# Another AC disconnect question

Solar Expert Posts: 64 ✭✭✭✭
This is a case where there is a sub-panel used as an AC combiner to combine the output from three inverters. Each inverter feeds a 20A breaker in a 100A sub-panel. The sub-panel connects to a 60A load side backfeed breaker in the main service panel through a separate 60A AC disconnect switch. According to 690.64(B)(2) [120% rule] the 100A sub panel and derated conductors feeding the main panel are correctly rated for 100A [60+60=120A, 120% of 100A = 120A].

My question is: does the 120% rule apply to he AC disconnect switch [should it also be rated at 100A and not 60A]?

Reading the 120% rule it refers to "the sum of the ampere ratings of overcurrent devices supplying power to a busbar or conductor shall not exceed 120%" etc.

Since the AC disconnect switch has no bus bar [as related to a panel] and the word "conductor" conventionally refers to a cable my guess that the AC disconnect rated at 60A would suffice.

Thanks, Brent

• Banned Posts: 17,615 ✭✭
Re: Another AC disconnect question

Not exactly the "120% rule" for the breaker bus bars being back-fed, but the standard NEC derating for constant current would apply to a disconnect the same as it would to a breaker or wire.

Although being Canadian in an area that uses "code of good practice" rather than NEC I'd appreciate confirmation of my interpetation from one of the US guys.
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Another AC disconnect question
Brent wrote: »
This is a case where there is a sub-panel used as an AC combiner to combine the output from three inverters. Each inverter feeds a 20A breaker in a 100A sub-panel. The sub-panel connects to a 60A load side backfeed breaker in the main service panel through a separate 60A AC disconnect switch. According to 690.64(B)(2) [120% rule] the 100A sub panel and derated conductors feeding the main panel are correctly rated for 100A [60+60=120A, 120% of 100A = 120A].

My question is: does the 120% rule apply to he AC disconnect switch [should it also be rated at 100A and not 60A]?

Reading the 120% rule it refers to "the sum of the ampere ratings of overcurrent devices supplying power to a busbar or conductor shall not exceed 120%" etc.

Since the AC disconnect switch has no bus bar [as related to a panel] and the word "conductor" conventionally refers to a cable my guess that the AC disconnect rated at 60A would suffice.

Thanks, Brent

The reason for the 120% rule is that if there are local loads in the same panel or attached to the same conductor, it could end up carrying the sum of the grid input power and the PV input power into those loads.
At a disconnect the power will either be going one way or the other and there is no way that the two contributions could add up.
SMA SB 3000, old BP panels.
• Solar Expert Posts: 64 ✭✭✭✭
Re: Another AC disconnect question

Good answer inetdog, thanks. I suspected as such but with the CEC one never is quite sure.

Brent