Determine minimum conductor size

[ekbrunn]

When determining the MINIMUM size of a feeder conductor for a specific OCPD do you -- 1. look in Table 310.16 and then apply the derating required by 310.15 (B)(2)(a) or do you -- 2. apply the derating factors to the conductor and then call this your MINIMUM sized conductor. For example, I have a 225 amp feeder that has 5 current carrying conductors (3 ungrounded and two grounded, provided for nonlinear load harmonics). For scenario #1, if I consider the 75?C column #4/0 THWN conductor as my minimum size, then apply 310.15(B)(2)(a) my NEW size is a #300 kcmil. On the other hand, for scenario #2, if I know I have 5 current-carrying conductors and multiply my 225 amp rated OCPD by 125% (the inverse of the required 80% derating), I still get a #300 kcmil conductor, BUT now I consider this my MINIMUM sized required ungrounded conductor. I will not need to increase the size of my equipment grounding conductor to compy with 250.122(B) using the second scenario! The problem really has to do with sizing the EGC to comply with 250.122(B)

 

[charlie b]

Doesn?t matter. 250.122(B) is not a consideration in either case. What you are doing, using either of your methods is determining the size of conductor needed for your application. The conductor must have sufficient ampacity for the amount of calculated load current. But the definition of ?ampacity? includes the concept of ?conditions of use.? In your case, the ?conditions of use? include having 5 current-carrying conductors in a raceway. That changes the ampacity of the conductors. As a result, the smallest size conductor with sufficient ampacity is the 300 MCM. Conclusion, your ungrounded conductors are not being ?increased in size,? in the context of 250.122(B). Rather, they are being properly ?sized.?

 

[bob]

I have a 225 amp feeder that has 5 current carrying conductors (3 ungrounded and two grounded, provided for nonlinear load harmonics). For scenario #1, if I consider the 75?C column #4/0 THWN conductor as my minimum size, then apply 310.15(B)(2)(a) my NEW size is a #300 kcmil.)

When using table 310.15 you may use the 90C col when adjusting the ampacity requirements. Required ampacity = 225/0.80 = 281.25. In the 90C
col 250 kcm = 290 x.80 = 232 amps which is adequate for you requirements.

On the other hand, for scenario #2, if I know I have 5 current-carrying conductors and multiply my 225 amp rated OCPD by 125% (the inverse of the required 80% derating), I still get a #300 kcmil conductor, BUT now I consider this my MINIMUM sized required ungrounded conductor. I will not need to increase the size of my equipment grounding conductor to comply with 250.122(B) using the second scenario! The problem really has to do with sizing the EGC to comply with 250.122(B)

I don't think 250.122B applies in either case. Generally you will use 250.122B
when you increase conductor size due to voltage drop. You are increasing
conductor size because the listed ampacity in table 310.16 is not correct for your conditions.