90C cable ampacity

[Immerkaer]

Hi

Am I right on this?

If current is continuous you will always have to multiply load current with 125% both for 75C and 90C.
So load of 100A have design current of 125A.

40C amb:
If you choose 75C cable:
125A/0,88 = 142A --> 1/0 AWG

If you choose 90C cable:
125A/0,91 = 138A --> 1 AWG. Check with 75C cable: 125A, no added correction = 1 AWG --> 1 AWG is OK as 90C cable

Thanks

 

[david luchini]

Hi

Am I right on this?

If current is continuous you will always have to multiply load current with 125% both for 75C and 90C.
So load of 100A have design current of 125A.

40C amb:
If you choose 75C cable:
125A/0,88 = 142A --> 1/0 AWG

If you choose 90C cable:
125A/0,91 = 138A --> 1 AWG. Check with 75C cable: 125A, no added correction = 1 AWG --> 1 AWG is OK as 90C cable

Thanks

The 125% applies before the application of any correction or adjustment factors.

For the application of the correction factor, the conductor need an ampacity of 100A for the load, but would need an ampacity greater than 110A to be protected by the next size up rule from 240.4.

The #1 awg would be OK for a 75deg conductor or a 90deg conductor.

 

[JoeStillman]

Be careful when relying on the 90°C ampacity. It is a rare OCPD indeed that has 90°C lugs. Check out 110.14(C).

The only situation where I've used the 90°C ampacity is before a more-than-three-conductors derating calculation.

 

[Immerkaer]

The 125% applies before the application of any correction or adjustment factors.

For the application of the correction factor, the conductor need an ampacity of 100A for the load, but would need an ampacity greater than 110A to be protected by the next size up rule from 240.4.

The #1 awg would be OK for a 75deg conductor or a 90deg conductor.

Okay, that is not how I understand the use of 125% rule. I am of the understanding that 100A continuous load will have a design current of 125A.
And that we do have to apply correction and adjustment factors to design current.

 

[Immerkaer]

Be careful when relying on the 90°C ampacity. It is a rare OCPD indeed that has 90°C lugs. Check out 110.14(C).

The only situation where I've used the 90°C ampacity is before a more-than-three-conductors derating calculation.

I am using the conductor with a 75degC CB. So I need to ensure that I choose the correct sized/smallest possible 90degC conductor.

 

[oldsparky52]

I am using the conductor with a 75degC CB. So I need to ensure that I choose the correct sized/smallest possible 90degC conductor.

If your final calculated load is 125A, then the conductor at 75C needs to be rated at 125 or higher. You can use the 90C rating for derating calculations, but whatever conductor you choose, the 75C rating needs to be 125A or higher.

 

[david luchini]

Okay, that is not how I understand the use of 125% rule. I am of the understanding that 100A continuous load will have a design current of 125A.
And that we do have to apply correction and adjustment factors to design current.

This is a common misconception. The 125% of continuous load is used to find the minimum conductor size and minimum OCPD size.

In your example, the 100A*1.25=125A means that the minimum conductor size will be #1 awg (130A allowable ampacity) and that the minimum circuit breaker size will be 125A.

When applying the correction factor, the conductor only needs to have an ampacity sufficient for the load to be served, which is 100A. But as I noted, the conductor must also be protected at its ampacity, or the next size up as applicable.

So in this example:
#1 awg, 75 deg, corrected for 40degC ambient: 130A*0.88= 114A. 114A is larger than the load to be served (100A) and is properly protected by a 125A c/b (next size up) so #1 awg, 75 deg is acceptable.

#1 awg, 90 deg, corrected for 40degC ambient: 145A*0.91= 132A. 132A is larger than the load to be served (100A) and is properly protected by a 125 c/b so #1 awg, 90 deg is acceptable.

 

[Immerkaer]

This is a common misconception. The 125% of continuous load is used to find the minimum conductor size and minimum OCPD size.

In your example, the 100A*1.25=125A means that the minimum conductor size will be #1 awg (130A allowable ampacity) and that the minimum circuit breaker size will be 125A.

When applying the correction factor, the conductor only needs to have an ampacity sufficient for the load to be served, which is 100A. But as I noted, the conductor must also be protected at its ampacity, or the next size up as applicable.

