How does a generation panel comply with NEC 705.12(b)(2) without derating?

[mokonack]

I've been researching electrical line diagrams for a 10kW solar system with 2x Powerwalls and how to incorporate that with 200amp service without derating a main breaker or doing a line-side tap for solar. There's a lot of information and references to bus rating requirements in NEC 705.12(b)(2)(3) which I think I understand, but also claims that using a "generation panel" connected to the Tesla gateway will meet those requirements without requiring any derating, but not explaining why. Generally those posts show one set of the TEG backup lugs connected to a 200amp load panel and the other backup lugs connected to a 200amp generation panel.

For example, this is what my configuration would look like following that:

             [200amp meter with 200amp service disconnect breaker]
                                    | |
                             Tesla energy gateway
              | | (1st backup lugs)                        | |  (2nd backup lugs)
[200 amp main breaker Load panel]         [200 amp main breaker generation panel]
      various load circuits                    60amp solar, 2x30amp Powerwall

I understand logically that the Gateway bus can't be overloaded because:
If the load panel pulled more than 200 amps, the load panel OCPD would trip.
If the generation panel was pulling power (e.g., Powerwalls charging from grid) and that combined with the load panel exceeded 200 amps, the service disconnect would trip.

I understand the load and generation panel each meet the NEC requirements individually, but don't understand how the Gateway does. Is it because the 2017 NEC bus rating requirement doesn't explicitly call out feed thru lugs? This is more explicit in the 2020 requirements.

 

[wwhitney]

In your schematic, typically 2020 NEC 705.12(B)(3) is only applied to the generation panel. It qualifies under 705.12(B)(3)(3), the sum of all breakers rule. (The 200A main breaker in the generation panel is superfluous and not required).

The Gateway is just treated as a spot on the feeder from the service to the 200A load panel, and as such the interconnection complies with 705.12(B)(1)(b) (which requires the 200A main breaker in the load panel). The load panel itself has only one source of supply, so it is not subject to 705.12(B) (beyond that requirement for the 200A main breaker).

It's not clear to me why the Gateway (without internal panelboard) is not considered "distribution equipment . . . capable of supply multiple branch circuits or feeders," which would require its busbars to comply with 705.12(B)(3). But it is a good thing in practice, as none of the allowances in 705.12(B)(3) cover the topology of the Gateway busbars, even though the design works to prevent overload in the intended installation configurations.

The 2023 NEC addresses this issue by limiting the 705.12(B) requirements to "distribution equipment with no specific listing and instructions for combining multiple sources," which excludes the Gateway.

Cheers, Wayne

 

[holeydonut]

I've been researching electrical line diagrams for a 10kW solar system with 2x Powerwalls and how to incorporate that with 200amp service without derating a main breaker or doing a line-side tap for solar. There's a lot of information and references to bus rating requirements in NEC 705.12(b)(2)(3) which I think I understand, but also claims that using a "generation panel" connected to the Tesla gateway will meet those requirements without requiring any derating, but not explaining why. Generally those posts show one set of the TEG backup lugs connected to a 200amp load panel and the other backup lugs connected to a 200amp generation panel.

For example, this is what my configuration would look like following that:

             [200amp meter with 200amp service disconnect breaker]
                                    | |
                             Tesla energy gateway
              | | (1st backup lugs)                        | |  (2nd backup lugs)
[200 amp main breaker Load panel]         [200 amp main breaker generation panel]
      various load circuits                    60amp solar, 2x30amp Powerwall

I understand logically that the Gateway bus can't be overloaded because:
If the load panel pulled more than 200 amps, the load panel OCPD would trip.
If the generation panel was pulling power (e.g., Powerwalls charging from grid) and that combined with the load panel exceeded 200 amps, the service disconnect would trip.

I understand the load and generation panel each meet the NEC requirements individually, but don't understand how the Gateway does. Is it because the 2017 NEC bus rating requirement doesn't explicitly call out feed thru lugs? This is more explicit in the 2020 requirements.

Yeah my problem was my gateway had a Eaton 200A breaker for the lugs that the utility feeder connected to. So I had a huge pain/difficulty getting people to accept the reality that you and Wayne are describing where 2020 NEC 705.12(B)(3) shouldn’t apply to the Tesla Gateway 2.

Looking for, the stupid thing that can allow me to remove the Eaton 200A breaker on a TEG2

I don't even want to try and explain why*... but it'll help me out if I can find the part with the green arrow on it. If I understand things correctly (which is probably not the case since I suck), if I had that part I could remove the 200A whack-job Eaton breaker from my TEG2. So the utility...

teslamotorsclub.com

I wish Wayne would become lead solar and ESS inspector for all of California.

 

[mokonack]

The Gateway is just treated as a spot on the feeder from the service to the 200A load panel, and as such the interconnection complies with 705.12(B)(1)(b) (which requires the 200A main breaker in the load panel).

Okay I think this makes sense. I assume it remains true even if there is a service disconnect breaker between the meter and the Gateway?

It's not clear to me why the Gateway (without internal panelboard) is not considered "distribution equipment . . . capable of supply multiple branch circuits or feeders," which would require its busbars to comply with 705.12(B)(3).

I guess this is the confusing part as the answer is basically “that section of code doesn’t apply to the Gateway for ‘reasons.’” I’m worried AHJ would not understand this and I don’t have a code section to point to that explains it.

The 2023 NEC addresses this issue by limiting the 705.12(B) requirements to "distribution equipment with no specific listing and instructions for combining multiple sources," which excludes the Gateway.

Isn’t this still unclear? Why is the Gateway not “distribution equipment?”

 

[wwhitney]

Okay I think this makes sense. I assume it remains true even if there is a service disconnect breaker between the meter and the Gateway?

Yes. If there isn't, the Gateway needs a service disconnect breaker installed within it on its line side connection, an option which it supports. Logically, it's the same either way.

I guess this is the confusing part as the answer is basically “that section of code doesn’t apply to the Gateway for ‘reasons.’” I’m worried AHJ would not understand this and I don’t have a code section to point to that explains it.

I agree. If the Gateway doesn't have breakers, then most will not consider it to be the type of distribution equipment covered by 705.12(B)(3), even though it has busbars. This is supported by the fact that all of the options under 705.12(B)(3) refer to rules about breakers, so implicitly 705.12(B)(3) is about distribution equipment with breakers.

Isn’t this still unclear? Why is the Gateway not “distribution equipment?”

If it is distribution equipment, it does have a "specific listing and instructions for combining multiple sources." So under the 2023 NEC, it would be exempt from the 705.12(B) requirements.

Cheers, Wayne