Amount of conductors in a wireway...

[Kells81]

Hello,

I am new here and have a question that seems to be an odd one.

Cannot go into complete specifics but will offer up as much info as possible.

In an office building type setting using a wireway how many Conductors total can be put into a wireway before deration begins? I want to say 30 plus grounds (15 line, 15 neutral, then 15 grounds for 45) . Each circuit includes its own neutral and ground. Would this assumption be correct or am I way off? I know you have 20% fill to deal with but that's not the topic in hand. Reading the below text has me leaning more towards 30 line, 30 neutral, 30 ground.


This is pulled from the NEC 2008 (only digital copy I currently have)

This is from 376.22
(B)Adjustment Factors. The adjustment factors in
310.15(B)(2)(a)shall be applied only where the number of
current-carryingconductors, including neutral conductors
classifiedas current-carrying under the provisions of
310.15(B)(4),exceeds 30. Conductors for signaling circuits
orcontroller conductors between a motor and its starter and
used onlyfor starting duty shall not be considered as
current-carryingconductors.

This is from 310.15(B)(4)
(a) Aneutral conductor that carries only the unbalanced
current fromother conductors of the same circuit
shall not berequired to be counted when applying the provisions
of310.15(B)(2)(a).



Table 310.15(B)(2)(a) Adjustment Factors for More Than
Three Current-Carrying Conductors in a Raceway or Cable
Number of
Current-Carrying
Conductors
Percent of Values in Tables
310.16 through 310.19 as
Adjusted for Ambient
Temperature if Necessary
4?6 80
7?9 70
10?20 50
21?30 45
31?40 40
41 and above 35

 

[augie47]

Derating actually begins at your 4th current carrying conductor as you chart shows (4-6 =80%).
In your situation that would mean you would technically derate on your second circuit (2 phases +
2 neutrals = 4).
In practice, assuming an install with #12 wire, by using THHN conductors at their 90? rating (for derating purposes), the situation would become significant when you added your 5th circuit. At that time, using individual neutrals per circuit, you would have 10 current-carrying conductors with a derating factor of 50%. Your #12 THHN amapcity would the be 15 amps and you could not use a 20 amp breaker.

 

[Kells81]

Derating actually begins at your 4th current carrying conductor as you chart shows (4-6 =80%).
In your situation that would mean you would technically derate on your second circuit (2 phases +
2 neutrals = 4).
In practice, assuming an install with #12 wire, by using THHN conductors at their 90? rating (for derating purposes), the situation would become significant when you added your 5th circuit. At that time, using individual neutrals per circuit, you would have 10 current-carrying conductors with a derating factor of 50%. Your #12 THHN amapcity would the be 15 amps and you could not use a 20 amp breaker.

Wouldn't this completely defeat the purpose of using a wireway? 376.22(B) states that the adjustment factors (310.15(B)(4) shall only be applied when you exceed 30 CCC's.

 

[Dennis Alwon]

Wouldn't this completely defeat the purpose of using a wireway? 376.22(B) states that the adjustment factors (310.15(B)(4) shall only be applied when you exceed 30 CCC's.

You are correct. I believe Gus was thinking about conduit not a wireway

 

[augie47]

Yep. Ole man failed to read the words his eyes were viewing.
Sorry
Thanks Dennis.

 

[infinity]

Wouldn't this completely defeat the purpose of using a wireway? 376.22(B) states that the adjustment factors (310.15(B)(4) shall only be applied when you exceed 30 CCC's.

Yes you're permited to have up to 30 CCC's before you must begin derating.

 

[texie]

Another thing to consider that sometimes works to your advantage is that the 30 CCC's only count at a given cross section. You could have 30 in and out at one end and 30 in and out at the other without derating.

 

[Joe Villani]

Another thing to consider that sometimes works to your advantage is that the 30 CCC's only count at a given cross section. You could have 30 in and out at one end and 30 in and out at the other without derating.

Where does the code state this?

Thanks,


Joe

 

[texie]

Where does the code state this?

Thanks,


Joe

Well, I cop to this not being real clear as is the case for fill. It seems to me that common sense says that derating should not apply if the conductors are not running parallel with each other. In all my years, I've never seen it apply simply to all the conductors if they don't run together by any AHJ. The NEC Handbook is silent on this. Be interesting to see what others have to say, especially the inspector guys.

 

[infinity]

The section uses the word "where", admittedly it's not explicit.

