Interaction of (2020) 408.36 and 705.12(B)(1)

[wwhitney]

408.36 requires a panelboard to protected by an individual OCPD, or under the exception, two OCPD that sum to not more than the panelboard rating.

For a feeder to a panelboard, suppose a load side connection is made in the middle of the feeder under 705.12(B)(1). If the panelboard has a main breaker, then that satisfies 705.12(B)(1)(b) and 408.36 and all is well.

So when does 705.12(B)(1)(a) come in, where the load-side segment of the feeder is increased in ampacity? Does its use necessarily require the use of 408.36 Exception? An example would be a 150A service disconnect, a 150A feeder segment to the point of interconnection of a PV source from a 50A breaker, and a 200A feeder segment from the interconnection point to a 200A MLO panel. I don't see any other way to utilize 705.12(B)(1)(a).

BTW, if the service in the example is residential, then 310.15(B)(7)(3) applies. That seems to say the "200A" feeder segment need not be larger than 83% * 150A (the service rating), which definitely seem wrong.

Cheers, Wayne

 

[Hv&Lv]

I may be way off base here, but 705.12 B 1 would come into play with an inverter and a separate battery bank, No?

 

[jaggedben]

408.36 requires a panelboard to protected by an individual OCPD, or under the exception, two OCPD that sum to not more than the panelboard rating.

For a feeder to a panelboard, suppose a load side connection is made in the middle of the feeder under 705.12(B)(1). If the panelboard has a main breaker, then that satisfies 705.12(B)(1)(b) and 408.36 and all is well.

So when does 705.12(B)(1)(a) come in, where the load-side segment of the feeder is increased in ampacity? Does its use necessarily require the use of 408.36 Exception? An example would be a 150A service disconnect, a 150A feeder segment to the point of interconnection of a PV source from a 50A breaker, and a 200A feeder segment from the interconnection point to a 200A MLO panel. I don't see any other way to utilize 705.12(B)(1)(a).

BTW, if the service in the example is residential, then 310.15(B)(7)(3) applies. That seems to say the "200A" feeder segment need not be larger than 83% * 150A (the service rating), which definitely seem wrong.

Cheers, Wayne

The feeder sized to the sum of the sources could feed multiple downstream taps to multiple panelboards with main breakers. Probably a rare animal in the real world, although I did once happen upon and installation where it would have been relevant. Unfortunately these sections were not in the code yet and that installation got cancelled.

 

[jaggedben]

I may be way off base here, but 705.12 B 1 would come into play with an inverter and a separate battery bank, No?

No. Way off base.

 

[wwhitney]

The feeder sized to the sum of the sources could feed multiple downstream taps to multiple panelboards with main breakers.

Good example, thanks.

Do you agree that for the 2026 NEC (missed the 2023 PI deadline), 310.15(B)(7) needs some modification for the case of feeders with multiple sources of supply?

Cheers, Wayne

 

[Hv&Lv]

No. Way off base.

Then help me out here..

705.12(B)(1) Dedicated Overcurrent and Disconnect. Each source interconnection of one or more power sources installed in one system shall be made at a dedicated circuit breaker or fusible disconnecting means.

I’m seeing this as a PV and a battery bank needing separate dedicated circuit breakers

 

[wwhitney]

Wrong version, the title specifies 2020 NEC.

Wayne

 

[Hv&Lv]

Wrong version, the title specifies 2020 NEC.

Wayne

Yep, see that now...
Way off base.
haven’t read the 2020

 

[jaggedben]

Good example, thanks.

Do you agree that for the 2026 NEC (missed the 2023 PI deadline), 310.15(B)(7) needs some modification for the case of feeders with multiple sources of supply?

Cheers, Wayne

I don't know of a reason to agree. I think that probably the language could always be improved in clarity, but getting the CMP to agree on what's clearer is not always easy. What specific issue do you think is unaddressed or improperly addressed?

 

[wwhitney]

For a 100A service to a 100A main feeder segment meeting an 80A inverter output, 705.12(B)(1)(a) would require a 180A feeder segment from the point of interconnection to, say, a 200A MLO panel. But 310.15(B)(7)(3) can be taken to mean that the "180A" feeder segment really only needs to have an ampacity of 83A. So 310.15(B)(7) should have some language about multiple sources of supply; as written, it only contemplates a single source of supply, the grid.

Cheers, Wayne

 

[jaggedben]

I see your point but I think 90.3 already explains that 705 already modifies 310. That should address your worry that somehow 310.B(7)(3) overrides 705.12(B)(1)(a). It doesn't. What is the most concise way to edit 310 to clarify? BTW the relevant section is numbered 310.12 now.

We could argue that the feeder you describe only needs an ampacity of 83% of 180A, rather than 83% of 100A. That would be 149A and would mean something like 1 awg instead of 1/0, rather than 4awg instead of 1/0. The former difference doesn't worry me the way that the latter one does.