Conductor deration in service trough

[quogueelectric]

Old school mentality seems to have been that in a commercial service to trough out both top and bottom of all panels to give you the most convenient route of getting line and load in and out of panels. With new code requirements doesnt this run into conductor deration issues for example running 3 section panels all with separate neutrals creating 240 current carrying conductors leaving the trough out to respective loads in a 10X10 in trough?

 

[Dave58er]

Are you referring to 376.22?
I'm not sure which changes you mean but, with your install it's always been my understanding that as long as there are not 30 in a cross sectional area you wont have to de-rate. So if most CCC's are going straight thru you're good.

BTW I've paid close attention to every trough I've seen (existing) since I learned this rule and I would bet 75% are in violation. I have never seen this rule enforced.

 

[augie47]

If the E/C pays little or no attention to the conduit entries you are correct, it can create a derating nightmare. With a little effort trying to terminate above the panel where most of the circuit terminate you can obviously reduced the number of longitudinal runs and when you look at each section of the wireway (inch by inch) the number may be a lot closer to 30. As Dave58er mentions, I know of few E/Is that actually carefully count each area.
Locally, most E/Cs now install a coupling/chase nipple between panels
and route traveling circuits through them.

 

[calhaz]

Trough

Trough

If you have service entrance conductors in these troughs you cannot have load conductors in them. I agree that it is 30 CCC conductors and if done right it is possible. Takes a little planning.

 

[Joe Villani]

Trough

Trough

Where does 376.22(B) mention cross sectional area when applying derating factors?

It is my opinion that cross sectional area only applies to sizing. Why isn't cross sectional area mentioned in (B)?

In some jurisdictions in this area you get 30 period, then start derating.

When I worked in the field, I just started using big junction boxes.

Joe

 

[augie47]

Where does 376.22(B) mention cross sectional area when applying derating factors?

It is my opinion that cross sectional area only applies to sizing. Why isn't cross sectional area mentioned in (B)?

In some jurisdictions in this area you get 30 period, then start derating.

When I worked in the field, I just started using big junction boxes.

Joe

I wished you had not asked that question :grin:
It totally messes up the way I was instructed. I have always applied the 30 ccc as "in any cross sectional area" but you are correct in that such wording is not included in (B).

 

[Rockyd]

Where does 376.22(B) mention cross sectional area when applying derating factors?

It is my opinion that cross sectional area only applies to sizing. Why isn't cross sectional area mentioned in (B)?

In some jurisdictions in this area you get 30 period, then start derating.

When I worked in the field, I just started using big junction boxes.

Joe

Good question! I looked over in 310.15(b)(2)(a) and 310.15(B)(4) (are listed as reference in 376.22(B) ) and believe this is where the derating comes from.

 

[Dave58er]

Where does 376.22(B) mention cross sectional area when applying derating factors?

It doesn't.

It is my opinion that cross sectional area only applies to sizing. Why isn't cross sectional area mentioned in (B)?

IMO this is an opinion, one that does have some merit, and could be debated. Personally I live on the other side of this fence.

In some jurisdictions in this area you get 30 period, then start derating.

Fine with me. I will work with just about any interpretation as long as I know before hand. The next time I see an inspector open a wireway around here it would be the first, so I really have no idea what the prevailing rule is.

When I worked in the field, I just started using big junction boxes.

I'm picturing 3+ panels on one wall. That's a big JB! :grin:

 

[Dave58er]

BTW, here is my education on the subject from some guys with a little more knowledge than me.

http://forums.mikeholt.com/showthread.php?t=79897

 

[augie47]

It does seem the is some controversy on the wording. For the benefit of the E/Cs I imagine I will still look at it as cross sectional area.

 

[RB1]

This code section made better sense when there were exceptions. There was really only one rule: 20% fill or 30 current carrying conductors in any cross section of the gutter. Exception No. 3 "lifted the 30 current carrying conductor" limitation where the derating factors were applied. When the exceptions were converted to positive code text, the 30 current carrying conductor "fill limit" was converted to a limit where derating factors are required to be applied. Based on the first sentence, these requirements are applied to "any given cross sectional area" of the gutter. The first sentence states where the requirement applies, the second sentence states when derating is required.

I vote we bring back exceptions.

 

[infinity]

If this 10X10 trough is feeding only three panels isn't there a good chance that 310.15(A)(2)Exception would make this the whole point of this thread moot?

 

[RB1]

Rob,

I don't think that 310.15(A)(2) can be applied in this case. There is a specific requirement to derate the conductors. Using your logic there would be no need to derate conductors in a raceway that is ten feet or less in length if the total circuit length was 100 feet or more.

 

[infinity]

Rob,

I don't think that 310.15(A)(2) can be applied in this case. There is a specific requirement to derate the conductors. Using your logic there would be no need to derate conductors in a raceway that is ten feet or less in length if the total circuit length was 100 feet or more.

According to 310.15(A)(2) that is true, no deration is required for your example.

 

[RB1]

Rob,

I disagree, but I don't think I can prove you wrong. If I were to attempt it, I would say that the specific requirement of 366.23 takes precedent over the general requirement of 310.15(A)(2). Otherwise, I can hardly think of an instance where there would be a limit on the number of current carrying conductors in an auxiliary gutter.

What if there are three calculated or tabulated ampacities?

 

[RB1]

Ampacities are tabulated in accordance with 310.16 and modified as required by 310.15(B)(1) through (B)(6). Ampacities that are claculated are in accordance with 310.15(C).

 

[infinity]

Rob,

I disagree, but I don't think I can prove you wrong. If I were to attempt it, I would say that the specific requirement of 366.23 takes precedent over the general requirement of 310.15(A)(2). Otherwise, I can hardly think of an instance where there would be a limit on the number of current carrying conductors in an auxiliary gutter.

What if there are three calculated or tabulated ampacities?

I agree with you in principle but I can't find the words in the book to prove it.
If you take a look at 334.80 you'll see that the NEC specifically mentions that the exception to 310.15(A)(2) does not apply. IMO such verbiage regarding wireways and Aux gutters would be required to prove your point.