705.12(D)(2)(3)(c) Clarification

[integralsolar]

Coworker and I are having discussion about 705.12(D)(2)(3)(c). See following scenario.

A 200A, 120/240Vac, 1ph, 3W Service feeds 200A MB Meter combo with feed thru lugs to 200A MB Sub panel. The meter combo only has a couple loads lets say 50A/2P and a 15A/1p and the Sub panel is full. Is it possible to install PV 60A/2P CB in Meter combo using 705.12(D)(2)(3)(c).

Using the wording of the code: 50A + 15A + 60A = 125A

My Argument is that you cannot install the PV breaker in meter combo as you need to also account for sub panel overcurrent devices. Coworker's argument is that there will never be more then 200A since Sub panel has 200A MB.

Thoughts?

 

[ggunn]

Coworker and I are having discussion about 705.12(D)(2)(3)(c). See following scenario.

A 200A, 120/240Vac, 1ph, 3W Service feeds 200A MB Meter combo with feed thru lugs to 200A MB Sub panel. The meter combo only has a couple loads lets say 50A/2P and a 15A/1p and the Sub panel is full. Is it possible to install PV 60A/2P CB in Meter combo using 705.12(D)(2)(3)(c).

Using the wording of the code: 50A + 15A + 60A = 125A

My Argument is that you cannot install the PV breaker in meter combo as you need to also account for sub panel overcurrent devices. Coworker's argument is that there will never be more then 200A since Sub panel has 200A MB.

Thoughts?

The problem is that it is a feed through; you cannot ignore the loads that pull from the feed through lugs. I realize that the article is silent on the issue, but IMO the intent is clear.

 

[wwhitney]

A 200A, 120/240Vac, 1ph, 3W Service feeds 200A MB Meter combo with feed thru lugs to 200A MB Sub panel. The meter combo only has a couple loads lets say 50A/2P and a 15A/1p and the Sub panel is full. Is it possible to install PV 60A/2P CB in Meter combo using 705.12(D)(2)(3)(c).

Funny, I was just thinking about this issue in regards to another thread. Here's my take on it:

Physics-wise, it makes basically no difference during normal operations if the feeder to the subpanel is supplied by feed thru lugs or by a 200A circuit breaker. However, code language-wise, if the feeder is supplied by feed thru lugs, the meter combo panel complies with the wording of 705.12(D)(2)(3)(c). If supplied by a circuit breaker, the meter combo panel couldn't comply with either 705.12(D)(2)(3)(b) or (c).

Clearly the code languages makes a (perhaps unintentional) distinction that has no basis in physics. I wouldn't worry about taking advantage of that, since the 120% figure of 705.12(D)(2)(3)(b) is arbitrary. That figure could reasonably be 150% or 200% and the bus bar would still be protected.

BTW, even though it does not seem to be required by 705.12(D)(2)(3), it would be best practice to put the 60A PV breaker in the meter combo panel opposite the utility supply. Otherwise you really could overload the meter combo panel bus.

Cheers, Wayne

 

[shortcircuit2]

I think this is in violation of 705.12(B)(2)(3)(a)

The 200-amp breaker and the 60-amp will be feeding the sub-panel bussing and could be overloaded.

 

[wwhitney]

I think this is in violation of 705.12(B)(2)(3)(a)

The 200-amp breaker and the 60-amp will be feeding the sub-panel bussing and could be overloaded.

If the subpanel were MLO, then there would be a violation of 408.36, and possibly of 705.12(D)(2)(1). But in the OP's example, the subpanel has a main breaker, so those issues do not arise.

Cheers, Wayne

 

[ggunn]

If the subpanel were MLO, then there would be a violation of 408.36, and possibly of 705.12(D)(2)(1). But in the OP's example, the subpanel has a main breaker, so those issues do not arise.

Cheers, Wayne

But if he is trying to qualify the bus under 705.12(D)(2)(3)(c), then the rating of the main breaker of the subpanel should count toward the breaker sum. I know the article doesn't say that explicitly, but what is the difference between a load drawing current from a breaker on the bus and drawing it from the feedthrough lugs?

 

[wwhitney]

But if he is trying to qualify the bus under 705.12(D)(2)(3)(c), then the rating of the main breaker of the subpanel should count toward the breaker sum.

