2023 NEC 705.12(A)(2)(b) & 705.12(B)(5)

[tortuga]

Say you have a 200A meter/main, 200A bus bar with feed thru lugs.
The existing feeder is 4/0 AL XHHW-2, the sub panel has a 200A main.
And we want to add a inverter with a 32 amp nameplate.

705.12(B)(5) says the feed-through conductors (feeder) needs to be sized per 705.12(A),
705.12(A)(1) is met.
Next we have 705.12(A)(2)
We can choose a or b
705.12(A)(2)(a) says the ampacity in this case would need to be 100% of the main + 125% of the inverter; 200 + 40 = 240A (350kcmil AL)
705.12(A)(2)(b) says we can have a main breaker in the sub panel not greater than the ampacity of the feeder.

705.12(A)(2)(b) does not contain any next standard size up language, so the breaker for the 4/0 AL would need to be 175A ?
Diagram attached

Any thoughts?

 

[jaggedben]

Ahh. Stupid.
(b) should instead say "shall protect the feeder in accordance with 240.4"

 

[wwhitney]

Any thoughts?

Yes, I agree that like the 240.21(B) tap rules, 2023 NEC 705.12(A)(2)(b) precludes the use of 240.4(B).

Cheers, Wayne

 

[wwhitney]

Ahh. Stupid.

Why is it stupid? 2023 NEC 705.12(A)(2)(b) is akin to an unlimited length tap rule, and all the tap rules preclude the use of 240.4(B).

Cheers, Wayne

 

[jaggedben]

Why is it stupid? 2023 NEC 705.12(A)(2)(b) is akin to an unlimited length tap rule, and all the tap rules preclude the use of 240.4(B).

Cheers, Wayne

Well, the tap rules preclusion of 240.4(B) is pretty suspect, too. But in this particular case the parallelling of sources upstream is really no reason at all not to allow 240.4(B). The available fault current doesn't change by a significant figure.

 

[jaggedben]

We also shouldn't have to throw out 310.12 because of all this.

 

[tortuga]

Thanks for the feedback, so either the 4/0 conductors get swapped out to 250kcmil AL (or 3/0 copper) or a 175A breaker in the sub panel.

There is also the case where the service panel is empty other than the inverter tie in and 100% of the dewlling is fed from the sub-panel, then the dewlling table in 310.Iforget could be used for the feeder ampacity.

 

[jaggedben]

You're thinking of 310.12 and unfortunately the way it is worded nowadays is that conductors may be used at 83% of ampacity. So the 'not greater than ampacity of the feeder' language trumps that as well. Under older versions of that section (the old 310.15(B)(7)) it was ambiguous if the rule actually changed the official ampacity.

 

[letgomywago]

This needs revisited I think but nothing will happen for 6 years atleast.