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I have a PV inverter that is rated at max output of 25A, and the backfeeding breaker in the main panel is a 35A. This is appropriate considering the 25A * 1.25 and round up sizing formula. However, I'd really like to move an additional size up to a 40A breaker so that I can replace the existing one with a quadruplex breaker to free up a space in the panel.

The main panel is rated 200A so I should be allowed up to a 40A PV breaker. The conductors to the inverter are #8 THHN in conduit.

Is there anything that should prevent me from swapping this 35A breaker out for a 40A quadruplex?

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  • Will the new breaker still provide common-trip to the PV inverter? – Harper - Reinstate Monica May 18 '19 at 13:58
  • Will there be a means to clamp down this quadplex breaker so it cannot rock out in the normal way breakers are removed? The current breaker either is clamped down, or is required to be. – Harper - Reinstate Monica May 18 '19 at 14:22
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Short answer. Your fine to upgrade to a 40A breaker. Remember the breaker is being used to protect the feeder conductors. If you feel you need to use the breaker to protect your inverter you should install protection just before the unit.

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Wire size looks OK

#8 wire is certainly good for 40A, so that is no reason to stay at 35A. The only thing that would forbid a bump from 35 to 40A is if the labeling and instructions on the inverter forbid 40A and required a 35A. Do check -- but I'd consider it rather unlikely, since 35A breakers are an odd duck, and not necessarily available for every panel (particularly obsolete ones).

Being out of spaces is an intolerable situation

You're in the place a lot of people are: out of breaker panel spaces. That's much worse than it sounds. Yeah, you can struggle around it -- but it forces you into compromises that have consequences - just like this one. Another compromise is the inability to use AFCI breakers to keep your family safe, or GFCI breakers where they would be more appropriate than receptacles.

It's much better to "take the pressure off yourself" and give yourself a free hand to install whatever you want in the future. Put a subpanel near this panel (or wherever, really). And think big (in terms of spaces) so you never have to do it again!

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    Note that breakers for PV inverters actually do not need to be strapped down (see NEC 705.12(B)(5)) – ThreePhaseEel May 19 '19 at 23:21
  • @ThreePhaseEel Did not know that. I assume this applies to grid-tied PV inverters which are deliberately designed so they shut off if mains power fades? That would make sense, if they were bumped out, they would shut off. (I don't have the latest NEC, can't look it up). – Harper - Reinstate Monica May 20 '19 at 0:35
  • Yes -- it's for "interactive power sources", but 99% of those you'll run into are PV inverters – ThreePhaseEel May 20 '19 at 0:53
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    @ThreePhaseEel OK, so "interactive" meaning that it is actively listening to the grid for sync and to assure mains power is reaching it, and cannot/will not backfeed unless present... that makes sense that it doesn't need a tiedown. – Harper - Reinstate Monica May 20 '19 at 1:02

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