Per NEC 376.22 I can have up to 30 current carrying condutors in a wireway with no derating of the conductors but when I add the 31st conductor I must derate per 310.15(B)(2)(a) which requires derating all conductors contained in the wireweay to 40% of conductor ampacity. Am I reading this right? It doesn't seem to make a lot of sense to me.
wireway derating
- Thread starter bsh
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Pierre C Belarge
Senior Member
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- Westchester County, New York
Does this mean that the ampacity adjustments of 310.15(B)(2)(a) are to be used regardless of how long the wireway may be as long as there are more than 30 current carrying conductors (CCC)?
Picture a wireway about 4 feet long. There are a number of raceways entering the top of the wireway. Below the wireway are 2 panels.
The wireway contains a total of 35 CCC. The conductors do not travel the wireway horizontically, just vertically. So, at any one cross sectional area there is no more than 20% wireway fill.
The wireway has more than 30 CCCs. Do I need to perform ampacity adjustment?
Picture a wireway about 4 feet long. There are a number of raceways entering the top of the wireway. Below the wireway are 2 panels.
The wireway contains a total of 35 CCC. The conductors do not travel the wireway horizontically, just vertically. So, at any one cross sectional area there is no more than 20% wireway fill.
The wireway has more than 30 CCCs. Do I need to perform ampacity adjustment?
chris kennedy
Senior Member
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- Miami Fla.
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- 60 yr old tool twisting electrician
Pierre C Belarge said:
I see no exceptions to the second sentence of 376.22. Exceed 30 and you derate.
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- New Jersey
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- Journeyman Electrician
chris kennedy said:
I agree, although I hate this rule.
e57
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- San Francisco, CA
I disagree, and thankfully many of the Inspectors on my jobs do as well. And a fairly common practice in my area.
The way I read it is 'cross-sectional area' as the second sentence is tied to the first on fill. The code name is "Number of Conductors", the first sentence describes it as a percentage of fill in cross-sectional, and the second sentence what point de-rating applies to that cross-sectional area IMO. The code also recognizes the use of wireways as a pull or junction box in 376.23(B) & 376.56, and it would be in the way Pierre C Belarge describes it. Used in this way, it is no different than a rectangular shaped j-box when not actually used as a raceway.
I would like to know if someone has seen an interpretation of this?
The '05 also allows terminal blocks as well as spices for a total fill in that section not to exceed 75%....
The way I read it is 'cross-sectional area' as the second sentence is tied to the first on fill. The code name is "Number of Conductors", the first sentence describes it as a percentage of fill in cross-sectional, and the second sentence what point de-rating applies to that cross-sectional area IMO. The code also recognizes the use of wireways as a pull or junction box in 376.23(B) & 376.56, and it would be in the way Pierre C Belarge describes it. Used in this way, it is no different than a rectangular shaped j-box when not actually used as a raceway.
I would like to know if someone has seen an interpretation of this?
The '05 also allows terminal blocks as well as spices for a total fill in that section not to exceed 75%....
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In re-reading the following, I have changed my mind about no derating required for lengths less than 24".
(2) Adjustment Factors
(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
In other words, if you had a wireway with 31 CCC in the cross section for 1" derating is required. Seems like a big jump from 0% to 40% derating going from 30 to 31 CCC.
(2) Adjustment Factors
(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
In other words, if you had a wireway with 31 CCC in the cross section for 1" derating is required. Seems like a big jump from 0% to 40% derating going from 30 to 31 CCC.
e57
Senior Member
- Location
- San Francisco, CA
spsnyder said:
If the wire way were used as a "raceway" then yes. Gutters, through's AKA J-boxes are steel boxes, and so is a section of 'wire way' when not used as a raceway.
Pierre C Belarge describes what sounds like a 6X6"X4' J-box. As many also enjoy dual listing as such.
- Location
- New Jersey
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- Journeyman Electrician
e57 said:
But how do you determine if the trough is used as a JB, a wireway, or a gutter? Is this defined in the NEC?
e57. I see what you mean. I have changed my mind again back to my original thinking.. In the definitions of all the various eq. like RMC, EMT, etc. it explicitly states "raceway". Not so for Wireway.
376.2 Definition
Metal Wireways. Sheet metal troughs with hinged or removable covers for housing and protecting electric wires and cable and in which conductors are laid in place after the wireway has been installed as a complete system.
So the wireway has to have 31 conductors in a cross section for more than 24" before derating. I suppose you could have 31 conductors for 18" then 30 CCC for 12" then 31 CCC for 18" and so on and so forth and not have to derate.
. In the definitions of all the various eq. like RMC, EMT, etc. it explicitly states "raceway". Not so for Wireway.376.2 Definition
Metal Wireways. Sheet metal troughs with hinged or removable covers for housing and protecting electric wires and cable and in which conductors are laid in place after the wireway has been installed as a complete system.
So the wireway has to have 31 conductors in a cross section for more than 24" before derating. I suppose you could have 31 conductors for 18" then 30 CCC for 12" then 31 CCC for 18" and so on and so forth and not have to derate.
I don't believe so.spsnyder said:
Note that "..., or where single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways, ..." in 310.15(B)(2)(a) is separated by commas as a complete clause. Wireways, by definition, are raceways (ref: 100-I)... so the clause quoted above has no effect on derating in wireways. The scenario you have depicted would require derating on all conductors passing through the wireway section having 31 or more ccc's.
As to whether a dual listed wireway/junction box can forego the derating requirement, I have to ask: when is a trough only a junction box and not a wireway?
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