Sum of all breakers rule question

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photonboy

Member
Location
Berkeley, CA, USA
Occupation
Ex roof monkey, current desk jockey
I was under the impression that the sum of all breakers rule is another way of saying the 120% rule, which says the MB and PV breaker cannot exceed 120% of bus bar rating. One of our installers is pushing back on this. There is no PCS in this system, just a SolarEdge inverter with no PCS capabilities.

Scenario:

200A bus
200A Main Breaker
1 existing 30A load breaker
1 new 60A PV backfeed breaker

The installer is claiming that the "sum of all breakers rule" means that you just add up the sum of the breakers present on the bus, minus the Main Breaker, and if that is less than the bus rating then good to go.

Here are the pics they uploaded to make their point and my marked up response, which is correct?:

NEC Sum of all breakers.jpgPurcell sum of breakers.png


NEC Sum of all breakers.jpg
 
photonboy said:
I was under the impression that the sum of all breakers rule is another way of saying the 120% rule
Click to expand...
No, they are different rules and either one suffices to protect the busbar. Your picture is an example that complies with the sum of all breaker rule but not the 120% rule. The panel shown is fine.

What's supplying this 200A panel? Each upstream panel needs to comply with one of the rules in 705.12.

Cheers, Wayne
 
wwhitney said:
No, they are different rules and either one suffices to protect the busbar. Your picture is an example that complies with the sum of all breaker rule but not the 120% rule. The panel shown is fine.

What's supplying this 200A panel? Each upstream panel needs to comply with one of the rules in 705.12.

Cheers, Wayne
Click to expand...
This is the MSP, so supplied by the utility. Can you direct me to the code section/language for the sum of all breakers rule? When I search it all of the results I get say that the sum of all breakers rule IS the 120% rule, sounds like I need to educate myself here. Thanks.
 
There are feed through lugs on this panel, seen in the photo. Which complicates the discussion considerably. (Although it explains why there are only two branch/feeder breakers in the MSP.)

As written, in my opinion, the code still allows the sum of all breakers rule to be used. However since the feed through lugs are a load which would ordinarily count against the sum if on an overcurrent device, this is an unintended loophole. Also you would need to comply with 705.12(B)(1) which requires (probably) an overcurrent device downstream of the feed through lugs.
 
photonboy said:
This is the MSP, so supplied by the utility. Can you direct me to the code section/language for the sum of all breakers rule? When I search it all of the results I get say that the sum of all breakers rule IS the 120% rule, sounds like I need to educate myself here. Thanks.
Click to expand...
No, the sum of all breakers rule is not the 120% rule. The sum of all breakers rule was new in (I believe) 2014, and does not include the breaker protecting the busbar in the calculation. The 120% rule does include the busbar OCPD and requires that the source breaker be installed at the opposite end of the busbar from the feed from the service. They are and always have been very different.

In the 2023 NEC, the 120% rule is 705.12(B)(2) and the sum of all breakers rule is 705.12(B)(3).
 
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