Trouble interpreting 705.12 (B)(3)(3) in NEC 2020

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Solar Noob

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Location
California
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Electrical Engineering (in training)
Hi everyone,

I am having trouble deciding if this needs to be included in the PV forum or this one, but this specifically applies to NEC so I figured I would ask the question here. According to the code mentioned in the title above, "The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar." From this description, which follows both 705.12 (B)(1) and 705.12 (B)(2), it seems that I can have multiple points of interconnection for PV, not just on the load side but also the supply side as well. Any help with making sure this is a correct interpretation, or providing an interpretation that is more sound would be appreciated.
 
2020 NEC 705.12 covers only load side interconnections; any interconnections on the supply side of the service disconnecting means may be ignored when applying its provisions.

Cheers, Wayne
 
Solar Noob said:
Hi everyone,

I am having trouble deciding if this needs to be included in the PV forum or this one, but this specifically applies to NEC so I figured I would ask the question here. According to the code mentioned in the title above, "The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar." From this description, which follows both 705.12 (B)(1) and 705.12 (B)(2), it seems that I can have multiple points of interconnection for PV, not just on the load side but also the supply side as well. Any help with making sure this is a correct interpretation, or providing an interpretation that is more sound would be appreciated.
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Total number of connections on a premise is not what 705.12 (B)(3) is talking about. It's talking about a single panelboard or switchboard.

This provision was put in the code mainly to permit 'combiner' panels for multiple inverter outputs to be sized to the total inverter output rather than having to also account for the utility source under the 120% rule (705.12(B)(2)). But it also allows other configurations where the source exceeds the 120% rule or where you have multiple source outputs, in some cases.
 
Perhaps I've been mistaken, but I thought this also applied to load centers/AC combiners?

As in 80 amp rated bus enphase combiner, max 4 20 amp backfed breakers.
 
I have 2 questions:
1/ For 705.12(B)(3)(1) and 705.12(B)(3)(3), as I understand that we can place the breakers of the PV inverters anywhere in the bus for both codes. So when should we apply 705.12(B)(3)(1) or 705.12(B)(3)(3)? I am thinking to apply both codes and then select the lower bus rating (for cheaper price)?
2/ For 705.12(B)(3)(3), does it matter to the 80% rated and 100% rated breaker? For example, if the inverter output current is 100A. Then we can use either 100A 100% rated breaker or 125A 80% rated breaker. And if a panel board has 5 inverters and 1 load of 40A, then in the case of using 80% breaker, the bus is >= (5*125+40)=665A. In the case of using 100% rated breakers, then the bus is >= (5*100+4)=540A. So, can we select 550A bus if using 100% rated breaker?
 
binhphuong5487 said:
I have 2 questions:
1/ For 705.12(B)(3)(1) and 705.12(B)(3)(3), as I understand that we can place the breakers of the PV inverters anywhere in the bus for both codes. So when should we apply 705.12(B)(3)(1) or 705.12(B)(3)(3)? I am thinking to apply both codes and then select the lower bus rating (for cheaper price)?
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This is a design question and depends on the circumstances. However 705.12(B)(3)(3) would be more appropriate for the sort of design you describe in your next question.

2/ For 705.12(B)(3)(3), does it matter to the 80% rated and 100% rated breaker? For example, if the inverter output current is 100A. Then we can use either 100A 100% rated breaker or 125A 80% rated breaker. And if a panel board has 5 inverters and 1 load of 40A, then in the case of using 80% breaker, the bus is >= (5*125+40)=665A. In the case of using 100% rated breakers, then the bus is >= (5*100+4)=540A. So, can we select 550A bus if using 100% rated breaker?
Click to expand...

It does not matter under 705.12(B)(3)(3). However my understanding is that 100% rated AC panelboards aren't really a thing. Just single breaker enclosures. So you should probably plan on using breakers at 80% of rating.
 
jaggedben said:
This is a design question and depends on the circumstances. However 705.12(B)(3)(3) would be more appropriate for the sort of design you describe in your next question.



It does not matter under 705.12(B)(3)(3). However my understanding is that 100% rated AC panelboards aren't really a thing. Just single breaker enclosures. So you should probably plan on using breakers at 80% of rating.
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Thank you for your answer. I am not understand when you say "100% rated AC panelboards aren't really a thing" and "Just single breaker enclosures". As I know, we can just buy the 100% rating breaker (more expensive than 80% rating) but still possible. In 690.9(B)(2), it is acceptable to use 100% rating breaker. So, if we use 100% rating breaker, is it OK to use a smaller bus rating?
 
binhphuong5487 said:
... As I know, we can just buy the 100% rating breaker (more expensive than 80% rating) but still possible. ...
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I think that is incorrect. A breaker at 100% rating must be listed together with the enclosure. Note that every instance of such an exception in the NEC uses the phrase "an assembly" followed by something like "together with its overcurrent devices". The entire assembly - that is, the enclosure, panel and breakers - would have to be listed for use at 100% of continuous rating. You cannot just replace the breakers in an off-the-shelf panel with different ones, that's not how these exceptions work.

I'm open to being proven wrong, but my understanding is that you can't buy such at thing for typical AC panelboards or switchboards in the US (meaning up to 480/277V), because nobody makes that. You might find some DC combiner solar equipment that is listed at 100%, because all the fuses come with it as an assembly from the factory that got listed at 100%. But AC panelboards are a different animal; you choose the breakers and they are not tested in all configurations at 100%. Again I'm open to being proven wrong but that's my understanding.

So, if we use 100% rating breaker, is it OK to use a smaller bus rating?
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Yes, theoretically. But only if you can find equipment - panel and breakers together - that say they can be used at 100%.
 
Side question but still related to the topic of discussion. Do I still have to account for the main breaker if it is in a different section on the switchboard or can I just count the OCPD to my inverters?
 
JonBrogden said:
Side question but still related to the topic of discussion. Do I still have to account for the main breaker if it is in a different section on the switchboard or can I just count the OCPD to my inverters?
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Probably should be a new topic. What do you mean by 'account for'? And where is your inverter OCPD in relation to it?
 
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