Article 376.22 (B) - Adjustment Factors (Metal Wireways)

[tnt8197]

Article 376.22 (B) Adjustment Factors states "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."

Table 310.15(B)(2)(a) - Adjustment factors, has the number of current-carrying conductors in a list and the percent of values in Tables 310.16 through 310.19 as adjusted for Ambient Temperature if Necessary in an adjacent list.

The question I have, is the NEC requiring that the derating of the 31 - 40 current-carrying conductors, referred to in 376.22 (B), be derrated at 40%?

If I read the NEC correctly its stating that the derration of a #8 THHN Copper Conductor goes from 55 Amps (90 Deg) when it falls within the 30 current carrying conductors provisions of 376.22 (B) & once the 31st wire is added, its ampacity drops to 22 Amps (90 Deg)?

I am looking for some clarification on the intent of the NEC for the derration of the 31st conductor as referred to in 376.22 (B)?

 

[infinity]

You can up to 30 CCC's at any cross section without derating. Go over 30 CCC's and you need to derate according to 310.15(B)(2)(a) as you've noted. What's the application?

 

[tnt8197]

Wireways in an interstitial space, feeding a multitude of areas in a hospital. Basically the major branch runs throughout the hospital.

Do you believe my assumptions are accurate?

 

[LarryFine]

Do you believe my assumptions are accurate?

Basically, but remember that you have loads to serve. You have to determine what size of wire you need after derating due to CCC count.

In other words, you don't reduce the OCP to keep a given conductor size, you enlarge the conductor to maintain that circuit's required ampacity.

Added: If you need to use #8 to keep a circuit's ampacity at 20a, then you have to calculate the conduit size to house that size and quantity of wires.

 

[benaround]

tnt8197,

Remember that it is 30 CCC in a cross sectional area, not just 30 CCC's. If the circuits

were piped, for example, into the bottom of the WW and straight out the top , then you

can put as many pipes as you want and you won't have more than the fill of the most full

pipe in a 'cross sectional area'. I hope you can follow that !!

 

[tkb]

tnt8197,

Remember that it is 30 CCC in a cross sectional area, not just 30 CCC's. If the circuits

were piped, for example, into the bottom of the WW and straight out the top , then you

can put as many pipes as you want and you won't have more than the fill of the most full

pipe in a 'cross sectional area'. I hope you can follow that !!

What about the cross section of side to side?