Enphase microinverter branch max circuit size?

[jaggedben]

.58 in table 310.15(B)(1)?

So the AHJ's in your area have determined an ambient temperature of 150-158 degrees F?

I agree it is very hot on the roof due to solar radiation. What data is used to determine this ambient?

They've changed around the rooftop temp adder section in 310.15(B)(3) since then, but the worst case scenario still basically requires that. Say 100F ambient plus 60F ambient if not installed 7/8" above roof. Basically the attitude of those AHJs was 'we're going to enforce the worst case scenario rather than split hairs over exact ambient temps used or whether you maintained consistent spacing over the roof." Regarding the latter, if I'm being honest, we used 1" blocks under EMT so given a little hump in the roof or a warped stick of EMT and the conduit was under 7/8" high in some small place or another on most every job.

 

[shortcircuit2]

They've changed around the rooftop temp adder section in 310.15(B)(3) since then, but the worst case scenario still basically requires that. Say 100F ambient plus 60F ambient if not installed 7/8" above roof. Basically the attitude of those AHJs was 'we're going to enforce the worst case scenario rather than split hairs over exact ambient temps used or whether you maintained consistent spacing over the roof." Regarding the latter, if I'm being honest, we used 1" blocks under EMT so given a little hump in the roof or a warped stick of EMT and the conduit was under 7/8" high in some small place or another on most every job.

That makes sense. Given that caculated ambient, they do not allow PVC?

 

[jaggedben]

That makes sense. Given that caculated ambient, they do not allow PVC?

I've never asked or installed PVC on a rooftop.

 

[Designer101]

A string of microinverters is always connected in parallel. You can have as many on a string as the spec sheet allows--typically, as in the OP, the spec sheet listed a maximum 20A OCPD and that determines the maximum count.

Cheers, Wayne

one question here if we don't use combiner box that's made by enphase and instead adopted to use Load center can we use more than 20A breaker here, per branch circuit i always wonder why they limit the max inverters per Branch to 20A max in parallel.

 

[jaggedben]

one question here if we don't use combiner box that's made by enphase and instead adopted to use Load center can we use more than 20A breaker here, per branch circuit i always wonder why they limit the max inverters per Branch to 20A max in parallel.

No, the trunk cable must be protected by a 20 breaker. The whole reason is the trunk cable is 12awg. The enphase combiner has nothing to do with the limitation and isn't any different from a generic load center in any important way. It's just an Eaton panelboard that they put in a box with the Envoy.

 

[wwhitney]

The whole reason is the trunk cable is 12awg.

Is that the whole reason? I mean, if you could make your own #10 trunk cable, would it be OK to use a 30A branch breaker? Or do the microinverters themselves specify that they need to be protected by a maximum 20A breaker?

Cheers, Wayne

 

[Designer101]

Is that the whole reason? I mean, if you could make your own #10 trunk cable, would it be OK to use a 30A branch breaker? Or do the microinverters themselves specify that they need to be protected by a maximum 20A breaker?

Cheers, Wayne

i have designed the system for independent electrical contractor where he used his own trunk cable i think he basically rewired(modified) adapter cable with larger wires AWG #10 and higher and used 30A circuit per branch, I am not sure he passed the inspection but i never got revision request, assuming he did, it was more than one year ago.

 

[jaggedben]

Is that the whole reason? I mean, if you could make your own #10 trunk cable, would it be OK to use a 30A branch breaker? Or do the microinverters themselves specify that they need to be protected by a maximum 20A breaker?

Cheers, Wayne

Didn't we have more or less the exact same conversation before? Yes, we did, I looked. See posts 29, 30 and 33. (I made a typo in the last one: I meant to say you could spec your own cable if the manufacturer allowed it. But they don't).

The spec sheet does specify the 20A max, so even if you wanted to argue that fabricating your own cable doesn't violate 110.3(B), you'd still have to argue your way around that instruction. That said, I still believe the cable size is the only reason the spec sheet states a 20A limit and not something else.

 

[Steve16]

Is that the whole reason? I mean, if you could make your own #10 trunk cable, would it be OK to use a 30A branch breaker? Or do the microinverters themselves specify that they need to be protected by a maximum 20A breaker?

Cheers, Wayne

Enphase specifies max 20 amp breaker.

Heres a good one. What size breaker for 9 enphase micros @ 1.21A?

 

[jaggedben]

Enphase specifies max 20 amp breaker.

Heres a good one. What size breaker for 9 enphase micros @ 1.21A?

20A.

Okay 15A is fine, too.

 

[Steve16]

20A.

Okay 15A is fine, too.

If your inverter output circuit amperage is 10.89A x 125% why would you size up to a 20A breaker? Your wire size is a 12 for voltage drop.

