Does 705.12(B)(2)(c) while relocating loads allow for larger PV back feed in downstream load centers?

Goatprod

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Following multiple threads, it appears that 705.12(B)(2)(c) would allow for a whole world of backfeed possibilities.

Hypothetically:

100 amp max rated MSP with 100 amp breaker.
Filled with standard household loads.

If one were to add a sub panel, and reconfigure the MSP to only have two loads, call it 2x50 amp breakers, then:

With the new sub panel having a max rating of 125 amps (on the bus bar), as well as the relocated house loads, would this allow the use of a 40 amp max backfeed PV breaker located at the opposite end of ?

The 120 percent rule allows for 100 amp backfeed breaker on said busing. (125 x 1.2 - 50 = 100).

The sum of the ampere ratings of all overcurrent devices equals the rating of the bus bar within the MSP.

And the backfeed breaker within the sub panel follows the 120 percent rule.

The 120 rule within the MSP would only allow for a 20 amp backfeed breaker.

This scenario would allow for a larger sized pv system to be installed without having to perform a line side tap or a main panel upgrade.

Looking forward to hearing what I’ve misinterpreted.
Thank you for your knowledge people.
 

jaggedben

Senior Member
Following multiple threads, it appears that 705.12(B)(2)(c) would allow for a whole world of backfeed possibilities.

Hypothetically:

100 amp max rated MSP with 100 amp breaker.
Filled with standard household loads.

If one were to add a sub panel, and reconfigure the MSP to only have two loads, call it 2x50 amp breakers, then:

With the new sub panel having a max rating of 125 amps (on the bus bar), as well as the relocated house loads, would this allow the use of a 40 amp max backfeed PV breaker located at the opposite end of ?
If I'm understanding your description correctly, yes, you can backfeed up to a 40A inverter output (requires 50A after 125% factor) through the subpanel.
The PV breaker in the subpanel does not necessarily need to be at the opposite end because the sum of the 50A breaker feeding it and the 50A PV breaker does not exceed the 125A rating of the sub.
The main qualifies under 705.12(B)(2)(c). The sub qualifies under 705.12(B)(2)(a).

Your limiting factor in this configuration is actually the 50A breaker feeding the sub. You can't backfeed more than that breaker allows no matter how you qualify the sub for backfeeding. You could backfeed the main with even more if, say, you moved all the loads out of the MSP onto a single sub fed by a 100A breaker. Then you could backfeed up to 80A inverter output (requires 100A after 125% factor), if, say, you used a 200A rated sub and the 120% rule.
 

jaggedben

Senior Member
While possible it seems like the work involved would be more than just installing a new MSP.
That really depends. Sometimes AHJs or utilities will not allow a simple swap out without demanding upgrades of otherwise unrelated features of a service that no longer meet their standards. For example in PG&E territory where you have a gas meter in proximity to an underground service riser, you are going to be looking for an alternative.
 
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