Subpanel capacity

[Open Neutral]

WA State
PSE
Building with two 200A disconnects with appropriate gauge to front & rear PB's.

One feeds an existing 100A PB with a 100A Main {It was the building's main before upgrades....}
And 20A breaker to solar supply. Others to misc. loads.

The client was told that any further array expansion would require replacing the PB with a 200A one.
Thinking about, I wonder why.
If you can consume 80% of 100A, can't you source that into the panel?

 

[jaggedben]

It is often correct that a 100A panelboard with a 100A main breaker can not be backfed by more than a 20A breaker. This is the traditional "120% rule" that limits how much interactive inverter output can be interconnected to a panelboard. It is no longer the only rule, but seems to be the one that is being cited.

(Strictly speaking the backfeed limit is not about the solar breaker but is 125% of nameplate inverter output. So the limit on the panel you describe would be 16A inverter output, which requires a 20A breaker.)

Replacing the panelboard with a 200A is not the only possible option. Other options could be:
- See if the sum of breakers in the panelboard is 100A or less.
- Replace with a 125A panel (would allow backfeeding 40A on a 50A breaker).
- Downsize the breaker feeding the panel if the calculated load does not require it to be 100A. (Not possible for a 100A service disconnect due to other rules, but I believe you're describing a downstream feeder breaker).

As for why, don't overthink it, just read 705.12. Exact numbering has changed in each cycle so tell us your code cycle for a more specific reference.

 

[Open Neutral]

The issue is the labor cost of replacing that PB with a larger one.

I'll read up re: 705.12; thank you.