Meter main feeder sized at 83%?

[jaggedben]

I don't follow this purported argument at all. The #1 to the 125A non-backed up panel is obviously a tap (and subject to 705.12(B)(2)(2)). But the 200A feeder from the Gateway to the Backup Panel is just a continuation of the 200A feeder from the meter main to the Gateway. 705.12(B)(2)(1)(b) allows the power source interconnection in the Gateway like that because of the 200A main breaker in the backed up loads panel.

Now if the Backuped Panel had no main breaker (not sure if that's possible because of 408 requirements, I didn't check), then 705.12(B)(2)(1)(a) would govern, and the required ampacity would be ~325A. This is definitely a situation where 705.12(B)(2)(1)(a) should overrule 310.15(B)(7)(3), and perhaps 310.15(B)(7)(3) should have some language in it to make that explicit.

It's arguably a tap because it's not protected from overload by the multiple sources that are capable of feeding it simultaneously. It meets the 240.21 definition in that respect. Further, it would seem that 705's language about taps requires you to consider the current from all sources when sizing the conductors to the backed up panel just the same as to the non-backed up panel. It doesn't seem like there's an important difference between those two feeders here.

Are you referring to the feeder from the 200A meter main to the Backup Gateway? Why wouldn't 310.15(B)(7) apply to that?

Cheers, Wayne

Sorry, I was unclear. What I meant was in regard to backfeeding calculations only. If your backfeeding circuit current per 705 were, say, 190A, it's not clear that you can apply 310.15(B)(7) 'backwards' and use 2/0 cu conductors. Where as if your calculated load is 190A then that's what 310.15(B)(7) is clearly meant to apply to.

 

[wwhitney]

It's arguably a tap because it's not protected from overload by the multiple sources that are capable of feeding it simultaneously. It meets the 240.21 definition in that respect. Further, it would seem that 705's language about taps requires you to consider the current from all sources when sizing the conductors to the backed up panel just the same as to the non-backed up panel. It doesn't seem like there's an important difference between those two feeders here.

I agree that it is like a tap in that it is protected against overload by the main breaker in the panel supplied, rather that at it's source of supply. But I would not say that it is a 240.21 feeder tap, because I don't see any language in 705.12(B) requiring it to comply to the length limits in 240.21 [perhaps because as far as short circuit current goes, the contribution of the other power sources is negligible, unlike a usual feeder tap.]

If the intention were for 705.12(B) to require that it comply with the 240.21 tap rules, then 705.12(B)(2)(1)(b) would refer to it as a tap and would incorporate 705.12(B)(2)(2). The facts that 705.12(B)(2)(2) is a separate section says to me that the 240.21 tap rules only apply if you actually decrease the wire size of the feeder.

Sorry, I was unclear. What I meant was in regard to backfeeding calculations only. If your backfeeding circuit current per 705 were, say, 190A, it's not clear that you can apply 310.15(B)(7) 'backwards' and use 2/0 cu conductors. Where as if your calculated load is 190A then that's what 310.15(B)(7) is clearly meant to apply to.

But the "backfeeding" current already includes a 125% factor. That means the actual continuous inverter current would be 152A, below the 166A ampacity required for a 200A dwelling unit service/main feeder. So there's no problem.

Cheers, Wayne

 

[jaggedben]

I agree that it is like a tap in that it is protected against overload by the main breaker in the panel supplied, rather that at it's source of supply. But I would not say that it is a 240.21 feeder tap, because I don't see any language in 705.12(B) requiring it to comply to the length limits in 240.21 [perhaps because as far as short circuit current goes, the contribution of the other power sources is negligible, unlike a usual feeder tap.]

If the intention were for 705.12(B) to require that it comply with the 240.21 tap rules, then 705.12(B)(2)(1)(b) would refer to it as a tap and would incorporate 705.12(B)(2)(2). The facts that 705.12(B)(2)(2) is a separate section says to me that the 240.21 tap rules only apply if you actually decrease the wire size of the feeder.

The counterargument would be that if the CMP intended to modify 240.21 in the manner you suggest, they would have been more explicit about it. After all Chapter 2 remains in force unless modified by 705. And there's no conflict between 705 and 240 here, it is possible to apply both.

But the "backfeeding" current already includes a 125% factor. That means the actual continuous inverter current would be 152A, below the 166A ampacity required for a 200A dwelling unit service/main feeder. So there's no problem.

Cheers, Wayne

Again, a simple counter argument would be that 705 has a requirement for conductor size, and the conductors have to meet that. And in this case, it's the opposite to the above. There's a conflict, so 705 rules.

In truth, I think both are grey areas, and I quite doubt that the CMPs truly considered their positions on these fine points.

 

[wwhitney]

Again, a simple counter argument would be that 705 has a requirement for conductor size, and the conductors have to meet that. And in this case, it's the opposite to the above. There's a conflict, so 705 rules.

Looking at 705.60(B), the language used is "The circuit conductors and overcurrent devices shall be sized to carry . . .," and does not refer to "ampacity". So I don't see a conflict, nor do I see a prohibition in using the 0.83 factor in going from rating to ampacity, when applicable under 310.15(B)(7).

In truth, I think both are grey areas, and I quite doubt that the CMPs truly considered their positions on these fine points.

Definitely agree.

Cheers, Wayne

 

[wwhitney]

Looking at 705.60(B), the language used is "The circuit conductors and overcurrent devices shall be sized to carry . . .," and does not refer to "ampacity". So I don't see a conflict, nor do I see a prohibition in using the 0.83 factor in going from rating to ampacity, when applicable under 310.15(B)(7).

OK, let me expand on that. From a brief survey, the "shall be sized to carry" language is used in multiple places in the NEC. And that language matches the definition of "ampacity" in Article 100.

What's unusual is the language in 310.15(B)(7), which says in subsection (1) for example ". . . shall be permitted to have an ampacity not less than 83 percent of the service rating."

So what is "rating" and how does it differ from "ampacity"? (Which it must, as their ratio can be non-unity.) It's really unclear, as there is no Article 100 definition. In other places I checked "rating" is used to mean things like "maximum amps the equipment can carry" or "maximum amps the the utilization equipment will draw" or just as a synonym for ampacity.

Basically I'm inclined to treat 310.15(B)(7) as a redefinition or remapping of ampacity for dwelling units, that only applies in the region from 83% to 100% of the service rating. I think the section has simply never been coordinated with the sections on "other power sources".

Cheers, Wayne