COMMERCIAL BUILDING PHOTOVOLTAICS - LOAD SIDE CONNECTION

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PE (always learning)

PE (always learning)

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Saint Louis
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Professional Engineer
I've got a project where a 25 KW solar array is being provided on the load side of a panel rated for 600 amps. The incoming service is 120/208, 3 phase, 4 wire and I want to verify that my main panel bus rating is adequate for the 25 KW solar array. I don't deal with solar that often and I just want to make sure my assumptions are correct. For this issue, I would defer to 705.12. The solar array has a FLA of 69 amps and I have located the breakers for the main and the PV source on opposite ends of the bus bar. If I calculate according to 705.12 (B), I would take (1.25*69 amps) + 600 = 686.25 amps. This is less than 1.2*600 = 720 amps. Everything checks out and the 600 amp bus should be adequate. Please let me know if I have misinterpreted anything.

Best Regards
 
engineerintraining12 said:
I've got a project where a 25 KW solar array is being provided on the load side of a panel rated for 600 amps. The incoming service is 120/208, 3 phase, 4 wire and I want to verify that my main panel bus rating is adequate for the 25 KW solar array. I don't deal with solar that often and I just want to make sure my assumptions are correct. For this issue, I would defer to 705.12. The solar array has a FLA of 69 amps and I have located the breakers for the main and the PV source on opposite ends of the bus bar. If I calculate according to 705.12 (B), I would take (1.25*69 amps) + 600 = 686.25 amps. This is less than 1.2*600 = 720 amps. Everything checks out and the 600 amp bus should be adequate. Please let me know if I have misinterpreted anything.

Best Regards
Click to expand...
Is this panelboard protected by a 600 amp OCPD or have a 600 Amp main breaker? Also what is ahead of this panel?
 
You've understood it correctly. 1.25*I + M <= 1.2*B, where I is the total inverter FLA, M is the main supply OCPD, and B is the busbar ampacity. 120% is allowed, only if it is at the opposite end of the busbar. M could be less than B, but it is commonly the case that M equals B. If this is a main-lug-only panelboard, you would have to have an external overcurrent device from which this panel is supplied, to take the place of M in this calculation.

Note that kW/Voltage/sqrt(3) doesn't necessarily get you the FLA of the inverter(s). It's a good starting point to get a general idea, but you'll ultimately have to check the datasheet to make the final call. Some inverters have a value that is slightly larger than what you expect, from its kW rating alone.

In your case, you can have up to 96A worth of total inverter FLA current interconnected on the opposite end of the busbar, which would correspond to a 125A breaker. At one point, the language of the 120% rule specified to use the interconnection breaker rating in this calculation, which would've limited your case to a 110A breaker since that is the previous standard size, but the rule was revised in NEC2014. As of NEC2014, instead of using the interconnection breaker rating in the calculation, you use the calculation that determines that breaker prior to rounding up. So 96A*1.25 + 600 <= 120% of 600, is the calculation that governs compliance.
 
sjddota065 said:
What could be the issue?
Click to expand...
Two things. 1. OP said "panel rated for 600 amps.". Technically we need to know what the bus rating is and what the OCPD rating protecting the bus is. Sure a factory main breaker panel is usually assumed to have the same breaker and bus rating* but OP didn't state specifics.

2. The 120% rule applies to everything upstream, so of there is upstream equipment it would need to be evaluated too.

So we can give responses that may be true most of the time or are based on assumptions, but often the devil is in the details.

* for load centers that might not be actually true but is hard to prove, true panelboards it's more likely to have a higher bus than MB rating and is easier to prove -i e. If you order a Siemens "200A MB panel board" you will get a P1 series which has 250A bus.
 
electrofelon said:
Two things. 1. OP said "panel rated for 600 amps.". Technically we need to know what the bus rating is and what the OCPD rating protecting the bus is. Sure a factory main breaker panel is usually assumed to have the same breaker and bus rating* but OP didn't state specifics.

2. The 120% rule applies to everything upstream, so of there is upstream equipment it would need to be evaluated too.

So we can give responses that may be true most of the time or are based on assumptions, but often the devil is in the details.

* for load centers that might not be actually true but is hard to prove, true panelboards it's more likely to have a higher bus than MB rating and is easier to prove -i e. If you order a Siemens "200A MB panel board" you will get a P1 series which has 250A bus.
Click to expand...


To clarify, the main breaker protecting the bus is rated for 600 amps.

Also, this panel is the main service entrance panel for the building and is served by it's own utility transformer. There is not any other equipment upstream of this panel.
 
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