90 Degree Column

Status
Not open for further replies.
M

mkgrady

Senior Member
Location
Massachusetts
When can we actually use the 90 degree column for conductor ampacity? I consider it when derating for more than 3 ccc's and for temperature adjustments but I have never used it otherwise.

I am getting ready to change the service at a small church. Right now it has a very old 200 amp service and I'm looking to not only replace it but I'd like to get a little more out of it if possible. The feed from the meter socket is relatively new 3/0 copper use-2 and I plan to reuse it. When I look up the ampacity of that wire I find it rated 225 amps in the 90 degree column.

Does that say I can use a MCB panel with a 225 amp main breaker?
 
mkgrady said:
When can we actually use the 90 degree column for conductor ampacity? I consider it when derating for more than 3 ccc's and for temperature adjustments but I have never used it otherwise.

I am getting ready to change the service at a small church. Right now it has a very old 200 amp service and I'm looking to not only replace it but I'd like to get a little more out of it if possible. The feed from the meter socket is relatively new 3/0 copper use-2 and I plan to reuse it. When I look up the ampacity of that wire I find it rated 225 amps in the 90 degree column.

Does that say I can use a MCB panel with a 225 amp main breaker?
Click to expand...


Conductors with derate factors (of either kind) are sized for the 90C column, if the conductor is rated at 90C.

Conductors in the TERMINATIONS on manufactured equipment are (usually) sized for the 75C column. 100A and less are by default rated for 60C, but that is mostly an academic rule, as it is common for them to be listed and marked otherwise for 75C. Over 100A, and the default is 75C. You implement this without any derate factors.

It is rare that you get to use the 90C column for terminations.
 
With 75°C-rated terminations, your circuit ampacity is lesser of the 75°C column value (200A) or the 90°C derated value.
 
jumper said:
Here is a pretty good thread on the subject.

Iwire has a good sketch in post #5 showing where it is possible to use the 90 degree ampacity.

http://forums.mikeholt.com/showthread.php?t=114595
Click to expand...

That is a very helpful thread, especially Iwire's example. Thanks

In my case the meter socket probably doesn't have 90 degree terminals and I guess my MCB would not either, so I guess I am limited to a 200 amp service.
 
jumper said:
I would see this as a problem since all the other terminations in the enclosure are likely rated for only 75C.
Click to expand...

Correct. But the 90C temperature isn't going to propagate throughout the enclosure.
Or do the NEC/UL require that you assume it does, out of ignorance for a comprehensive thermal model?
 
mkgrady said:
When can we actually use the 90 degree column for conductor ampacity? I consider it when derating for more than 3 ccc's and for temperature adjustments but I have never used it otherwise.

I am getting ready to change the service at a small church. Right now it has a very old 200 amp service and I'm looking to not only replace it but I'd like to get a little more out of it if possible. The feed from the meter socket is relatively new 3/0 copper use-2 and I plan to reuse it. When I look up the ampacity of that wire I find it rated 225 amps in the 90 degree column.

Does that say I can use a MCB panel with a 225 amp main breaker?
Click to expand...
Assuming you have 90 degree rated conductors and 75 degree terminals, for terminals, derate the ampacity in the 75 degree column for continuous use if appropriate, and for insulation, derate the ampacity in the 90 degree column for conditions of use. The smaller of the two is the ampacity for those conductors.
 
ggunn said:
Assuming you have 90 degree rated conductors and 75 degree terminals, for terminals, derate the ampacity in the 75 degree column for continuous use if appropriate, and for insulation, derate the ampacity in the 90 degree column for conditions of use. The smaller of the two is the ampacity for those conductors.
Click to expand...

Can you give an example of this?
Considering 3 circuits each with thier own neutral and a EGC in a common raceway for example.

JAP>
 
ggunn said:
Assuming you have 90 degree rated conductors and 75 degree terminals, for terminals, derate the ampacity in the 75 degree column for continuous use if appropriate, and for insulation, derate the ampacity in the 90 degree column for conditions of use. The smaller of the two is the ampacity for those conductors.
Click to expand...
That's incorrect. The continuous load is factored 125% to make the connected load (noncontinuous plus continuous) proportionately less than the 75°C column value. The table value is the number to use for comparison.
 
jap said:
Can you give an example of this?
Considering 3 circuits each with thier own neutral and a EGC in a common raceway for example.

JAP>
Click to expand...

OK, say you have a circuit that needs 100A continuous to 75 degree terminals on a rooftop in direct sunlight less than 3-1/2 inches off the roof and with 4 conductors in conduit, and the ASHRAE 2% maximum ambient is 100 degrees F.

