[RE-wrenches] Congested Panel Problem

[Peter Parrish]

We occasionally run into a dual headache: a congested AND underpowered
service panel.

Case in point a 100A Zinsco main panel with no room for a two pole breaker
for inverter back feed.

Normally we pull two 20A/120V branch circuits out of the main panel into a
subpanel, back feed the subpanel with the inverter output and feed the main
panel from the subpanel. 

The inverter in question is a SMA SB4000. Minimum over current protection is
16.66A (e.g. 20A continuous duty breaker). Otherwise protection needs to be
20.83A (e.g. 25A regular breaker).

So we installed a 100A subpanel, pulled in the two 20A circuits, backed this
subpanel with the inverter via a 20A continuous duty breaker, and fed the
main panel via #8 wire and a 40A breaker.

The inspector has a problem with this approach. Doesn't have a problem with
the 20A breaker in the subpanel, but he objects to the 40A breaker in the
main panel. His reasoning includes applying NEC 790.64(B). I believe the
last sentence of (B)(2) stipulates that the breaker back feeding the
subpanel is the one that counts, not what eventually what breaker back feeds
the main panel.

This inspector also has a problem with the production meter we installed. He
wanted to see the "UL Listing" on the meter. I called Austin International
(our vendor) about the meter in question (Itron Centron C1S) and they said
that UL doest test meters as they (normally) are not used on the customer
side of the service. Is there another listing for meters that would satisfy
a nit-picky inspector? To make matters worse, we now have in stock both
Itron Centron C1S meters as well as Schlumberger Centron C1S meters. Did
Schlumberger buy out Itron? Or vice versa?

Any help on the main panel calculation and acceptable listings for meter
would be greatly appreciated. 


 
Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885