NEC 705.12 Panelboard sizing with inverters

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Sol

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Atlanta, GA
We have seven (7) 36kW(ac) inverters feeding a single panelboard.
For a 3-phase 480V 36kW(ac) inverter, the output current is 43.4A.
An OCPD of 54.2A is required (as per the 125% rule).
However, the next readily available circuit breaker size is 60A.
Here's where it gets curious:
7 x 60A = 420A which would require the next panelboard size up which would be 600A.
But 7 x 54.2A (or 55A) = 379.4A would mean that a 400A panelboard would suffice.

As per 2017 NEC 705.12(B)(2)(3)(c):
"The sum of the ampere ratings of all OCPD on the panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar."
However, at the beginning of the 2017 NEC 705.12(B)(2), it says "One hundred twenty-five percent of the power source output circuit current shall be used in ampacity calculations for the following:"
That would mean that it's okay to use the 54.2A value perhaps? That would allow us to use a 400A panelboard instead of jumping to a 600A panelboard.

Also, if an SPD is added into this panelboard, should the OCPD used to connect it be used in the calculations as well (even though it's load under normal operation is effectively zero?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
We have seven (7) 36kW(ac) inverters feeding a single panelboard.
For a 3-phase 480V 36kW(ac) inverter, the output current is 43.4A.
An OCPD of 54.2A is required (as per the 125% rule).
However, the next readily available circuit breaker size is 60A.
Here's where it gets curious:
7 x 60A = 420A which would require the next panelboard size up which would be 600A.
But 7 x 54.2A (or 55A) = 379.4A would mean that a 400A panelboard would suffice.

As per 2017 NEC 705.12(B)(2)(3)(c):
"The sum of the ampere ratings of all OCPD on the panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar."
However, at the beginning of the 2017 NEC 705.12(B)(2), it says "One hundred twenty-five percent of the power source output circuit current shall be used in ampacity calculations for the following:"
That would mean that it's okay to use the 54.2A value perhaps? That would allow us to use a 400A panelboard instead of jumping to a 600A panelboard.

Also, if an SPD is added into this panelboard, should the OCPD used to connect it be used in the calculations as well (even though it's load under normal operation is effectively zero?
That's the code. Qualifying the bus under section (c) of 705.12(D)(2)(3) you must use the breaker ratings while under (a) and (b) you can use 125% of your nameplate maximum inverter current. Where we do business the AHJ's don't count the breaker connecting the SPD toward the section (c) calculation but if it would put you over 100% of your bus rating you'd better check.

Here's something, though: if your three phase inverter bank consists of several single phase inverters phase to phase or phase to neutral, you only count the breakers connected to each busbar, not the total of all the breaker ratings in the panel.
 
Hey, found you, Sol Goodman!

That is a good question. Definitely one of those areas where the code isn't clear. The thing is though, it isn't how YOU interpret the code, it's how the inspector interprets it (and which side of the bed they woke up on). In that scenario I would probably just order to 600A panelboard or call the inspector and ask them what they are comfortable with.
 
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