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I'm renting a garage, and it gets too cold in the winter to work comfortably. I ordered an electric 30 Amp rated space heater but realized that I have a 50 Amp, NEMA 6-50r receptacle. Obviously I cannot just use a straight-through adapter or different plug because that would defeat the safety of the expected 30 Amp breaker circuit. However, would it be safe to create an adapter where it goes from a 6-50p pigtail, into a circuit breaker box with a 30 Amp breaker, which is wired to a NEMA 6-30r receptacle? All this would be using properly sized wire and grounding of course. Being a rented space, I don't want to be installing any permanent fixtures.

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Yes. Your plan will absolutely work, and be safe. Make sure you properly use flexible cable (likely SOOW) with the breaker box you put together, and consider strain relief where the cable exits the box.

However, the cheaper route you mentioned, isn't as dangerous as you think.....

Consider any device you plug into a normal 15amp outlet. Very often, the circuit, and breaker are 20amp, even when the outlet has a 15 amp style receptacle. Say you're plugging in a cellphone charger. It needs 0.1 amp. Yet your circuit breaker exposes it to 200 times that! Omg let's freak out and build a special subpanel at the outlet to provide a breaker appropriate for the cell phone charger. Wait. Nobody does that.

Plugging a 30 amp device into a 50 amp outlet is no worse than plugging a 0.1 amp device into a 15 amp outlet. Just change the plug and be done with it.

But if you enjoy building sub panels, enjoy this project! If you build a special sub panel just for this, you should consider adding 20 amp breakers and outlets to it also. Could end up being super convenient to have.

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  • If the heater would short out internally, isn't it the case that the 50-A breaker would trip before any significant heating of the (30-A rated) power cord or internal conductors would occur? So the only condition which would cause damage and be a risk of fire would be if the heater or its cord would fail and start passing say 48 A. Of course, if there was somehow a fire and the instructions for the heater in fact say to use a max 30 A breaker, then there could be some legal consequences. – Jim Stewart Dec 24 '17 at 16:05
  • Breakers in your breaker panel AREN'T FOR protecting things plugged in to the outlet from going over current. Breakers are for protecting the fixed writing and receptacles permanently installed in the structure. That said, a dead short inside the appliance means thousands of amps, and would likely trip the breaker. If the heater instructions say anything if the sort, it would likely be 'do not modify'. In which case of you really care, buy or make an adapter cord with a male 50a and female 30a. Make sure both of their screw terminals are rated to accept the gauge wire you're using. – Billy C. Dec 24 '17 at 16:22
  • So I decided to go ahead and build a box with a breaker. Even if it doesn't really add much safety it's a good project for me to get familiar with home wiring. I ended up only finding 10-3 SOOW cable, which should be fine since I have a 30 Amp breaker. The breaker is a double slot two circuit model rated for 30 Amps. I assume then that I put the black("hot") and white("neutral") through each side of the breaker and hook the ground up to the ground bar, and ground the metal boxes. Does it matter which side of the 6-30 I hook the black up to? – Jeff S Dec 29 '17 at 2:40
  • You need two breakers because you're using an underrated infeed cable. And could add more loads too the panel in the future. Both breakers should be double pole. Never connect a neutral to a breaker. The 10-3 should have a red and a black hot. Those go to the terminals on one breaker. The 6-30's two hots go to the two terminals in the other breaker. Grounds so go to the ground bar. Neutral to a separate, insulated neutral bar. – Billy C. Dec 29 '17 at 2:46
  • I misspoke when I said neutral, was using the terminology used with the plug instructions but it's actually two hot wires and one ground wire. Isn't a single double pole circuit breaker (the style rated for 240v that occupies two breaker slots) what is needed? One hot going through one pole, the other hot going through the other? There will not be any other circuits added to this box in the future, it is using a breaker box with only room for the one breaker. – Jeff S Dec 29 '17 at 6:28
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Another simple and inexpensive fix is to down-size the breaker. And then simply replace the receptacle.

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