Enphase OCPD and disconnect sizing

[solarken]

This is in regards to 2023 NEC. My question is does 220.5 rounding to the nearest whole number apply to microinverter output conductor sizing and OCPD of backfed breaker?

With 17 enphase IQ8M-72-2-US microinverters, each having max continuous output current of 1.35A, per 705.28(A), the total power source max current is calculated as:
17 x 1.35A = 22.95A

And per 705.28(B), the conductor ampacity is calculated as:
22.95A x 1.25 = 28.6875A, or 28.69A using appropriate significant digits.

For 18 microinverters, the total max current per 705.28(A) is:
18 x 1.35A = 24.30A

And the conductor ampacity is:
24.30 x 1.25 = 30.375A, or 30.38A

Does 220.5(B) permit the dropping of the 0.38 fraction of an ampere to arrive at 30A? And would this permit a 30A rated disconnect, and a 30A rated OCPD backfed breaker to be used for this?

705.30(B) specifies OCPD sized not less than the maximum currents calculated in 705.28(A). 705.28(A) and 705.28(B) both contain the wording "Where not elsewhere required or permitted in this code". If 220.5(B) which is "permitted elsewhere in the code" for feeder conductors, can be applied after the 705.28(A) calculation that arrives at 24.3A, with the .3 dropped and the value used is 24A, then the conductor ampacity and the OCPD required would be 24 x 1.25 = 30A.

Doing so would permit 30A disconnect, and a 30A backfed breaker, instead of a 60A disconnect and 35A breaker.

This seems logical to me, what do you say?

 

[jaggedben]

Practical answer...
With Enphase PCS now, who cares? Set the PCS output to the rounded-down number. Output will rarely reach that number anyway.

I suppose before PCS I might have submitted it to the AHJ the way you say and then if they kicked it back I'd cross that bridge.

 

[TX+ MASTER#4544]

NEC 2023 TX
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TX+MASTER#4544

 

[pv_n00b]

The problem that we can run into is that Art. 220 is for load calculations and technically power sources are not loads. Also, I don't think many people in PV are rounding so if the AHJ sees a lot of PV designs and no one is rounding but you that will raise a flag. There's nothing that says you can't try though.

 

[ggunn]

The problem that we can run into is that Art. 220 is for load calculations and technically power sources are not loads. Also, I don't think many people in PV are rounding so if the AHJ sees a lot of PV designs and no one is rounding but you that will raise a flag. There's nothing that says you can't try though.

I saw a system recently that used (5) PV inverters that each had 48.25A max output current. (1.25)(48.25A) = 60.3A, and 60A inverter OCPDs were used per 220.5(B). For the combined inverter output, however, (5)(1.25)(48.25A) = 301.6A, so no rounding; a 350A OCPD was used.

 

[pv_n00b]

Practical answer...
With Enphase PCS now, who cares? Set the PCS output to the rounded-down number. Output will rarely reach that number anyway.

"Output will rarely reach that number anyway."
I remember in the early days of grid tied PV that I would fairly often see someone trying to make the argument that if they had an inverter that had a DC/AC ratio less than one that they should be able to size the interconnection based on the AC output amps the inverter could produce with the attached PV, which would be less than the inverter rating. I never knew of anyone winning with this argument, but I thought the attempt was grand.

 

[jaggedben]

"Output will rarely reach that number anyway."
I remember in the early days of grid tied PV that I would fairly often see someone trying to make the argument that if they had an inverter that had a DC/AC ratio less than one that they should be able to size the interconnection based on the AC output amps the inverter could produce with the attached PV, which would be less than the inverter rating. I never knew of anyone winning with this argument, but I thought the attempt was grand.

All that has next to nothing to do with what I was saying. PCS is a valid way to limit the inverter output from a code compliance point of view. Effectively it raises the DC/AC ratio. My only point in the comment about output was that in the OP's situation the production/economic impact would be negligible given the numbers.