Solar Overcurrent Protection

[Kjc59]

I am no solar expert so I’m hoping someone is. I have a 4,000 switchboard and a 317kW PV system will be tied into a breaker in the board. I am pretty sure the breaker in the switchboard needs to be 1,200 Amps. Is this system too large for a 4,000A board? Size up buswork? To me I would think the ideal install would be ahead of everything but that is not the constraints I’m working with. Any feedback?


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[ggunn]

I am no solar expert so I’m hoping someone is. I have a 4,000 switchboard and a 317kW PV system will be tied into a breaker in the board. I am pretty sure the breaker in the switchboard needs to be 1,200 Amps. Is this system too large for a 4,000A board? Size up buswork? To me I would think the ideal install would be ahead of everything but that is not the constraints I’m working with. Any feedback?


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125% of your total inverter current cannot exceed 800A for a load side connection to a 4000A panelboard. You should interconnect the PV on the supply side of the main service OCPD.

 

[pv_n00b]

You can do it either by following ggunn with the 120% rule or doing it under engineering supervision without the 120% limitation. In my opinion it's always better to do a load side connection if that is possible, but a supply side is also an option.

 

[jaggedben]

There are possibilities beyond the 120% rule these days, but you need to fully read 705.12 or understand about Power Control Systems.

 

[pv_n00b]

There are possibilities beyond the 120% rule these days, but you need to fully read 705.12 or understand about Power Control Systems.

I wish that 705.12(B) had a line that included PCS as a specified alternative to comply. There is a bit of hand waving around it right now. 705.13 says PCS can be used to limit current and loading on busbars but not how that is used to comply with 705.12(B). It looks like PCS is only allowed in connection with the engineering supervision allowed by 705.12(B)(6).

 

[wwhitney]

705.13 says PCS can be used to limit current and loading on busbars but not how that is used to comply with 705.12(B).

Seems to me that means it can be used to limit the current values used as input to any of the compliance checks in any of the methods of 705.12(B).

Cheers, Wayne

 

[pv_n00b]

Seems to me that means it can be used to limit the current values used as input to any of the compliance checks in any of the methods of 705.12(B).

Cheers, Wayne

There is that implication and I'm sure people are doing that. But it is not listed as one of the ways to comply with 705.12(B) and I wish it were, to provide better guidance to AHJs.

 

[jaggedben]

There is that implication and I'm sure people are doing that. But it is not listed as one of the ways to comply with 705.12(B) and I wish it were, to provide better guidance to AHJs.

The 2020 and 2023 NECs both refer to using the PCS inputs in 705.12, in different ways. 2020 mentions it directly at the beginning of 705.12, and 2023 does it by referencing 705.28(A), which in turn mentions EMS.

 

[pv_n00b]

The 2020 and 2023 NECs both refer to using the PCS inputs in 705.12, in different ways. 2020 mentions it directly at the beginning of 705.12, and 2023 does it by referencing 705.28(A), which in turn mentions EMS.

But the description is very simplistic, hinting at a single max current limit set in the PCS. And that's not how we want a PCS to work. Take the 2023 705.12 for instance.

Currents from power source connections to feeders or busbars shall be based on the maximum circuit currents calculated in 705.28(A).

So I still need to use 705.(B)(1) to (6) to qualify a panelboard. I can use the maximum current from the PCS to do that and not the maximum current of the source feeding the PCS. But 705.(B)(1) to (5) still puts limits on that current.
For instance if I want to use 705.(B)(2) then the PCS max current would be limited to the 120% rule. But if the PCS is monitoring the loading on the bus I want to be able to increase the PCS output over that 120% limit, up to the full bus rating. That's kind of a major advantage of PCS. The only way to take full advantage of what a PCS can do is by using 705.(B)(6). To allow a PCS to work as designed without involving an engineer there needs to be an addition to the list that covers how to use PCS to protect the interconnection and not simply use it as a fixed max current limiter. That's what I was getting at.

 

[wwhitney]

From the 2026 NEC (Article 120 is formerly Article 220):

(1) Controlled Loads.
Controlled loads shall be based on the monitoring by the PCS to provide overload control and the PCS control configuration and shall comply with one or both of the following:

If the PCS monitors only controlled loads, the control setting of the PCS shall be used in place of the controlled loads in load calculations.
If the PCS monitors both controlled and noncontrolled loads, the minimum operating current of the controlled loads shall be used in place of the controlled loads in load calculations.

Informational Note: Minimum operating current is a value greater than or equal to zero representing the minimum current of the controlled loads.

It does seem like something similar belongs in 705 with respect to controlled sources to permit "dynamic" PCS overload control of a busbar.

Cheers, Wayne