376.22 Number of Conductors and Ampacity. The num-
ber of conductors and their ampacity shall comply with
376.22(A) and (B).
(A) Cross-Sectional Areas of Wireway. The sum of the
cross-sectional areas of all contained conductors at any cross
section of a wireway shall not exceed 20 percent of the inte-
rior cross-sectional area of the wireway.
(B) Adjustment Factors. The adjustment factors in
310.15(B)(3)(a) shall be applied only where the number
of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of
310.15(B)(5), exceeds 30. Conductors for signaling circuits
or controller conductors between a motor and its starter and
used only for starting duty shall not be considered as
current-carrying conductors.

 

[Kells81]

Yes you're permited to have up to 30 CCC's before you must begin derating.

Do you count the neutral in this to get your 30 CCC's? My thinking is 15 Line, 15 Neutral to get the 30. In 310.15(B)(4) it kind of goes against this thought as each circuit in that transition from a conduit to the wireway has 1-line, 1-neutral, 1-ground. I know the neutral is always current carrying but it would appear even so that in this situation you do not count it as a CCC.

BTW wireway is 30' long so were not dealing with a small run of it. Not worried about the 20% fill aspect of it just the deration issue.

 

[Kells81]

The section uses the word "where", admittedly it's not explicit.

Which code cycle is this from? The numbers do not seem to be lining up. I don't have my 2011 here in Paki.

from the 2008 which is different from the one you have listed.

376.22 Number of Conductors and Ampacity.
The number
of conductors and their ampacity shall comply with
376.22(A) and (B).
(A) Cross-Sectional Areas of Wireway.
The sum of the
cross-sectional areas of all contained conductors at any
cross section of a wireway shall not exceed 20 percent of
the interior cross-sectional area of the wireway.
(B) Adjustment Factors.
The adjustment factors in
310.15(B)(2)(a) shall be applied only where the number of
current-carrying conductors, including neutral conductors
classified as current-carrying under the provisions of
310.15(B)(4), exceeds 30. Conductors for signaling circuits
or controller conductors between a motor and its starter and
used only for starting duty shall not be considered as
current-carrying conductors.

 

[Kells81]

From the 2008 book. It does not have a 310.15(B)(3)(a) It has a (B)(4)(a) I am thinking there is a crossing of code cycles.
376.22 Number of Conductors and Ampacity.
The number
of conductors and their ampacity shall comply with
376.22(A) and (B).
(A) Cross-Sectional Areas of Wireway.
The sum of the
cross-sectional areas of all contained conductors at any
cross section of a wireway shall not exceed 20 percent of
the interior cross-sectional area of the wireway.
(B) Adjustment Factors.
The adjustment factors in
310.15(B)(2)(a) shall be applied only where the number of
current-carrying conductors, including neutral conductors
classified as current-carrying under the provisions of
310.15(B)(4), exceeds 30. Conductors for signaling circuits
or controller conductors between a motor and its starter and
used only for starting duty shall not be considered as
current-carrying conductors.


From your posting

376.22 Number of Conductors and Ampacity. The num-
ber of conductors and their ampacity shall comply with
376.22(A) and (B).
(A) Cross-Sectional Areas of Wireway. The sum of the
cross-sectional areas of all contained conductors at any cross
section of a wireway shall not exceed 20 percent of the inte-
rior cross-sectional area of the wireway.
(B) Adjustment Factors. The adjustment factors in
310.15(B)(3)(a) shall be applied only where the number
of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of
310.15(B)(5), exceeds 30. Conductors for signaling circuits
or controller conductors between a motor and its starter and
used only for starting duty shall not be considered as
current-carrying conductors.

 

[texie]

Do you count the neutral in this to get your 30 CCC's? My thinking is 15 Line, 15 Neutral to get the 30. In 310.15(B)(4) it kind of goes against this thought as each circuit in that transition from a conduit to the wireway has 1-line, 1-neutral, 1-ground. I know the neutral is always current carrying but it would appear even so that in this situation you do not count it as a CCC.

BTW wireway is 30' long so were not dealing with a small run of it. Not worried about the 20% fill aspect of it just the deration issue.

The determination of how many CCC you have is separate and apart from Art 376. But by your description you have 30 CCC. If they run from end to end, then derating would not apply. If you exceed 30 CCC then derating would apply.

 

[azhapp]

2011 Code.....30 Conductors at any cross-section

2011 Code.....30 Conductors at any cross-section

Using 2011 NEC 1.When does 30 conductors apply? "at any cross-section" NEC Article 376.22
?(A) Cross-Sectional Areas of Wireway. The sum of the cross-sectional areas of all contained conductors at any cross section of a wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway. 2. Neutrals: The circuit descriptions have not been supplied. If the circuit neutrals are current-carrying conductors such as with non-linear loads (C-E, computers, lighting), then the neutrals COUNT as currenty-carrying conductors. If it's simply 120v linear loads, then they don't. Take a look at 310.15(B)(5) to qualify the branch circuit neutrals.

