I have a single inverter generator with a floating ground that connects to my house panel using a manual interlock and double breaker. It works well. I have a garage and shop on a separate meter and panel about 20 feet away. It is a separate feed from the power company transformer. Same transformer that feeds the house. Can I add an interlock and generator inlet box to the shop panel and feed both at the same time using a splitter cord on the generator? My main concern is that there are two separate grounds. Could I create a potential between those grounds? Any other hazards?
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1Is the garage panel fed from the house panel? If so, just keep the breaker to the shop panel on. If not, please describe the feed to that garage panel.– FreeManCommented Jan 9 at 12:06
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No, the garage panel is fed from the same pole mounted transformer as the house. Two service drops to two meters.– Steve BCommented Jan 9 at 14:38
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1That would be very useful info to edit into your question. Not everyone will read all the comments, and they might get deleted in the future.– FreeManCommented Jan 9 at 14:41
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1Thanks. My first post, so I will edit. I tried to communicate that it was a separate feed from power company saying it had a separate meter but maybe that wasn’t clear. Thanks for the advice.– Steve BCommented Jan 9 at 14:44
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1 Answer
I'm not certain whether there's a literal violation of electric code in this scheme but I do see a violation of principle.
The problem is this: you'll be tying the neutrals of the two services together. A two-pole circuit breaker interlock type transfer breaks only the line conductors, so regardless of whether the present interlock is selecting generator or grid power, the neutral is connected from transformer to panel to generator. If you now add a feeder from the generator to the shop building you'll create a ring of neutral. Current drawn by a load in the house will return to the house panel, where it will divide so that part returns to the transformer on the house service and part returns to the transformer via the generator and shop service. If one of the services is smaller than the other there could be a possibility of overloading the smaller neutral as well.
I believe if you used a 3- or 4-wire transfer switch at both locations rather than circuit breaker interlock everything would be okay.
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Yeah, you need to switch the neutral at both panels for this to be OK (otherwise, you wind up with what's effectively paralleled/looped neutrals, violating the intent of 300.3(B) and 310.10(H). See my Q&A in the ATS context for details Commented Jan 10 at 2:55
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That makes sense. I’ll operate the two panes with independent generators or time share power to a panel one at a time. Thanks– Steve BCommented Jan 10 at 13:52
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@SteveB Time sharing with interlock breakers won't fix the problem - the problem is the non-switched neutral. Independent generators, 3-/4- wire transfer switch, or cord-and-plug "transfer" all avoid the parallel/loop neutral. To be clear, with cord-and-plug, you'd have a male inlet socket that wires to the interlocked breaker, and all of that duplicated for the second building. Then you use a cord from the generator to the output to the inlet of the building you want to power. This is common with portables; not so much with permanent standby generators. Commented Jan 10 at 15:19