NEC 2020 - 705.12(B)(3) - Center Fed Switchgear - Some only apply to panelboards?

[Elect117]

I got sent something with a center fed switchgear line up. 4000A bus and 3600A long time trip setting on the existing main.

They are putting 1200A OCPD for solar on the far end of one of the center fed sections. ~1100A for the 125% of output circuit current.

They were using 705.12(B)(3)(2) but I had issue with it since it is center fed and therefore the main and the solar breaker wouldn't be on opposite ends of the switchgear.

If there was a 4700A load on the section that doesn't have solar then the solar + main could feed it without tripping and exceed the 4000A bus rating.

But then I got confused by the fact that (B)(3)(3), (B)(3)(4) and (B)(3)(6) only applied to panelboards? Isn't the theory the same regardless?

Because it would meet (B)(3)(4) if it was in a dwelling and was a panelboard. And I could move loads around to meet (B)(3)(3) but that is only allowed for panelboards. I think I am just confused as to why switchgear and panelboards are treated differently and why (B)(3)(3) wouldn't apply to switchgear.

 

[jaggedben]

What code cycle? 2020? The numbering and nuances here have shifted with every cycle, it seems.

I think the general answer is that they've been willing to be less stringent with smaller systems where the tie in is likely to be on a panelboard, and the 'engineering supervision' provision is how you get to stretch the rules on bigger gear.

 

[pv_n00b]

Center fed switchgear interconnections are to be done under engineering supervision. SWGR, SWBD, and panelboards are treated differently because they are listed under different standards and have different design requirements. I think the center fed dwelling panelboard rule was added simply because without it a lot of homeowners would not have been able to install PV. It kind of proves the requirment for mounting PV interconnection CBs on the opposite end of the bus from the main is actually bunk since it is violated for the bus across from the center feed point in center fed panelboards.
So, as an engineer how do you approach this? There are several ways, you could decide to apply the center fed panelboard rules if you felt that provided a safe installation. You can install a PCS that will monitor the current going to main OCPD and the opposite side bus and reduce the PV output if either exceeds the bus rating. You could placard the SWGR to limit the installed loading. Let your PE flag fly!