690.64 solar grid tie in

[highvolts582]

My question is regarding 690.64 I understand the concept of this calculation concerning putting breakers in the house panel or doing a line side tap if the calculation works out over the rating of the bus bar in the branch circuit main panel. My question is does my AC combiner panel also have to be sized the same way. so if i have two 30 amp breakers and 60 main in a 125 amp combiner panel do I have to up the size to 150 or can I leave the 125 amp combiner panel the same as long as i do a Line side tap to feed this panel.

Thanks
guys

 

[highvolts582]

clarification

clarification

the calculation goes 30 breaker plus 30 breaker is 60 then 100 main is 160 then 160 divided by 1.2 close to 133 amps bus bar is rated for 125 . if this was the main house panel this would not work. but if I combined in a separate main panel apart from the house panel and tie in on the Line side of the main house panel. is that legal to not up size this panel to.

 

[highvolts582]

any takers

any takers

does anyone want to take a whack at this one?

 

[ggunn]

does anyone want to take a whack at this one?

Well, there's common sense and then there's code... ;^)

Common sense says (to me, anyway) that if there are no load breakers present in the AC aggregation panel, there can never be any more than the sum of the inverter max outputs (something less than 30A + 30A, right?) passing through the the main breaker and busbar under any circumstances, so a 60A main should be fine and there's no reason the busbar should have to have a higher rating than that, either. The 20% rule should not apply, since the inverters can never load the main at any more than their idle current in the dark and there are no other loads present. If the tap is through a fused disconnect, why would you need a main breaker on the panel at all?

Or am I missing something?

 

[highvolts582]

added fun with solar

added fun with solar

(solar system 33 kw). My engineer specified a 400 amp busbar load center for the ac breakers with a 225 amp single phase 240v breaker serving as the main for this 400 amp panel. the sum of the breakers in that panel is 150 amps this building has a 600 AMP SERVICE. This is a solar combiner ac panel only. Not the main of the building. He says it still has to be sized that way because technically someone can add other branch circuits to this system. and would not let me do a 225 amp. rated bus bar. this solar system is a 33 kw. regarding 690.64 (b)(2) I put the 400 amp panel with Bolt on breakers. in the price after he would not Budge but I have only done smaller residential systems 10 kw and less. So I see how I have been passing fine on other jobs using a 125amp. sub panel but if i did not have this engineer and designed this larger system myself I would I would have put 225 amp bus bar rated panel and thought it was OK. I still think this is OK can anyone make me believe otherwise?

 

[BillK-AZ]

See NEC Art 690.64(B)(2)

See NEC Art 690.64(B)(2)

The general rule (NEC Art. 408.36) is that you can not feed a panel with overcurrent protective devices (OCPD) rated higher than the panel, but NEC Art 690.64(B)(2) allows the sum of all breakers to not exceed 120% of the panel rating provided the PV OCPD is placed opposite the main OCPD.

You state that the PV breakers in the AC Combiner Panel total 150A (typical for a 33 KW PV inverter). With these breakers, you will need a OCPD feeding the panel of 150A, therefore the sum is 300A. Standard panel ratings are 225A & 400A. A 225A panel can be used with a sum of up to 270A, so it looks like you need a 400A panel.