Installing 9500 watt, 30 amp generator. I have two 200 amp service boxes coming from the same meter. I plan to run the generator to one box and install an interlock device on that service box and relocate the needed circuits/breakers from the other panel to the generator powered panel and move the circuits not needed in a blackout to the unpowered service box. I understand I can extend the wires from each circuit to move them from box to box through conduit and need to move both the hot and neutral wires. I am guessing the grounds should also be transferred accordingly also. Any thoughts and input greatly appreciated.
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One thing is good about your plan: breaker panels are also junction boxes, so you're okay to make splices in there as needed. Where I'm less certain is if it's okay to have generator power heading through another panel.– KMJCommented Jan 24, 2023 at 0:26
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How many circuits are you looking to back up?– ThreePhaseEelCommented Jan 24, 2023 at 1:51
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30A @ 240V Only adds up to 7200W - must have 2300W of "marketing magic" power you can't use.– EcnerwalCommented Jan 24, 2023 at 2:12
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You are right. It is a 9500 watt surge with 7200 watts. Didn't mean to mislead. Thank you to all that have replied.– AndrewCommented Jan 28, 2023 at 0:03
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1 Answer
You are correct, there is no way to feed 2 panels with 1 generator (except for some arcana using an isolation transformer).
Electric current flows in a loop. On 120V circuits, neutral is an equal partner in the loop (Even if its role is underplayed). Thus neutral must come over with the hot(s). This is unlike safety ground, which only handles current during (hopefully) momentary fault conditions. Ground can simply land in the panel that the Romex cable enters. Hot(s) and neutral must be extended.
When someone installs 2 panels next to each other, it is wise to install several short conduit pass-throughs between them. If this is done, then the circuit can be extended to the other panel simply with two THHN wires white and black. (Or 3 if it is a MWBC or 120/240V circuit).
Keep hot/neutral pairs identified or twisted together, and make sure any given circuit's hot and neutral go through the same conduit.
answered Jan 24, 2023 at 0:34
Harper - Reinstate MonicaHarper - Reinstate Monica
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Interesting edge case with ground: Smart switches where they are UL/ETL listed to use ground for mA current return where neutral is not available. If you knew you had such equipment would you be obligated to move grounds for those circuits? Or is the current so little compared to the actual current used by the switched equipment that nobody really cares? Commented Jan 24, 2023 at 0:47
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1Harp,. you a bright guy and I respect and agree with nearly all of your posts....been giving you many UVs over the last couple of years. But in your above answer I have to disagree. Theoretically the first panel will be fed by the generator and of course with the appropriate automatic or manual lockouts (maybe plate style) MTS. Then the second panel could also have a MTS (plate type lockout) fed by a breaker from the first panel. Now this all just theoretical In practice, a 30 amp generator seems pretty wimpy to supply 2 200 amp panels w/o turning of HOLE BUNCH of circuits. Comment? Commented Jan 24, 2023 at 2:25
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@GeorgeAnderson The problem is neutral on the cross-feed from generator-powered panel A to panel B. If you omit neutral, you violate 300.3 because panel B neutral is getting back to panel A via the meter pan. If you include neutral, now your neutral has a redundant route (loop). That's why I mentioned the transformer, it breaks that loop. Commented Jan 24, 2023 at 5:40
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1@manassehkatz there's something to that, but we are talking <0.5mA per switch. I have to assume UL accounted for all the wonky grounding arrangements that can exist in the field, e.g. retrofit grounds. Commented Jan 24, 2023 at 5:43
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@Harper-ReinstateMonica That's a good point. Hadn't thought of that. Even though all the neutrals are connected, you're right , any neutral current would end up going thru the meter base neutral connections ...not a good thing. Commented Jan 24, 2023 at 6:04