Place yourself on Table 310.15(B)(16) by multiplying 100A by 1.25 (it's continuous) to get 125A. Pick AWG#1, which is 130A in the 75 degree column.

For terminals, derate the 75 degree ampacity for #1 wire for continuous use by dividing 130A by 1.25 to get 104A.

For the wire, derate the 90 degree ampacity for #1 wire for conditions of use. The derate for CCC's is 0.8 (Table 310.15(B)(3)(a)) and the derate for temperature (100 degrees F + 60 degrees F rooftop adder = 160 degrees F from Tables 310.15(B)(3)(c) and 310.15(B)(2)(a)) is 0.5. The 90 degree ampacity for AWG#1 is 145A, so (145A)(0.8)(0.5) = 58A

Your ampacity is the smaller of the two numbers, which is 58A. 58A < 100A, so your wire isn't big enough. Pick a larger size and do the second calc again (you are going up in size so you know that the first calc will be good). Repeat until it is greater than or equal to 100A.
 
Carultch said:
Correct. But the 90C temperature isn't going to propagate throughout the enclosure.
Or do the NEC/UL require that you assume it does, out of ignorance for a comprehensive thermal model?
Click to expand...
Exactly.

There's a new "clause" likely to appear in the 2017 requirements for circuit ampacity determinations which makes it quite clear the higher-temp terminations cannot be in the same enclosure.
 
ggunn said:
OK, say you have a circuit that needs 100A continuous to 75 degree terminals on a rooftop in direct sunlight less than 3-1/2 inches off the roof and with 4 conductors in conduit, and the ASHRAE 2% maximum ambient is 100 degrees F.

Place yourself on Table 310.15(B)(16) by multiplying 100A by 1.25 (it's continuous) to get 125A. Pick AWG#1, which is 130A in the 75 degree column.

For terminals, derate the 75 degree ampacity for #1 wire for continuous use by dividing 130A by 1.25 to get 104A.
Click to expand...
This part is unnecessary. 130A is the circuit ampacity limitation for this size wire.

ggunn said:
For the wire, derate the 90 degree ampacity for #1 wire for conditions of use. The derate for CCC's is 0.8 (Table 310.15(B)(3)(a)) and the derate for temperature (100 degrees F + 60 degrees F rooftop adder = 160 degrees F from Tables 310.15(B)(3)(c) and 310.15(B)(2)(a)) is 0.5. The 90 degree ampacity for AWG#1 is 145A, so (145A)(0.8)(0.5) = 58A

Your ampacity is the smaller of the two numbers, which is 58A. 58A < 100A, so your wire isn't big enough. Pick a larger size and do the second calc again (you are going up in size so you know that the first calc will be good). Repeat until it is greater than or equal to 100A.
Click to expand...
You can determine the appropriate size faster this way: (100A)/(0.8)/(0.5) = 250A and find the 90° column ampacity which equals or smallest that exceeds this value... and that'd be 4/0 copper at 260A.

The downside using the faster method is that you then have to calculate the derated ampacity when the value is not equal (e.g. 250A calculated vs 260A from table).... so (260A)(0.8)(0.5) = 104A (the lesser value when compared with the 75°C column value of 4/0 copper).
 
ggunn said:
Your ampacity is the smaller of the two numbers, which is 58A. 58A < 100A, so your wire isn't big enough. Pick a larger size and do the second calc again (you are going up in size so you know that the first calc will be good). Repeat until it is greater than or equal to 100A.
Click to expand...

You'd have to repeat until it is greater than 110A so that the conductor would be properly protected per 240.4(B).

For less than 3.5" above the roof, the adder should be 40°, making the correction factor 0.71, not 0.5.
 
Last edited:
Smart $ said:
That's incorrect. The continuous load is factored 125% to make the connected load (noncontinuous plus continuous) proportionately less than the 75°C column value. The table value is the number to use for comparison.
Click to expand...
Yes, of course; I come from solar where everything is continuous. In the rest of the world only multiply the continuous part of the load by 1.25.
 
david luchini said:
You'd have to repeat until it is greater than 110A so that the conductor would be properly protected per 240.4(B).

For less than 3.5" above the roof, the adder should be 40°, making the correction factor 0.71, not 0.5.
Click to expand...
Yep. I read the wrong number off the chart, sorry. And yes, if the breaker is 125A, the conditions of use 90 degree ampacity must be >110A, not >100A.
 
Status
Not open for further replies.
Top