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My old house is wired as follows:

  1. A 100A service to a meter and main panel on the outside of the garage.
  2. Main panel has a single 100A breaker.
  3. A 125A rated sub panel inside the garage with house load breakers connected with approximately 30Ft of Aluminum wires.

I am planning on adding a PV breaker in the sub-panel. According to my understanding of the NEC 120% rule for busbars, I should be able to add upto a 125*1.2-100=50A breaker in the sub-panel.

Does the 120% rule still apply normally to this scenario when the 100A main breaker is in another panel? Does the 120% rule also apply to the wires connecting the main panel to the sub-panel?

I haven't checked what gauge those wires are. Do they become part of the bus-bar, so to speak, and could the grid and the PV breaker cause the current on the wire between the panels to exceed 100A?

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I am planning on adding a PV breaker in the sub-panel. According to my understanding of the NEC 120% rule for busbars, I should be able to add upto a 125*1.2-100=50A breaker in the sub-panel.

Yes, you have done that properly.

Does the 120% rule still apply normally to this scenario when the 100A main breaker is in another panel? Does the 120% rule also apply to the wires connecting the main panel to the sub-panel?

Yes.

What they're concerned with is the panel buses being overloaded because the sum of solar power + utility power being able to exceed the panel bus rating. No circuit breaker exists to protect from that.

If you could rearrange it so there was, then you'd have no limit and could go 100A solar.

So for instance if you had a subpanel in between the main panel and this panel, which had ONLY a 100A breaker to this sub and a 100A breaker to the solar, then - diagram it out if you like - no combination of current movement would exceed 100A in any direction without tripping a breaker.

The 120% rule requires that the two supply breakers be on opposite ends of the bus, with the loads between them. There is an exception for "combo meter+main+breakers" panels with the main breaker in the middle.

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  • Thanks for the detailed response. I also found [1] in searching this topic on the web. According to that article, the feeder wires to the sub-panel don't need to be replaced as the PV current cancels out the grid current on the feeder. Is that right? [1] iaeimagazine.org/columns/photovoltaic/pv-questions/…. Mar 10, 2023 at 17:35
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    Never mind, you already answered that in your hypothetical scenario: no combination of current movement would exceed 100A in any direction without tripping a breaker. Mar 10, 2023 at 17:57

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