So in this example:
#1 awg, 75 deg, corrected for 40degC ambient: 130A*0.88= 114A. 114A is larger than the load to be served (100A) and is properly protected by a 125A c/b (next size up) so #1 awg, 75 deg is acceptable.

#1 awg, 90 deg, corrected for 40degC ambient: 145A*0.91= 132A. 132A is larger than the load to be served (100A) and is properly protected by a 125 c/b so #1 awg, 90 deg is acceptable.

Thanks for clearing this out for me.
I this case I find that 90deg cable can be even smaller. Correct?

So for 75deg cable:
125% rule: 100A x 1,25 = 125A à 1 AWG
40deg amb: 100A/0,88 = 114A à 2 AWG
Choose larger one = 1 AWG

For 90deg cable:
125% rule:100A x 1,25 = 125A à 2 AWG
40deg amb: 100A/0,91 = 110A à 3 AWG
Choose larger one = 2 AWG
To comply with 75deg CB:
Check with 75deg cable à 2 AWG = 115A. <- No correction here?!

So 90deg 2 AWG cable is adequate

Think that I am getting to the point now.

 

[david luchini]

Thanks for clearing this out for me.
I this case I find that 90deg cable can be even smaller. Correct?

So for 75deg cable:
125% rule: 100A x 1,25 = 125A à 1 AWG

So 90deg 2 AWG cable is adequate

Think that I am getting to the point now.

The #1 awg is the minimum conductor size, so a # 1 awg 90 deg conductor would be required.

 

[oldsparky52]

Be careful when relying on the 90°C ampacity. It is a rare OCPD indeed that has 90°C lugs. Check out 110.14(C).

The only situation where I've used the 90°C ampacity is before a more-than-three-conductors derating calculation.

I am using the conductor with a 75degC CB. So I need to ensure that I choose the correct sized/smallest possible 90degC conductor.

Under the 75C column of Table 310.15(B)(16), the smallest conductor that has the 125-amp rating you are looking for is a #1 wire. A 90C rated conductor can help you with ambient temperature corrections, conduit fill corrections, etc. but the bottom line is you may not use a conductor smaller than #1 becausee of 110.14(C). Did you read that section carefully?

 

[Immerkaer]

Under the 75C column of Table 310.15(B)(16), the smallest conductor that has the 125-amp rating you are looking for is a #1 wire. A 90C rated conductor can help you with ambient temperature corrections, conduit fill corrections, etc. but the bottom line is you may not use a conductor smaller than #1 becausee of 110.14(C). Did you read that section carefully?

So , there is really no advantage of using 90C conductors when they are terminated at 75C terminals. Unless we have parallel adjustment and temperature correction applied?
For single conductors at max 40degC ambient temp is will always be 125% rating that are the higher one, giving the largest conductor.

 

[oldsparky52]

So , there is really no advantage of using 90C conductors when they are terminated at 75C terminals. Unless we have parallel adjustment and temperature correction applied?

Pretty much yes. I ran into this wiring docks with "W" cable. W cable has a much larger temperature rating, even at 75C than standard building wiring. But because of 110.14 we could not use the smaller conductors w/out some fancy termination schemes that made it more of a PITA.

For single conductors at max 40degC ambient temp is will always be 125% rating that are the higher one, giving the largest conductor.

I don't think I understand what you are saying.

 

[oldsparky52]

Pretty much yes. I ran into this wiring docks with "W" cable. W cable has a much larger temperature rating, even at 75C than standard building wiring. But because of 110.14 we could not use the smaller conductors w/out some fancy termination schemes that made it more of a PITA. I don't think I understand what you are saying.

I meant to write, W cable has a much larger ampacity rating

 

[Immerkaer]

For single conductors at max 40degC ambient temp is will always be 125% rating that are the higher one, giving the largest conductor.


I don't think I understand what you are saying.

Just stating that it will always be 125% rule that gives the largest conductor, unless many parallel conductors are installed

 

[david luchini]

Just stating that it will always be 125% rule that gives the largest conductor, unless many parallel conductors are installed

For your specific example, yes. But not in all cases.

And don't forget, the 125% rule only applies to continuous loads.