You wouldn't ocpd an 7.6kw 240v inverter at 50 amps

 

[ggunn]

If your inverter output circuit amperage is 10.89A x 125% why would you size up to a 20A breaker? Your wire size is a 12 for voltage drop.

You wouldn't ocpd an 7.6kw 240v inverter at 50 amps

But you could if the wiring were sized appropriately. We connect microinverter branches with 20A breakers irrespective of how many micros are on the branch.

 

[jaggedben]

If your inverter output circuit amperage is 10.89A x 125% why would you size up to a 20A breaker? Your wire size is a 12 for voltage drop.

Because if I just standardize on 20A then I reduce the likelihood of copy/paste errors by our designer and which-breaker-do-I-need errors by our crew, and I hopefully never have an inspector tell me I need to call for reinspection after I've changed the breaker or resubmitted plans. This may sound like a hypothetical, but it's not.

Also I can just stock hella 20A breakers and not worry so much about the 15s. Which is an issue when you need the hold down types with IQ8s and batteries.

You wouldn't ocpd an 7.6kw 240v inverter at 50 amps

Well, I suppose you're right that with that particular combination I'd put a 40 on the plans, and it's not likely I would use a 50 instead, other than temporarily if I dropped the 40 and broke it, or something like that.

But I sure as hell would breaker a 6kW inverter at 40A on the plans if the manufacturer allowed it, instead of 35A, because they don't make quad-combos with 35A. That one has saved me extra work multiple times.

 

[ggunn]

Because if I just standardize on 20A then I reduce the likelihood of copy/paste errors by our designer and which-breaker-do-I-need errors by our crew, and I hopefully never have an inspector tell me I need to call for reinspection after I've changed the breaker or resubmitted plans. This may sound like a hypothetical, but it's not.

Also I can just stock hella 20A breakers and not worry so much about the 15s. Which is an issue when you need the hold down types with IQ8s and batteries.


Well, I suppose you're right that with that particular combination I'd put a 40 on the plans, and it's not likely I would use a 50 instead, other than temporarily if I dropped the 40 and broke it, or something like that.

But I sure as hell would breaker a 6kW inverter at 40A on the plans if the manufacturer allowed it, instead of 35A, because they don't make quad-combos with 35A. That one has saved me extra work multiple times.

Agreed. The NEC only specifies the minimum OCPD for an interconnection, and some inverters specify a maximum OCPD which is typically significantly more than 125% of the maximum output current. Within those bounds one is free to use any OCPD as long as the conductors are sized so that the OCPD protects them.

It is important to note that the purpose of the OCPD is to protect the conductors from fault current from the service, not from the inverter; inverters are current limited by design and the conductors are sized so that inverter current cannot endanger them. The minimum OCPD rating is set to 125% of the inverter maximum output current by the NEC to prevent nuisance tripping.

We occasionally encounter inspectors who do not comprehend this and who will decree that the minimum OCPD rating is also the maximum, and they cannot be convinced otherwise even though the NEC is (some say uncharacteristically) very clear on this point. Heavy sigh.

 

[tortuga]

That said, I still believe the cable size is the only reason the spec sheet states a 20A limit and not something else.

I'd wager its not the rating of the cable but the internal inverter output circuitry has chokes, filters, and caps that all have max AIC ratings similar to consumer electronics before the internal fuse (if there is an internal fuse). Say one of the 'C1-4' EMI suppression cap in the diagram fails shorted, that's a across the line short if you have larger than 20A breaker thats going to have a different trip curve and AIC let thru than a 20A which those components are designed to tolerate per the 'UR' symbol. In the case of that failure it would not matter if the inverter electronics is current limited from the DC side you'd still have a possible fire if it were on a larger than 20A circuit, as the breaker is still the line side short circuit and ground fault protection of those internal components.

 

[jaggedben]

I'd wager its not the rating of the cable but the internal inverter output circuitry has chokes, filters, and caps that all have max AIC ratings similar to consumer electronics before the internal fuse (if there is an internal fuse). Say one of the 'C1-4' EMI suppression cap in the diagram fails shorted, that's a across the line short if you have larger than 20A breaker thats going to have a different trip curve and AIC let thru than a 20A which those components are designed to tolerate per the 'UR' symbol. In the case of that failure it would not matter if the inverter electronics is current limited from the DC side you'd still have a possible fire if it were on a larger than 20A circuit, as the breaker is still the line side short circuit and ground fault protection of those internal components.
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I'd wager that while you are right on some level about the AIC ratings, the cable is still the reason the number is 20A and not higher. Or perhaps it's both, in which case all the more inertia to keep them from changing it.