750 kcmil Rating

[tkb]

What would the ampere rating be for 750kcmil THHN/THWN-2 at 35°c ambient?

Would you use the 90°c rating of 535amps (TABLE 310.15(B)(16)) multiplied by 0.96 (TABLE 310.15(B)(2)(a)) 90° column.

Using this method I come up with an ampacity of 513.6 amps.
Then using the 75°c column I can use the ampacity of 475 amps for the terminations.

Is this correct?

 

[infinity]

If your numbers are correct (don't have the book handy) then it looks good. You would use the lower of the two numbers for your final ampacity which in this case is 475 amps.

 

[Carultch]

What would the ampere rating be for 750kcmil THHN/THWN-2 at 35°c ambient?

Would you use the 90°c rating of 535amps (TABLE 310.15(B)(16)) multiplied by 0.96 (TABLE 310.15(B)(2)(a)) 90° column.

Using this method I come up with an ampacity of 513.6 amps.
Then using the 75°c column I can use the ampacity of 475 amps for the terminations.

Is this correct?

In practice, the terminations are usually rated for 75C, and they are for >100A unless listed/marked otherwise. However, there is a work-around to allow you to use the 90C rating, even at the terminations, provided that you splice it to 75C sized wire to meet the equipment. Page 4 of this document explains it in further detail. One reason you might do this, is if you make or inherit a mistake, and need to restore code compliance. It might also be a value-engineering decision, if the smaller main run wire can justify the splices.
http://www2.schneider-electric.com/...7/en_US/Wire Terminations 0110DB9901R2-02.pdf


If you are just asked for the ampacity of the wire itself at conditions of use, with no explanation of what you are connecting it to, then the answer comes from the 90C column with applicable derates. The instant you terminate it to most common equipment without the aformentioned work-around, it is the minimum of either 75C without derates or 90C with derates.

 

[tkb]

This application is a connection between a 2000kva padmount transformer and a piece of outdoor switchgear. They are about 10' apart.
We are planning to parallel 5 sets for a total ampacity of 2375amps.
The circuit breaker has an electronic trip and will be set for 2300 amps.

 

[JFletcher]

This application is a connection between a 2000kva padmount transformer and a piece of outdoor switchgear. They are about 10' apart.
We are planning to parallel 5 sets for a total ampacity of 2375amps.
The circuit breaker has an electronic trip and will be set for 2300 amps.

If you need 2000A, why not 5 sets of 600MCM vs 750? 600MCM is 420A@75*C, for 2100A. According to this link, 750MCM is $14.80-$18.70/ft for THWN-2/XHHW, vs $9.17-$11.19/ft for 600MCM in THWN-2/XHHW. 10' apart will be ~20' actual length each conductor, x 15-20. 300-400 ft of 600MCM would be $5.50-$7.50/ft cheaper, or $1500-$3000 cheaper.

or run 8 sets of 350MCM AL. 250A @75*, 2000A total. $1.37/foot, $7.80 cheaper/foot than the 600MCM copper. 24-32 wires @ 20' ea = 480-640'. 480x1.37 = $657.60, vs 300' of 750 MCM copper at 14.80/foot... 300 x 14.80 = $4440, for a $3700+ wire saving cost.

eta: was thinking 2000A not 2300. One more set of 600 cu would give 2520A and still be well over 1k cheaper for THWN-2. 10 sets of 350MCM Al, 2500A, still over 3k cheaper.

 

[tkb]

If you need 2000A, why not 5 sets of 600MCM vs 750? 600MCM is 420A@75*C, for 2100A. According to this link, 750MCM is $14.80-$18.70/ft for THWN-2/XHHW, vs $9.17-$11.19/ft for 600MCM in THWN-2/XHHW. 10' apart will be ~20' actual length each conductor, x 15-20. 300-400 ft of 600MCM would be $5.50-$7.50/ft cheaper, or $1500-$3000 cheaper.

or run 8 sets of 350MCM AL. 250A @75*, 2000A total. $1.37/foot, $7.80 cheaper/foot than the 600MCM copper. 24-32 wires @ 20' ea = 480-640'. 480x1.37 = $657.60, vs 300' of 750 MCM copper at 14.80/foot... 300 x 14.80 = $4440, for a $3700+ wire saving cost.

eta: was thinking 2000A not 2300. One more set of 600 cu would give 2520A and still be well over 1k cheaper for THWN-2. 10 sets of 350MCM Al, 2500A, still over 3k cheaper.

I'm locked in at 5 sets due to the conduits already installed.

 

[infinity]

This application is a connection between a 2000kva padmount transformer and a piece of outdoor switchgear. They are about 10' apart.
We are planning to parallel 5 sets for a total ampacity of 2375amps.
The circuit breaker has an electronic trip and will be set for 2300 amps.

Your plan is fine, 750's in 10' of raceway is workable if you have the right setup.

 

[Dennis Alwon]

I assume the neutral is not a current carrying conductor? If it is then you need to multiply by 80% also

 

[Carultch]

I assume the neutral is not a current carrying conductor? If it is then you need to multiply by 80% also

Unless it is just an imbalance current carrying conductor.

 

[jumper]

Unless it is just an imbalance current carrying conductor.

It is "unbalanced".

310.15(B)(5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a)

 

[Dennis Alwon]

It is "unbalanced".

310.15(B)(5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a)

of course we don't know if it unbalanced although in most cases it will be. Generally we don't have have a major portion of the loads as nonlinear loads, especially on a 2000 amp service ,

(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor.

 

[Dennis Alwon]

Follow up question--- The code does not mention a Delta setup--- I assume the neutral there qualifies as a current carrying conductor because of the high leg. Yes? No?

 

[jumper]

of course we don't know if it unbalanced although in most cases it will be. Generally we don't have have a major portion of the loads as nonlinear loads, especially on a 2000 amp service ,

True.

I was kinda nit picking Carulth for his "imbalance" word usage.:angel:

 

[jumper]

Follow up question--- The code does not mention a Delta setup--- I assume the neutral there qualifies as a current carrying conductor because of the high leg. Yes? No?

Pretty sure it is "no". The high leg is irrelevant in regards to the neutral since no 1 pole circuits use it and the the neutral is treated as if it was a regular 120/240 circuit. The non-linear load rules would determine if it is a CCC or not.

 

[Dennis Alwon]

True.

I was kinda nit picking Carulth for his "imbalance" word usage.:angel:

I know you were correcting "imbalance"-- I was adding some info

 

[tkb]

There is a neutral but it is not used in this situation.

What is the code reference for using the 90ºc column for derating and then using the 75ºc column for the termination. I can't seem to find it right now.

 

[Dennis Alwon]

There is a neutral but it is not used in this situation.

What is the code reference for using the 90ºc column for derating and then using the 75ºc column for the termination. I can't seem to find it right now.

110.14(C) ...........

(C) Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified
for terminations shall be permitted to be used for ampacity
adjustment, correction, or both.

 

[tkb]

110.14(C) ...........

Thanks, I was looking in 310.