going the other way around and connecting a 30A RV cord to a 50A outlet via an adapter (or a 15A to a 30A outlet) is more troublesome in theory, as the adapters generally don't have any sort of overcurrent protection. I looked at a 30A male to 15A female adapter the other day out of curiosity, and it had molded into it in smallish lettering instructions that it was not to be used on circuits with greater than 15A overcurrent protection. I guess that makes it legal as per code, since any use on an actual 30A circuit would be theoretically disallowed ("not installed and used as per manufacturer's instructions"), but the chances of it being used that way in actual practice are about zero percent.