Subpanel fed breaker size

[Ericdh]

Hello,
I'm designing a system using Enphase microinverters of various string sizes feeding a subpanel. The 208V 3ph amperage output is relatively small, between 4A and 10A based on 6 and 15 per branch circuit. The mfr recommends using 20A breakers for the subpanel. If I have five 20A breakers in the sub, but the circuit amperages total only 36A, what size should the PV subpanel breaker be on the main switchboard? Do I just take 36A x 125% and round up to 50A? I don't think adding up the 20A breakers (100A) is the way to go. Is there a limit to how large a breaker it can be?
I can't find this discussed in the NEC for PV book. Please help.
Thanks!

 

[jaggedben]

...Do I just take 36A x 125% and round up to 50A?

Yes.

The code doesn't really make it clear but it hardly prohibits that

Is there a limit to how large a breaker it can be?

No, but the panel busbar needs to meet the rules for interconnection, e.g. 120% rule. Making the feeder breaker smaller may help with that.

If you're on the 2011 code you'll have to use all those 20A breakers for calculating the 120% rule. If you're on the 2014 code you can use 36A x 125%.

Some AHJs might not require you to follow the 120% rule if the subpanel is reserved for solar. The 2014 code gives you a couple alternatives, too.

 

[Ericdh]

Thank you!