[RE-wrenches] Busbar 120% rule

[William Korthof]

In situations where the supply wires can be tapped, it's reasonable to tap in as much as 50% of the service rating. 

But in California, we mostly seem to have combo meter/main/distribution services, precluding service taps in most projects.

1) 200 amp service w/ 40 amp * 80% 
= 7.68 kW maximum inverter output, which matches with up to around 9kW of solar array 

2) 100 amp service (with 125 A rated bus bars) is a great situation we run into fairly often... NEC allows up to 50A * 80% = 9.6 kW maximum inverter output and around 11 kW of solar. 

3) Dual main service--some panels have provision for 2 sets of main breakers. When the solar breaker is in the second position, it can be equal to the rating of the main distribution breaker (100A is common). That equates hypothetically to a 19.2 kW ceiling. 

4) 200A or 225A panel with derated main breaker. Lower capacity main breakers can be substituted into many of the Murray, Cutler Hammer, GE, and SQD panel main breakers, which then allows a larger solar input breaker... 
200 amp panel with 150 A main allows up to 90A solar backfeed breaker (17.3 kWac). Load monitoring or load calculations will show that most homes on 200A service can easily and safely be served by 150A or 175A of utility supply. 

/wk

William Korthof
714.875.3576
Sustainable Solutions
#956904

On Apr 3, 2014, at 9:18 AM, Phil Forest <pforest at southmountain.com> wrote:

Troy,
This is my first post. 
We install 32 amp rated output inverters, reduce the main breaker to 175amp, or line side tap. 

Phil Forest
South Mountain Company

On Apr 2, 2014, at 1:40 PM, Troy Harvey <taharvey at heliocentric.org> wrote:
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