705.12 (B)(2)(3)(C) and surge arrestors

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am1954

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Location
NY
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contractor
I'm trying to understand how much PV I can load onto a dedicated panel as per 705.12(B)(2)(3)C) while still taking into account CBs I will need to dedicate to monitoring and surge arrestors. If I have a 400a bus bar and PV circuits are 20a each, I could install 20 circuits (20a x 20 circuits = 400a). If I wanted to add a 20a breaker for the monitoring system I assume I would need to sacrifice one of my PV circuits in order to make it fit. But what about a surge arrestor? If I want to have a SA on it's own dedicated breaker should I assume I need to lose yet another PV circuit, or should that not count in the calculation since it's not a load? We are in 2017 NEC. Thanks for any thoughts.
 
am1954 said:
I'm trying to understand how much PV I can load onto a dedicated panel as per 705.12(B)(2)(3)C) while still taking into account CBs I will need to dedicate to monitoring and surge arrestors. If I have a 400a bus bar and PV circuits are 20a each, I could install 20 circuits (20a x 20 circuits = 400a). If I wanted to add a 20a breaker for the monitoring system I assume I would need to sacrifice one of my PV circuits in order to make it fit. But what about a surge arrestor? If I want to have a SA on it's own dedicated breaker should I assume I need to lose yet another PV circuit, or should that not count in the calculation since it's not a load? We are in 2017 NEC. Thanks for any thoughts.
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This is a case where the NEC's rules do not reflect reality. This sum of breakers excluding main supply rule, requires you to accumulate the phantom amps of rounding errors. (E.g. consider 12 qty 25A inverters on 35A breakers each; should it be a 400A or 600A bus?). Also insignificant loads like monitoring systems and surge arresters that are subtractive anyway. I'd recommend reform to this rule, based on the 120% rule in reverse, allowing load breakers to add up to 20% of the busbar rating. Try qualifying the panel under engineering supervision. I've never had a problem doing the same thing.

I understand the use case for "sum of breakers excluding main supply", when there is a mixture of sources and loads, and both are significant enough relative to the busbar rating. It's a good rule for a general case of a mixed use panel, but it's overly conservative when only the sources are significant.
 
To agree with Carultch, unfortunately if you encounter an AHJ who argues to the letter of the code, you will have to count any OCPDs for surge arrestors.

If you are able to use 'surgebreakers' or any other type of surge arrestor that doesn't require connection through an OCPD, you wouldn't have to count them. Or like, another work-around might be to tap the feeder ahead of the PV combiner panel to another panel, and put your surge arrestors there.
 
NEC 705.12(B)(2)(3)C) works great if you know the history and purpose for it. Before this was added to the code if you wanted to have a code compliant AC combiner panel it had the be very oversized for the inverter load because it had to comply with the 120% rule. NEC 705.12(B)(2)(3)C) was added so that we can have reasonably sized AC combiner panels, and it is great for that. Saves people a lot of money.
The problem is when people want to have an AC combiner panel that also supplies loads such as monitoring systems. It's really convenient to pop a CB in that combiner panel but people also want to load the panel up to the limit with inverters too. Usually an AHJ will let small loads slide. If they don't then we have to go by the code.
 
FWIW, most AHJs I have dealt with count monitoring loads but not surge arrestors toward the sum of all breakers rule.
 
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