I agree that would be a logical procedure, and perhaps 705.12(D)(2)(3)(c) should include language about subfeed lugs, but as you say:

the article doesn't say that explicitly

So I'm sticking with Charlie's Rule.

Cheers, Wayne

 

[ggunn]

I agree that would be a logical procedure, and perhaps 705.12(D)(2)(3)(c) should include language about subfeed lugs, but as you say:


So I'm sticking with Charlie's Rule.

Cheers, Wayne

Be that as it may, if a loophole or alternative interpretation enables me to build a code compliant system that is unsafe, I'm not doing it.

 

[wwhitney]

Be that as it may, if a loophole or alternative interpretation enables me to build a code compliant system that is unsafe, I'm not doing it.

Absolutely, so in the OP's situation it would be important that in the meter combo panel, the PV breaker is at the opposite end of the bus from the main breaker.

I guess my point is that despite the work that has occurred this decade on 705.12(D)(2), the current wording is still not very good. Here's a simple example:

Consider the following three configurations on a 120/240V service:

Panel A is a Meter Main with a 200A Service Disconnect, no other breaker spaces
Panel B is a 200A MLO panel with 6 spaces, assume 200A Breakers and SubFeed Lugs take 4 spaces
Panel C has a 200A MB and many branch circuit breakers


Configuration 1:

200A Feeder from Panel A
|
3-Port Insulated Connector -- 200A Feeder -- 100A Enclosed Breaker for PV
|
200A Feeder to Panel C


Configuration 2:

200A Feeder from Panel A
|
Panel B with 100A PV Breaker and 200A SubFeed Lugs
|
200A Feeder to Panel C


Configuration 3:

200A Feeder from Panel A
|
Panel B with 100A PV Breaker and 200A Breaker
|
200A Feeder to Panel C, which is now MLO


Physics-wise and safety-wise, these all look the same to me. The breakers involved and their connectivity graph is the same, as is the ampacity of their interconnections. The only things that differ are where the breakers are located, and whether they are interconnected by busbars or feeders.

But the 705.12(D)(2) treats them all differently:

Configuration 1: Allowed under subsection (1)(b)
Configuration 2: Panel B (probably) meets subsection (3)(b), the Feeder to Panel C meets subsection (1)(b)
Configuration 3: Panel B doesn't meet any part of subsection (3)

Cheers, Wayne

 

[jaggedben]

Be that as it may, if a loophole or alternative interpretation enables me to build a code compliant system that is unsafe, I'm not doing it.

Are you saying you think there's something unsafe about the OP's proposal?

I'm inclined to agree it's not allowed, but I think this is an interesting case.

 

[wwhitney]

Configuration 2: Panel B (probably) meets subsection (3)(b)

Typo, I meant (3)(c).

Cheers, Wayne

 

[ggunn]

Are you saying you think there's something unsafe about the OP's proposal?

Not necessarily; that comment was meant to be general in scope. I do, however, think anyone who is qualifying a bus under 705.12(D)(2)(3)(c) and does not consider the contribution to the current in the bus from feedthrough lugs is using the letter of the code to do an end run around the spirit of the code.

 

[wwhitney]

I do, however, think anyone who is qualifying a bus under 705.12(D)(2)(3)(c) and does not consider the contribution to the current in the bus from feedthrough lugs is using the letter of the code to do an end run around the spirit of the code.

I'll agree with that; it does seem to be a loophole. I would respect the spirit of 705.12(D)(2) more if it were logically consistent.

Cheers, Wayne

 

[electro7]

I do think there is something unsafe with the OP proposal according to the NEC (without downsizing the main breaker to 175A). If he lands a 60A PV breaker on the bus bar of the meter main combo there is potential that busbar will see 260A (200 from utility, 60 from inverter) which violates the 120% busbar rule. That is only because there are other branch circuits on that busbar and therefore potential.

I do not think this has anything to do with the feed thru lugs and 200A sub panel since the sub panel has a 200A main breaker on it. I think everything is up to code and safe there.

The solution here in my opinion, is downsizing the main breaker on the main panel to 175A.

The feeder tap rule is about taping into feeder conductors where there are not other loads also tapped into those conductors, if that makes sense.