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I need to add an EV level 2 charger. Among other reasons, I just overloaded the garage circuit, frying a GFCI outlet when charging my BMW i3 and golf cart at the same time.

I'd like the wiring to be non-visible, so no surface conduit. After some deliberation, the seemingly best solution I've came up with is to add a new load center onto my main feed from the transfer switch. The load center (shown in yellow below) would be located directly below the transfer switch in a wall void where I'd have easy access to wire everything up. 2x 6-50 outlets would be beside the load center.

The would avoid wiring through the exterior wall, tearing up insulation, notching studs, drywall repairs, etc. Also, the existing panels are generally full, so I'd have to decommission a circuit (possible), add a subpanel, or do some other trickery to make room regardless.

I'm generally familiar with wire sizing, grounding, conduit requirements, etc. Stepping down to 4awg feed to the new panel off the 500 mcm main wire is a little concerning, but I read this is just how these things are done, as the feeds to the other panels are also stepped down in conductor size from the 500 mcm.

The main issue I believe to be facing is adding a 100 amp branch off the 400 amp service when two 200 amp branches already exist. Does the service wire being fused make this OK? Is the generator wire not protected? (it's a 75kw generator)

What all am I missing here?

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  • Do you really want the chargers on the generator? – ThreePhaseEel Nov 7 '18 at 12:40
  • I'm mostly indifferent, but it would probably be a positive if they were to be on the generator. – crazyjncsu Nov 7 '18 at 12:44
  • The generator already cannot supply 400A: a 75Kw will be about 315A. – ratchet freak Nov 7 '18 at 16:41
  • As a sidenote: what's the rationale behind the combination of a 400A transfer switch and such a large generator? – ThreePhaseEel Nov 8 '18 at 3:33
  • I’m not sure, as the generator was already there when we bought the house. – crazyjncsu Nov 8 '18 at 20:42
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If you supply from the 500mcm you could use the 10' tap rule as listed in the NEC 240.21.1 chargers are a continuous load so you would need to size the wire at 125% of the nameplate for the 2 chargers as a minimum (the wire size can not be below 1/10 ampacity but you are fine here) so your plan sounds good.

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