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I am currently hunting for an AC disconnect to sit between my main service panel and the AC power that will be supplied by my solar panel microinverters.

Nearly every AC Disconnect I find online is rated at either 30 A or 60 A. It is unclear to me if that rating is the MAX breaker ampacity allowed, or if it is specific to that breaker size.

The max current used for sizing the wires from the microinverters is 33 A, so I am using #8 size THWN wire, which is rated for 50 A. This puts my current load and my protection requirement right between 30 A and 60A. Ideally, I'd like to use a 40 A breaker.

Does the AC disconnect ampacity rating refer to the max breaker ampacity? i.e., can I buy a 60-Amp disconnect and put a 40 Amp breaker in it?

Note: "AC" here means "Alternating Current", not "Air Conditioning", in case anyone was confused.

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  • Who's your utility? The utility specs I've seen for a DG (solar/...) disconnecting means require a visible blade disconnect, which your AC disconnect box doesn't qualify as. – ThreePhaseEel Nov 5 '19 at 23:59
  • @ThreePhaseEel I'm in NE Ohio. My utility is First Energy. What does "visible blade" mean? If that means externally accessible, i.e. with a throw switch, then yes, my disconnect will meet that. – rothloup Nov 6 '19 at 0:02
  • It means that the utility can open the front cover and see, positively, that the switch contacts (blades) are in the OFF position. – ThreePhaseEel Nov 6 '19 at 0:04
  • @ThreePhaseEel Ah. I'll check on that. Thank you. – rothloup Nov 6 '19 at 0:06
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    I was confused, sorry for messing that up. – Harper - Reinstate Monica Nov 6 '19 at 15:56
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The disconnect rating is the maximum amperage the disconnect is designed to handle. Yes, you can install a 60 Amp disconnect and feed it from a 40 Amp circuit breaker.

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  • So have I completely misunderstood what an AC disconnect does? I thought it had a breaker inside of it. Is it simply a big switch? – rothloup Nov 5 '19 at 19:39
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    @rothloup There are AC disconnects that have a 60 Amp breaker in them that protects against short circuit but not against overload. That's what the breaker in the main panel does. Most all AC disconnects just have jumpers hooked to a handle that you plug in to engage and pull out to disengage. They do not offer short circuit protection but again, that's what your breaker in the panel does. – JACK Nov 5 '19 at 20:02
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Good news: you can go ahead with your plan

Unlike some utilities, FirstEnergy does not require solar/DER utility disconnects for single phase, self-contained-metering customers to be of a visible blade type, which means that your idea of using an AC disconnect box for this application is workable. This is laid out in section 6.5 of the FirstEnergy Customer Interconnection Guide:

6.5 DER installed at a single phase electric service location utilizing a self-contained company meter (usually 400A service size, or less), shall have a disconnect switch meeting the requirements of the NEC between the Company meter and the DER PoC. The switch shall be clearly marked, "Generator Disconnect Switch”, with permanent 3/8 inch or larger letters. The preferred location of the switch is outside the building in the immediate vicinity of the electric meter to facilitate access by Company personnel.

As to the headline question: yes, that's true as well

Given that you are getting a fusible disconnect, as non-fusible disconnects are also a thing, you can put smaller fuses in then the disconnect's amp rating, up to a limit (if you want to put a 1A fuse in a 60A disconnect, you'd need a set of fuse reducers for that). Note that if you don't need to have overcurrent protection at that point, there's no reason not to get a non-fusible disconnect box vs. a fusible disconnect box as the non-fusible variety are somewhat cheaper and you don't have to worry about changing blown fuses in that case either.

Give me a sign, friend!

There is one thing you will have to do no matter what you buy, and that's go to your local trophy engraving shop and get them to engrave a plastic plate with the words "Generator Disconnect Switch" in letters 3/8" high or taller. You'll then need to get a small hand-riveter and a couple of pop rivets and rivet the plate to the front cover of your disconnect box; this'll satisfy the permanent marking requirement imposed by FirstEnergy.

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  • Thanks for the link to the customer interconnection guide! – rothloup Nov 6 '19 at 4:14

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