120% Rule on 400A and 600A Switchboard Panels

[msnell@sungenia.com]

I am trying to backfeed 200A of PV on 400A and 600A panels with a switchboard without derating the panel. For example, on a 400A panel with a 4 breaker location switchboard, I have two 200A breakers feeding two subpanels. I want to install a 200A PV backfeed breaker on the third breaker position and disable the fourth breaker location, or note that no additional loads be put on the switchboard. This would prevent the instance of any draw on the panel being larger than 400A regardless of utility or PV production sources. We're simply trying to prevent the main bus from seeing a current of >120% of the bus rating, right? This can be achieved via the 120% rule on production sources but can also be achieved by limiting the maximum possible load on the bus to 120% of the bus rating, that is, to limit the sum of all breakers to 120% or less. We do this with load relocation situations on 100A and 200A panels, shouldn't it work here.

 

[ggunn]

The 120% rule (705.12(B)(3)(2)) doesn't have anything to do with load breakers. If you want to qualify the bus with the total ratings of all breakers excepting the rating of the OCPD protecting the bus (705.12(B)(3)(3)), you are limited to 100% of the bus rating.

 

[wwhitney]

For example, on a 400A panel with a 4 breaker location switchboard, I have two 200A breakers feeding two subpanels. I want to install a 200A PV backfeed breaker on the third breaker position and disable the fourth breaker location, or note that no additional loads be put on the switchboard.

Assuming your 400A panel is protected by a 400A breaker, what you propose would protect the panel busbar, but is not a method that the NEC allows. I submitted a proposal to extend the 100% rule to cover the above case, but apparently it was too logical for the CMP.

What you can do is intercept one of the 200A feeders and interconnect there. Then the subpanel supplied by that feeder needs a main breaker. And the 400A panel needs to be labeled not to add breakers. And any upstream panels need to be checked for compliance with 705.12.

Cheers, Wayne

 

[pv_n00b]

I'm assuming since you are in CA this question will be based on the 2017 NEC, and that when you write "panels" in switchboards you actually mean switchboard sections. NEC 705.12(B)(2)(3) only applies to busbars in panelboards and not switchboards. The 2017 NEC 705 does not describe how to size busbar in switchboards for back feed and it is typically done under engineering supervision. There is no reason why you can't apply the 120% rule, but the NEC does not require it and it is overkill for switchboards that are built to different standards than a panelboard. The 2020 NEC changes this and switchboards, switchgear, and panelboards do fall under the 120% rule, but it also makes it clear that sizing of all the busbars can be done under engineering supervision.