120%

[Good guy]

The plans call for480/278, 200 amp main meter can then that feeds a 200 fused disconnect, the fused disconnect is labeled pv disconnect. From that disconnect it feeds a
400 amp 480/277 subpanel with a backfeed breaker of 150 amps and 200 amp that feeds a 75k transformer for the low voltage loads, I’m thinking that I can only backfeed 40 amps of solar based on my main meter can and 200 amp disconnect. Bussing is only 200 amps on both meter can and fused disconnect

 

[wwhitney]

The design as described is OK. The rules in 705.12(B) basically only restrict equipment that has at least 3 connections, two of which are separate power source connections.

The 400A subpanel is fine: it has a 200A grid supply, a 150A PV supply, and a 200A feeder for loads. So it has to comply with 705.12(B). It meets 705.12(B)(2)(3)(a) [125% * 150A (worst case) + 200A < 400A], which means it would also meet (b) [the 120% rule], but you aren't restricted in where the PV breaker is located within the panel.

The 200A meter and 200A disconnect are also fine. They have only two connections, so they are exempt from 705.12(B) per the first paragraph, as they don't separately feed other loads. [And if they were subject to 705.12(B), they would qualify under 705.12(B)(2)(3)(c).] Basically current can only flow one way through them. Up to 200A can flow from the grid, or up to 150A (worst case) can flow from the PV, but not simultaneously. In all cases the current is under the 200A rating.

Cheers, Wayne

 

[winnie]

In what scenario with the current design could you have more than 200A of current flowing on components only rated for 200A?

-Jon

 

[Carultch]

The plans call for480/278, 200 amp main meter can then that feeds a 200 fused disconnect, the fused disconnect is labeled pv disconnect. From that disconnect it feeds a
400 amp 480/277 subpanel with a backfeed breaker of 150 amps and 200 amp that feeds a 75k transformer for the low voltage loads, I’m thinking that I can only backfeed 40 amps of solar based on my main meter can and 200 amp disconnect. Bussing is only 200 amps on both meter can and fused disconnect

The busbar rating in the 120% rule, doesn't propagate to other equipment. It is only the busbar for the individual device you are examining that matters. Every busbar that could be fed from multiple sources, and draw loads as well, needs to satisfy at least one of the busbar protection rules, but since the bussing in the disconnect doesn't serve loads, you don't need to think about busbar protection rules affecting it. You also need to consider local load diminishing to zero if you backfeed a subpanel, since that backfeed could propagate to the main panel and backfeed it as well.

Your 120% rule for the 400A panelboard is as follows, assuming the 150A is opposite the 200A breaker:
150A + 200A = 120% of 400A

150A + 200A is less than 400A anyway, so this doesn't strictly require it to be on the opposite end of the bus.