Sol
New member
- Location
- 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?
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?