60 deg C column in Table 310.15 (B)(16)

[NOV]

For 100A and below, the conductor must be sized using the 60 deg C column in Table 310.15 (B)(16). If I'm reading this right, a 110A circuit would require a smaller conductor than a 100A circuit. Is that correct?

 

[Little Bill]

For 100A and below, the conductor must be sized using the 60 deg C column in Table 310.15 (B)(16). If I'm reading this right, a 110A circuit would require a smaller conductor than a 100A circuit. Is that correct?

You have to use the 60 deg column for 100A and below only if the terminations are not marked. If marked for 75 deg, then you can use that column.
You have it backwards on the wire sizing, 110A would require a larger conductor than 100A.

Welcome to the Forum!

 

[bwat]

I think his point was that you have to use #1 to get 100A at 60deg, but if it’s 110A you can use 75deg and therefor #2.

(If completely unmarked)

 

[NOV]

Thank you Little Bill!

Assume the 100A load terminals are 60 deg C and the 110A load terminals are 75 deg C, is my first post correct regarding conductor size?

 

[NOV]

I think his point was that you have to use #1 to get 100A at 60deg, but if it’s 110A you can use 75deg and therefor #2.

(If completely unmarked)

Exactly my thought bwat!

 

[augie47]

I don't want to negate what Bill states but I do see your point.
In actual practice, termination normally allow 75° ratings in the 100 amp range so it is rarely an issue but
after looking at the Tables, I agree with you and bwat.. rare occurrence, but yes

 

[Little Bill]

I wasn't "reading between the lines" enough. so if you are jumping columns, you would end up with a smaller conductor for the same load.

 

[NOV]

OK thanks. I just wanted to be sure I was reading it correctly. It seemed strange but I guess as you said, a rare occurrence.

 

[infinity]

OK thanks. I just wanted to be sure I was reading it correctly. It seemed strange but I guess as you said, a rare occurrence.

Yes you're correct, but it's not really an apple to apples comparison when you have two different temperature ratings on the terminals.

 

[NOV]

I wasn't "reading between the lines" enough. so if you are jumping columns, you would end up with a smaller conductor for the same load.

Sorry for the confusion Little Bill

 

[kwired]

If you had an application where you could actually use 90C for terminations (rare but not impossible) you could have even smaller conductor.

When it comes to panels, breakers, safety switches, motor controllers, etc. pretty much everything made past 40 years or even longer has 75C termination ratings and you can pretty much assume 75C for new installations will be ok. Older equipment can get you into trouble if you don't pay attention to any markings.

you don't get into trouble with general use switches and receptacles because of overcurrent protection rules in 240.4(D) leaves you with same size conductor you would use @ 60C anyway

 

[NOV]

Thanks kwired. So since I'm working with new installations only, it's fairly safe to stick with the 75 Deg C column then?

 

[kwired]

Thanks kwired. So since I'm working with new installations only, it's fairly safe to stick with the 75 Deg C column then?

I'd say you are pretty safe doing that. Particularly for 60-600 amp equipment like I mentioned. Probably safe to 2000 amps but if you are only seldom using that equipment might be good idea to check each time.

Like I said for 30 amp and less the small conductor rule makes you use same size conductor you would at 60C anyway so doesn't have an impact there. Motors could get you into trouble under 30 amps, but motor rated switches are typically 75C AFAIK.

 

[Carultch]

Thank you Little Bill!

Assume the 100A load terminals are 60 deg C and the 110A load terminals are 75 deg C, is my first post correct regarding conductor size?

60C sizing tends to be more academic, rather than practical. One place where it is practical, is when you use cable instead of conduit, such as NM cable or certain installations of SE cable. Most equipment in practice is listed and labeled otherwise for 75C. When you are taking an exam, you can't assume this, and have to follow the defaults that the code spells out. There may be a specific problem that specifies otherwise, or a blanket statement applying to all problems in the exam, but if there isn't, you follow 110.14's defaults.

100A and less: 60C is the default termination rating. More often than not, your equipment is listed and marked otherwise for 75C, but you do have the burden of proof if you intend to take credit for this. There are cable wiring methods like NM cable, that may require you to override a 75C rating, and use the 60C termination rating anyway.

Over 100A, not inclusive of 100A: 75C is the default termination rating. In theory, equipment over 100A could exist that specifically requires you to use 60C sizing, but the manufacturer has the burden of proof to list this product accordingly. I've never seen such a product, and it seems like an unlikely listing that a manufacturer would pursue.