30A plug to 50A receptacle

I am building a eHERMS and need to demostrate it at school. The problem is that they only have NEMA 6-50R receptacles and I have a NEMA 14-30R plug that has the GFCI inline with the cord. So I have to make some kind of adapter for this. So do I combine the Ground and Netural wires to go from the 4 wire grounding to the 3 wire grounding? Or should I do something else?

It depends, are you using the neutral for anything? for example are you splitting off one leg of the 240 to get 120? If that is the case than you are using the neutral for the current return path. If you are not using 120 anywhere in the system than you really don't need the neutral connected at all so you could just eliminate it from your adapter. If you are using the neutral and tie that to the ground line you could have one of 2 issues.

1. you could trip your inline gfci because current is being returned on the ground line.
2. you will put current onto the buildings ground line which could be really bad (could end up giving some a pretty good shock)

ajm163 said:

1. you could trip your inline gfci because current is being returned on the ground line.
2. you will put current onto the buildings ground line which could be really bad (could end up giving some a pretty good shock)

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Agreed.

A 2nd option (provided you really need the 120v in one connector), would be to make a bastard child adapter box where you'd have your mating connector spliced out to two separate plugs: #1) the 6-50 for both both ends of the 240 and ground, #2) a standard 120v plug where you just snag the neutral line and ground as well. Common the grounds inside the box for the additional safety factor. And finally - don't ever tell an electrician that I said this was 'ok' - but yes, it will work.

lordhazard said:

Agreed.

A 2nd option (provided you really need the 120v in one connector), would be to make a bastard child adapter box where you'd have your mating connector spliced out to two separate plugs: #1) the 6-50 for both both ends of the 240 and ground, #2) a standard 120v plug where you just snag the neutral line and ground as well. Common the grounds inside the box for the additional safety factor. And finally - don't ever tell an electrician that I said this was 'ok' - but yes, it will work.

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this if you need the 120.... not to code in the least but would work and don't tell anyone in building maintenance what you are doing a head of time