Another 250.122(B) Thread

[Flapjack]

When determining the minimum size that has sufficient ampacity for the intended installation, can I choose to derate only the 75C column even though the multi-conductor cable that will be specified is rated 90C?

A 60 HP, 460 V motor requires a cable that has a minimum ampacity of 96.25 A, which results in a 3 AWG. The EGC size would be 6 AWG per 250.122(D)(1). The client wants a particular type of tray cable and 3 AWG is not available. The ambient temperature leads to a 0.87 correction factor. Using the 90C column for derating results in the 3 AWG cable being sufficient. 250.122(B) would then cause the 2 AWG that I will specify to have a 4 AWG EGC.

However, if I derate only using the 75C column, then the 2 AWG would be the minimum size that has sufficient ampacity and 250.122(B) would not apply. Thus, no custom cable.

 

[acrwc10]

Your not really de-rating from the 75 degree column you are just upsizing the cable to an available size. If #3 isn't avalible #2 is the correct choice.

 

[iwire]

Your not really de-rating from the 75 degree column

I agree

you are just upsizing the cable to an available size. If #3 isn't avalible #2 is the correct choice.

That means the EGC must be upsized as well.

 

[luckylerado]

The ambient temperature leads to a 0.87 correction factor.

Are you sure that the ambient temp is not 114° instead of 113°?? It is still early but it seems like that solves your problem without getting into the weeds of 250.122(B).

Would that be considered half assed do you think?? I wouldn't think so.

 

[Flapjack]

Your not really de-rating from the 75 degree column you are just upsizing the cable to an available size. If #3 isn't avalible #2 is the correct choice.

I know what you are saying, but if the cable was only rated 75C then I wouldn't be upsizing the cable to an available size. I would be required to use a 2 AWG due to ambient temperature.

Because the cable is rated 90C it now would be considered just upsizing to an available size. So it sounds like you are saying if the cable is 90C, any derating that may be applicable must be applied to the 90C column?

 

[Flapjack]

Are you sure that the ambient temp is not 114° instead of 113°?? It is still early but it seems like that solves your problem without getting into the weeds of 250.122(B).

Would that be considered half assed do you think?? I wouldn't think so.

Haha. Unfortunately, it is more around 110 max. If the ambient was 114 then all of the other cables may need to be increased in size as well.

 

[Carultch]

When determining the minimum size that has sufficient ampacity for the intended installation, can I choose to derate only the 75C column even though the multi-conductor cable that will be specified is rated 90C?

A 60 HP, 460 V motor requires a cable that has a minimum ampacity of 96.25 A, which results in a 3 AWG. The EGC size would be 6 AWG per 250.122(D)(1). The client wants a particular type of tray cable and 3 AWG is not available. The ambient temperature leads to a 0.87 correction factor. Using the 90C column for derating results in the 3 AWG cable being sufficient. 250.122(B) would then cause the 2 AWG that I will specify to have a 4 AWG EGC.

However, if I derate only using the 75C column, then the 2 AWG would be the minimum size that has sufficient ampacity and 250.122(B) would not apply. Thus, no custom cable.

The underlying question here, is how are we required to calculate "the minimum size that has sufficient ampacity for the intended installation", when determining the need to upsize ground wires?

Remember that the intent of this rule is to keep the EGC's resistance within reasonable limits, when length is significant. So it is primarily voltage drop that should be the driving factor to determine the need for upsizing the wire and its corresponding EGC proportionally by the KCMIL ratio.

However, we do end up in situations where we need to use a larger wire for other reasons than voltage drop, when length is not significant. Availability of wire, termination capacity, using leftover wire from a previous job. And by the word of the code, you'd need to proportionally upsize the EGC, even though there is no physical reason you should have to.

Sort of a "loophole" I can see in this example, is that 110.14(C) specifies that your terminations are by default rated for 60C, even though they are probably listed and marked otherwise for 75C. So do you need to take advantage of the 75C termination rating, regarding your starting point for EGC sizing? IMO, you shouldn't need to. The #2 wire with a #8 ground would be sufficient if terminations were rated for 60C. The #2 wire still is *A* minimum size that has sufficient ampacity for the intended installation, if you were to ignore the termination markings for 75C. 110.14(C) says you CAN, but it doesn't say you have to, take advantage of higher temperature ratings for terminations and conductors.