The 90 Degree column.

[Ace18]

I have a co-worker who uses the 90? column for sizing all his wire. He thinks just because he uses THHN and THHN is listed in the 90 degree column, that he is using the correct wire size. I told him he should be using the 60? column. Can someone tell me the NEC code that coveres this so I can have proof for the next time i have this argument with him?

 

[roger]

See all of 110.14(C).

Roger

 

[Dennis Alwon]

I have a co-worker who uses the 90? column for sizing all his wire. He thinks just because he uses THHN and THHN is listed in the 90 degree column, that he is using the correct wire size. I told him he should be using the 60? column. Can someone tell me the NEC code that coveres this so I can have proof for the next time i have this argument with him?

You would use the column that is your weakest link. If you are running pipe and using thhn you can use the 90 c for derating but your CB connections etc are generally rated 75C, or less if it is a very old panel. In this case 75C is the max. amp you could use. If you use NM cable then you are limited to 60C but again the 90C can be used for derating as long as the final ampacity is not more than 60C

 

[MNWildcat]

Also remember that per 110.14.C.1 the terminations have to be marked to be able to use higher ratings, not just listed.

 

[Ace18]

Thanks for the replies guys.

 

[Ace18]

Where can i read information about derating?

 

[Dennis Alwon]

Where can i read information about derating?

Art. 310.15(B)

 

[raider1]

Art. 310.15(B)

Section 310.15(B)(2) to be precise.

Chris

 

[Ace18]

Lets assume there is only 3 conductors in a conduit, so no derating is needed for this load. The equipment is marked with "Minumum circuit Amps" of 60A. Even though I may be using THHN wich is rated for 90 degrees, I should still use the 75 degree column to choose my wire size, correct?

Or should I use the 60 degree column since the equipment is under 100 amps?

Section 110-14(c) requires the circuit conductors to be sized according to the 60?C column of Table 310-16 for equipment rated 100 amperes and less, unless marked otherwise, and equipment rated over 100 amperes must be sized to the 75?C column of Table 310-16.

 

[Dennis Alwon]

Lets assume there is only 3 conductors in a conduit, so no derating is needed for this load. The equipment is marked with "Minumum circuit Amps" of 60A. Even though I may be using THHN wich is rated for 90 degrees, I should still use the 75 degree column to choose my wire size, correct?

Or should I use the 60 degree column since the equipment is under 100 amps?

If the termination you are tying to are listed for 75C, most are today, then you can use the 75C column for the ampacity.

 

[Ace18]

I think I'm getting the hang of this now. But I am still confused as to why the 90 degree column even exists if you have to always us the 75 degree column. Can someone show me an example workup in detail using the 90 degree column?

 

[Dennis Alwon]

I think I'm getting the hang of this now. But I am still confused as to why the 90 degree column even exists if you have to always us the 75 degree column. Can someone show me an example workup in detail using the 90 degree column?

Suppose you have 9 #12 THHN in a conduit. According to 310.16 we can use 30 amps. Now we must de-rate for 9 conductors at 70%. 70% of 30 is 21 amps so we can still use a 20 amp breaker. If we had to use 20 amps (60C) and de-rate then we would have 14 amps so we would have to go to a larger size wire

 

[Ace18]

Do your example from a different approach. lets say I'm trying to figure out what size wire I need to use for this load. Suppose I have 3 3ph compressors as my load. They each have a rated-load ampacity of 50A. And I will run all 9 wires through one conduit. 50x125%=62.5A

Thats where I get stuck.....

 

[Ace18]

Ok tell me if i got this right. I need a conductor rated for 62.5A. #4 THHN is rated 95A in the 90* column. 70% of 95A = 66.5A

Since my equipment is rated for 75* I look at #4 in the 75* column and it says 85A. 66.5 is less than 85, so #4 would be the correct conductor size.

Right?????

 

[raider1]

Ok tell me if i got this right. I need a conductor rated for 62.5A. #4 THHN is rated 95A in the 90* column. 70% of 95A = 66.5A

Since my equipment is rated for 75* I look at #4 in the 75* column and it says 85A. 66.5 is less than 85, so #4 would be the correct conductor size.

Right?????

I would say that you have it correct.

Chris

 

[Dennis Alwon]

Ok tell me if i got this right. I need a conductor rated for 62.5A. #4 THHN is rated 95A in the 90* column. 70% of 95A = 66.5A

Since my equipment is rated for 75* I look at #4 in the 75* column and it says 85A. 66.5 is less than 85, so #4 would be the correct conductor size.

Right?????

The only correction I will make is if these compressors are rated mca 50 amps. If that is the case then the 125% is already figured in and you do not need to do it again.

Here is another way to do it without guess what size conductor to use and then proving it.

If in this case 50 amps is the mca then we have 50 /.7 = 71 amps.
You would still need #4 at 75C according to Table 310.16

 

[Ace18]

Thanks for all the help fellas. I now have the knowledge to prove my co-worker wrong when he tries to size everything using the 90? column without any de-rating.:grin: