PV mocroinverter

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hhsting

Senior Member
Location
Glen bunie, md, us
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Junior plan reviewer
I have enphase IQ7-60-2-US micro inverter.
Max power each inv: 240w
Max output voltage: 240V single phase

Attached sketch shows AC 240/120V single phase subpanel MLO which connects to line side of main service disco.

Subpanel has 6 breakers which feed respective string microinverters.

So I have (12)#10 awg in one conduit going to each respective string from the subpanel. If I take 15 microinverter string then max amps drawn is 15A. To size breaker IQ7 15x1.25=18.75A which is 20A.

However when I do ampacity correction of #10 for 12 of them 50% I get exactly 15A. However breaker is sized 20A.

Is #10 sized correctly or should it be next size up#8?

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Carultch

Senior Member
Location
Massachusetts
If your ampacity adjustments are an exact match to the previous standard size OCPD, 240.4(B) would not allow you to take credit for the "next size up rule". You need an ampacity that exceeds the previous standard OCPD rating, and has an ampacity for the load. For a 20A breaker, this means you need 15.1A worth of wire ampacity after derate factors are applied. It could also be 15.01A, but the point is, it needs to be greater than 15A not inclusive of 15A itself. Derate factors are usually applied to the 90C rating, due to most wire being 90C rated.

Your calculation for ampacity also needs to compound ampacity adjustment with temperature correction. I.e. both derate factors multiply together, if both apply. The most common temperature correction factor is 0.96, due to common ASHRAE design high temperatures being slightly higher than 30C.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
So I have (12)#10 awg in one conduit.. However when I do ampacity correction of #10 for 12 of them 50% I get exactly 15A.
See NEC 310.15(B) Second paragraph. Note: "Temperature rating of conductor"
*If #10 has no markings, or strpped from cable sheath, max rating is 60C
*If #10 wire is marked THWN, outdoor wet location max rating is 75C
*If #10 wire is marked THWN-2, outdoor wet location max rating is 90C

Take 50% of max-rated ampacity as marked on wire
 

hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
If your ampacity adjustments are an exact match to the previous standard size OCPD, 240.4(B) would not allow you to take credit for the "next size up rule". You need an ampacity that exceeds the previous standard OCPD rating, and has an ampacity for the load. For a 20A breaker, this means you need 15.1A worth of wire ampacity after derate factors are applied. It could also be 15.01A, but the point is, it needs to be greater than 15A not inclusive of 15A itself. Derate factors are usually applied to the 90C rating, due to most wire being 90C rated.

Your calculation for ampacity also needs to compound ampacity adjustment with temperature correction. I.e. both derate factors multiply together, if both apply. The most common temperature correction factor is 0.96, due to common ASHRAE design high temperatures being slightly higher than 30C.

Their is junction box in between from which (12)#10 separate not shown on sketch.


If you look at manufacturer instruction IQ7 micro inverter they say take max inverter output current which here is 15A and multiply by 1.25 which would be 20A breaker.

But its cable sizing thats problem since i need 15A cable ampacity because thats load drawn max but dilemma is 240.4 wont accept it while breaker size is per manufacturer instruction.

So then #10awg would be small for iq7?


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ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
..Your calculation for ampacity also needs to compound ampacity adjustment with temperature correction. I.e. both derate factors multiply together, if both apply. The most common temperature correction factor is 0.96, due to common ASHRAE design high temperatures being slightly higher than 30C.
310.15(B)(2) is a Good point. We have 100°F + summer days is Southern CA.

310.15(B)(3) (c) may also apply, if raceway is too close to roof.

That would be 3 correction factors that need to be multiplied against proper ampacity as marked on wire.
 

Carultch

Senior Member
Location
Massachusetts
Enphase also makes the Q-aggregator product, that allows you to parallel multiple 20A branch circuits, and land them on a larger breaker up to 60A, which could help cut down on the number of wires, by consolidating the branch circuits.

The NEC generally requires OCPD and disconnecting means any time you parallel inverters, but there is an exception for microinverter systems. Microinverters are allowed to be paralleled without OCPD/disconnecting means, as long as they are paralleled with equipment specifically listed as a system for doing so. In other words, that means you wouldn't be allowed to just wire-nut or use other general splice/tap products to parallel multiple Enphase branch circuits together in an otherwise-empty box, but rather you'd need the brand-specific parts that are listed as a system. You can parallel the branch circuits in third-party panelboards, with no issue. But paralleling without OCPD, needs a brand-specific match to the system.

That would be 3 correction factors that need to be multiplied against proper ampacity as marked on wire.

It is 3 correction terms, but 310.15(B)(3)(c) is not really a multiplier factor. It is a temperature adder. It tells you to add a temperature differential to the effective air temperature inside the conduit, and then apply it to the temperature correction table or formula.
 

Steve16

Member
Location
Ct
Occupation
Master electrician
Enphase strings call for #10 wire and a 20a breaker regardless of the amount of micros on the circuit.

I also thought they were 1.1-1.15amps per micro?

Best thing is to read what enphase has spec'd
 
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