---

On another note, I'm trying to figure out why the 2011 Handbook example for 376.22 didn't run the calculations FOR derating. And why the example Exhibit 310.3 has STEP 3....determining the size of the overcurrent protection device after figuring out the size of a DErated cable. If the original load was 16A, it's not going to get less with a DErated cable. Did I miss something there? And, why do the following numbers not continuously grow in Chapter 9 Table 1: 1 conductor 53%, 2 conductors 31%, 3 conductors 40%. Why isn't the number for 2 conductors between 40% and 53%? Does it have to do with heat, figuring each is a current-carrying conductor?

 

[GoldDigger]

Where does the code state this?

Thanks,


Joe

Although it is not explicitly applied to the derating language, the 20% fill provision does mention looking at each cross section separately.

(A) Cross-Sectional Areas of Wireway. The sum of the
cross-sectional areas of all contained conductors at any cross
section of a wireway shall not exceed 20 percent of the inte-
rior cross-sectional area of the wireway.

From that, the generally accepted understanding is that the number of conductors for derating purposes is counted in the same way. It is not an issue for raceways like conduits where what goes in one end must come out the other. But for things like boxes and gutters, it does make a difference.

 

[steve66]

And, why do the following numbers not continuously grow in Chapter 9 Table 1: 1 conductor 53%, 2 conductors 31%, 3 conductors 40%. Why isn't the number for 2 conductors between 40% and 53%? Does it have to do with heat, figuring each is a current-carrying conductor?

That's the conduit fill, and it has nothing to do wiht heating or derating. It's just a basic limit to keep the conductors from being damaged by being pulled into a conduit that is too small.

1 conductor at 53 percent fills up most of the conduit, leaving a nice even space around the conductor (if you picture the conductor in the middle of the conduit. If you picture it laying on the bottom, its not even all the way around, but 1 conductor still fits in the conduit best.

2 conductors - picture them side by side. There is lots of wasted space above and below the 2 conductors, so you have the lowest fill.

3 conductors in a triangle pattern- more efficient again, with the same amount amount of wasted space on either side. But there is a little more wasted space between the 3 conductors than with a single conductor. So the fill is somewhere between that for 1 or 2 conductors.

 

[azhapp]

That's the conduit fill, and it has nothing to do wiht heating or derating. It's just a basic limit to keep the conductors from being damaged by being pulled into a conduit that is too small..

I get the purpose and use, but......

1 conductor at 53 percent fills up most of the conduit, leaving a nice even space around the conductor (if you picture the conductor in the middle of the conduit. If you picture it laying on the bottom, its not even all the way around, but 1 conductor still fits in the conduit best...

Yes, and I'm ok with this one.

2 conductors - picture them side by side. There is lots of wasted space above and below the 2 conductors, so you have the lowest fill...

I follow your theory, but here's where I'm not ok....if there are 2 conductors, they're both current-carrying....whereas with 3 conductors, it could one OR 2 current-carrying conductors, given "most" branch circuits.

3 conductors in a triangle pattern- more efficient again, with the same amount amount of wasted space on either side. But there is a little more wasted space between the 3 conductors than with a single conductor. So the fill is somewhere between that for 1 or 2 conductors.

I get why this could easily be less than 1 conductor, but the 2-conductor one still just doesn't sit right with me.

 

[GoldDigger]

3 conductors in a triangle pattern- more efficient again, with the same amount amount of wasted space on either side. But there is a little more wasted space between the 3 conductors than with a single conductor. So the fill is somewhere between that for 1 or 2 conductors.

Also, although the NEC tables do not reflect it, when pulling three wires into a conduit under some fill and diameter combinations, you can get a wedge condition going around corners that raises the pulling force well beyond that for two conductors.
Pulling force calculators will generally alert you if you are in this situation.

 

[texie]

Another thing to consider that sometimes works to your advantage is that the 30 CCC's only count at a given cross section. You could have 30 in and out at one end and 30 in and out at the other without derating.

I think some may have felt that my opinion that the 30 CCC derating only counts at any cross section is not correct. I have since learned that for the 2014 NEC this language has been changed to make it clear that this is the case, the same as the 20% fill rule is based on any cross section.
The folks I spoke to from the code panel on this said this was always the intent and it just got muddied up over the years.