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I recently had solar PV installed. The electrician replaced my service entrance box. Power is fed to an indoor subpanel through a 100A breaker, mounted directly above the service disconnect breakers. The label on the box reads "A 100A max circuit breaker may be installed in the lowest position in the branch panel. All other positions are limited to 70A max circuit breaker." There is one slot for a 2-pole breaker underneath the service disconnect breakers. Did the electrician install it wrong by not using that slot?

box label inside box breaker labels

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Wow, that much weather damage already to the labeling. Is this door being left open, or did Square D really oversell the NEMA 3R claim?

Yes, I agree. This is a special panel specifically for solar. The solar breaker (up top) is bot part of the main bus, and gets its own connection hot-off-the-meter, as does the (notably, factory paralleled) main breaker. As a result, you do not need to (and should not) do the derating to avoid solar+main overloading the panel's bus bars.

The 100A breaker does indeed need to be in the bottom bus position. The stabs above are not rated for it.

You now have 2 empty holes in your panel. They sell blank cover plates, but they are flimsy as can be. The best blank hole filler is an actual breaker. Either Square D HOM or Eaton CL will do. Do not use Eaton BR or any other brand. Many will clip in, but you wouldn't want some fool using it as a breaker!

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    I suspect it's bending space, not the stab rating, that's limiting the slots above the main to 70A max. Note the black wire existing its load lug on the 100A breaker at an awkward angle (instead of straight on) – ThreePhaseEel Jan 18 at 0:11
  • So does the solar breaker still need to be bolted down, as it's not on the main bus? – zwiebelspaetzle Jan 18 at 1:30
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    @zwiebelspaetzle no, because solar breakers don't need to be bolted down generally as the inverter will stop supplying power if the breaker is pulled off the bus. See NEC 705.12(B)(5) for details. – ThreePhaseEel Jan 18 at 1:43
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Yes. The panel's labeling means exactly what you think it means, that breaker slot 5-6 and all above it are limited to 70A breakers max. That 100A breaker should be moved down to the bottom slot.

Additionally, that top breaker is being backfed by your solar setup, correct? In that case, it needs to be bolted down -- that's a code requirement whenever a breaker is backfed. You'll need a HOM4RK2HA bolt-down kit, which you (or your installer) will probably have to get from a proper electrical supply house.

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  • What about the 200A breaker? Why is no one mentioning that? – JPhi1618 Jan 17 at 19:09
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    @JPhi1618, that's factory installed, bolted down, and it is allowed by the labeling. Look at the diagram in the upper right -- it labels it service disconnect, and specifies 200A max. No problem there. – Nate S. Jan 17 at 19:10
  • Ok, thanks. I saw the 200A max on the lugs at the bottom, but couldn't quite read the rest. I see that the breaker numbering on the cover skips over that 200A service disconnect. – JPhi1618 Jan 17 at 19:12
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    Get your electrician back to correct this. Also, I have seen updated labels for panels stuck in boxes instead of being applied to panels so this might be right. – JACK Jan 18 at 14:58

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