Conductor Sizing for Subpanel Backfeed

[solarmol]

I want to backfeed 40A PV microinverter system into a 125A MLO garage subpanel. The subpanel is fed by #8cu from a 40A breaker in a 200A bus/200A main MSP. The garage is detached and the #8cu is trenched about 100ft, so to avoid needing to upsize the trenched wire, I am hoping that if I add a 40A main breaker in the garage subpanel, the #8 feeder wire won't need to be changed out.

Quoting 2014 NEC 705.12(D)(2)(3)(b):"Where two sources, one a utility and the other an inverter,
are located at opposite ends of a busbar that contains loads,
the sum of 125 percent of the inverter(s) output circuit current
and the rating of the overcurrent device protecting the busbar
shall not exceed 120 percent of the ampacity of the busbar."

...which I am interpreting to mean that wire only needs to be sized #4 (40A PV + 40A utility / 1.2 = 67A = #4cu @ 75°) at the garage, and #8 is sufficient from garage to the MSP as long as the garage subpanel has a main OCPD. Am I interpreting this correctly? Trying to avoid re-trenching the garage feeders.

 

[ggunn]

I want to backfeed 40A PV microinverter system into a 125A MLO garage subpanel. The subpanel is fed by #8cu from a 40A breaker in a 200A bus/200A main MSP. The garage is detached and the #8cu is trenched about 100ft, so to avoid needing to upsize the trenched wire, I am hoping that if I add a 40A main breaker in the garage subpanel, the #8 feeder wire won't need to be changed out.

Quoting 2014 NEC 705.12(D)(2)(3)(b):"Where two sources, one a utility and the other an inverter,
are located at opposite ends of a busbar that contains loads,
the sum of 125 percent of the inverter(s) output circuit current
and the rating of the overcurrent device protecting the busbar
shall not exceed 120 percent of the ampacity of the busbar."

...which I am interpreting to mean that wire only needs to be sized #4 (40A PV + 40A utility / 1.2 = 67A = #4cu @ 75°) at the garage, and #8 is sufficient from garage to the MSP as long as the garage subpanel has a main OCPD. Am I interpreting this correctly? Trying to avoid re-trenching the garage feeders.

705.12(D)(2)(3)(b) doesn't have anything to do with conductor sizing; it only pertains to busbars. The language you want to look at is in 705.12(D)(2)(1). When you land your connectors in a subpanel you are connecting to the opposite end of a feeder from its primary source OCPD.

 

[PWDickerson]

The detached garage sub-panel should have a main breaker in it, so correcting that should be the first order of business. Make sure you have a proper GEC to a GES while you are at it. Your interpretation of the 120% rule is incorrect. You have to apply this rule (or one of the others above or below it in the code book) to both the garage sub-panel and the house panel. For the garage panel, 1.2 x 125 = 150, 40A main breaker + 40A backfeed (assume 32A inverter output x 1.25) is less than 150A so you are OK. For the house panel, 1.2 x 200A = 240A, 200A main breaker + 40A backfeed (assume 32A inverter output x 1.25) = 240A so you are OK. The problem is that you may have too much voltage drop on your PV AC circuit conductors, which will drive up your inverter output voltage, and hence garage subpanel voltage, too high. You will need to do math there and make sure it won't be a problem. It sounds a little dicey.

 

[solarmol]

705.12(D)(2)(3)(b) doesn't have anything to do with conductor sizing; it only pertains to busbars. The language you want to look at is in 705.12(D)(2)(1). When you land your connectors in a subpanel you are connecting to the opposite end of a feeder from its primary source OCPD.

Thanks for the redirection. I'm still a bit confused, since 705.12(D)(2)(1) says "Where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device..." I assumed it didn't apply.

What is meant by "primary source overcurrent device"? In this situation would that be the 40A main breaker in the garage subpanel, or the first disconnect at the MSP?

I have always used [(Sum of PV breakers + Main Disconnect) / 1.2] to size the wire between a PV dedicated subpanel (used to consolidate + monitor microinverters) and the backfed breaker, but in this situation I'm concerned that the conductors from the garage sub to the MSP might also need to be sized that way.

 

[jaggedben]

Thanks for the redirection. I'm still a bit confused, since 705.12(D)(2)(1) says "Where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device..." I assumed it didn't apply.

What is meant by "primary source overcurrent device"? In this situation would that be the 40A main breaker in the garage subpanel, or the first disconnect at the MSP?

I have always used [(Sum of PV breakers + Main Disconnect) / 1.2] to size the wire between a PV dedicated subpanel (used to consolidate + monitor microinverters) and the backfed breaker, but in this situation I'm concerned that the conductors from the garage sub to the MSP might also need to be sized that way.

As ggunn mentioned, if you're on the 2014 or 2017 NEC the 120% rule no longer applies to conductors, only to panelboard busbars. In the 2011 code it was more ambiguous and some AHJs I worked with would have required the feeders to be upsized in your situation. I had one AHJ that would require a placard saying the feeder couldn't be tapped. But that's all irrelevant if your on the 2014 NEC.

The primary source overcurrent device is the utility side overcurrent device. Since you happen to have two in series because it's a detached building, you can use whichever is rated lower, if they are not the same. If they are both 40A then that's your number.

 

[ggunn]

Thanks for the redirection. I'm still a bit confused, since 705.12(D)(2)(1) says "Where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device..." I assumed it didn't apply.

You are correct; since you are connecting in the subpanel, you are at the other end of the feeder, so it doesn't apply. Or rather, it does apply in the sense that it shows that you do not have to increase the size of the feeder conductors because of the PV. We have, though, encountered an AHJ chief inspector (who will remain nameless) who doesn't understand that the PV's contribution to the current in the feeder is subtractive from the current drawn by the subpanel. We don't connect to subpanels in his jurisdiction.

 

[solarmol]

Understood! Thank you all.