Sum of all breakers rule question

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photonboy

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Location
Berkeley, CA, USA
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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
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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
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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.
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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).
 
You would need to add the breaker rating of the downstream load center at the other end of those feed thru lugs, and if including that breaker with the 30A load and 60A PV breakers does not exceed 200A, then the bus is not overloaded. So if the downstream load center has a 110A Main breaker or smaller, it is fine.

See 705.12(B)(5)
 
You all helped me with a similar situation with feed thru lugs a few years back, back then there was some confusion as to what "installed at the supply end of the feed-through conductors" meant, that has since been clarified to mean you can have a main breaker in the feeder panel that the feed thru lugs supply.
 
solarken said:
You would need to add the breaker rating of the downstream load center at the other end of those feed thru lugs, and if including that breaker with the 30A load and 60A PV breakers does not exceed 200A, then the bus is not overloaded. So if the downstream load center has a 110A Main breaker or smaller, it is fine.

See 705.12(B)(5)
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I agree with this interpretation, but what's your thoughts if the upstream load center has no loads? Essentially, the bus is protected by a 200 amp breaker on the utility side and another 200 on the sub-panel...
 
Legend said:
I agree with this interpretation, but what's your thoughts if the upstream load center has no loads? Essentially, the bus is protected by a 200 amp breaker on the utility side and another 200 on the sub-panel.
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You would qualify under 705.12(B)(3), but only if there are no loads in the panel with the feedthrough lugs if it is a 200A panel with a 200A MCB.
 
@photonboy whats on the other end of those feed thru lugs? A main lug panel or main breaker panel? CA is on the 2020 NEC still correct?
 
Legend said:
I agree with this interpretation, but what's your thoughts if the upstream load center has no loads? Essentially, the bus is protected by a 200 amp breaker on the utility side and another 200 on the sub-panel...
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FWIW, this arrangement is sometimes used as a cheaper alternative to a 200A fused disconnect. I most often saw it in rural locations where the utility meter was under a pole mounted transformer some distance from the building(s). Often it was a 320A meter can with two 200A feedthrough panels with no loads on a rack under the meter.
 
Legend said:
I agree with this interpretation, but what's your thoughts if the upstream load center has no loads? Essentially, the bus is protected by a 200 amp breaker on the utility side and another 200 on the sub-panel...
Click to expand...
If the upstream load center has no loads, has a PV backfed breaker, and has a main breaker of 200A, , and feed-thru lugs that land on a subpanel's 200A breaker, it still fails I guess, because the PV breaker would be >0A, and any >0A + 200A exceeds the 200A bus rating. This is assuming still using the sum of all breakers method. But it does not seem possible that the bus rating would be exceeded in this scenario. So good point.
 
solarken said:
If the upstream load center has no loads, has a PV backfed breaker, and has a main breaker of 200A, , and feed-thru lugs that land on a subpanel's 200A breaker, it still fails I guess, because the PV breaker would be >0A, and any >0A + 200A exceeds the 200A bus rating. This is assuming still using the sum of all breakers method. But it does not seem possible that the bus rating would be exceeded in this scenario.
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Right, you can't overload any panel where each busbar has at most 3 connections, each protected at no more than the busbar rating. But unfortunately that requires using 705.12(B)(6) engineering supervision.

Cheers, Wayne
 
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