3 Phase Feeders

[Tajon]

Is there a special calculation for 3 phase feeders? 400 kcmil for a 250A panel and 2 sets of 500 kcmil in parallel for a 600A panel seems like over kill. Am I missing something?

I used the 75 degree column from 310.15(B)(16) and applied the adjustment factor from 310.15(B)(3)(A)

Does anyone know of anything else that is applied to 3 phase feeders? Any help is greatly appreciated!

-Jose

 

[ptonsparky]

What is your load calculation?
The feeder and protection is not required to be as big as the panel rating.

We recently installed a 250 amp feeder to a 400 amp panel with full knowledge and acceptance by the owner. It was for specific loads and he was informed that. Three months later he is rethinking the cost savings. Such is life.

 

[ptonsparky]

Keep the feeders in separate conduits. 250 MCM is good for 250 amps. The Neutral and EG are not current carrying. Usually.

 

[infinity]

Is there a special calculation for 3 phase feeders? 400 kcmil for a 250A panel and 2 sets of 500 kcmil in parallel for a 600A panel seems like over kill. Am I missing something?

I used the 75 degree column from 310.15(B)(16) and applied the adjustment factor from 310.15(B)(3)(A)

Does anyone know of anything else that is applied to 3 phase feeders? Any help is greatly appreciated!

-Jose

Where are you getting these wire sizes from, an engineer? Why are adjustment factors being applied?

 

[Carultch]

Is there a special calculation for 3 phase feeders? 400 kcmil for a 250A panel and 2 sets of 500 kcmil in parallel for a 600A panel seems like over kill. Am I missing something?

I used the 75 degree column from 310.15(B)(16) and applied the adjustment factor from 310.15(B)(3)(A)

Does anyone know of anything else that is applied to 3 phase feeders? Any help is greatly appreciated!

-Jose

This sounds like the sizes for aluminum wiring. Are you sure it specifies copper?

Maybe distance is significant, and sizes are in excess of the minimum ampacity to mitigate voltage drop.

 

[Tajon]

Where are you getting these wire sizes from, an engineer? Why are adjustment factors being applied?

I came up with the sizes on my own.

This a 3 phase, 120/208 Wye, 4 wire circuit. To the best of my knowledge the neutral is considered a current carrying conductor. 220.61(C)(1)
I want to size the panel to the full capacity of the breaker supplying it. One is for 250A and the other is 600A.

310.15(B)(3)(a) states - 4-6 Conductors in a raceway use an 80% adjustment factor.

Circuit 1 (250A)

310.15(B)(16) 75 degree column states - 400 kcmil copper is good for 335A. 335 x .8 = 268A

Circuit 2 (600A) (2 sets of conductors ran in parallel to panel)

310.15(B)(16) 75 degree column states - 500 kcmil copper is good for 380A. 335 x .8 = 304A. (304A x 2 = 604A.)

Is there anything else I am missing?

 

[Tajon]

I guess I should of just asked how to size conductors for a 250 A / 3 phase / 120/208 / 4 Wire Circuit where the conductors will be installed in a raceway and for a 600 A / 3 phase / 120/208 / 4 Wire Circuit where the conductors will be installed in parallel in a raceway.

I'm nervous about purchasing the wrong wire. There is 7 panels in total and this is my first time running feeders in parallel. The wire is going to be super expensive and I can not afford to order and install the wrong size wire. I overthink things when it comes to thousands of dollars being spent. I also get lost in the code book looking for every single possibility that may apply to my design. Thanks for reading my post guys.

 

[augie47]

Note in 310.15, the Neutral does not count as a current carrying conductor unless the MAJOR portion of the load is non-linear, a rare situation.
In addition, if you are using a 90° conductor such as THWN or XHHW you can apply any adjustment factors to the 90° ampere rating.
Under normal conditions:
For a 250 amp panel, 250 kcmil Cu is fine.
For your 600 amp with parallel conduits 350kcmil Cu would be fine

 

[texie]

"conductors will be installed in a raceway". Do you mean all conductors in one raceway or each set in it's own raceway?

 

[infinity]

As Augie stated if you use a 90° C conductor you would that ampacity for adjustment. Also unless you're doing a data center or some other specific installation where there are a large amount of harmonics the neutral does not count as a CCC. Also the adjustment factor table is for CCC's in the raceway not the number of conductors.

 

[Carultch]

Also unless you're doing a data center or some other specific installation where there are a large amount of harmonics the neutral does not count as a CCC.

Is there any standard for what "large amount of harmonics" means in this context? As in, what measurable factor would I look for in the waveform when I see it on an oscilloscope?

 

[paulengr]

Is there any standard for what "large amount of harmonics" means in this context? As in, what measurable factor would I look for in the waveform when I see it on an oscilloscope?

Not in NEC. I’ve never seen it in practice.

 

[infinity]

Is there any standard for what "large amount of harmonics" means in this context? As in, what measurable factor would I look for in the waveform when I see it on an oscilloscope?

No standard that I know of. In the real world it probably doesn't exist to a point where the system would be over 50% harmonic.

 

[Tajon]

As Augie stated if you use a 90° C conductor you would that ampacity for adjustment. Also unless you're doing a data center or some other specific installation where there are a large amount of harmonics the neutral does not count as a CCC. Also the adjustment factor table is for CCC's in the raceway not the number of conductors.

I'm counting the neutral as a conductor as per 220.61(C)(1). Is this wrong? The incoming power source is 208Y/120 VAC, 3PH, 4W

 

[Tajon]

"conductors will be installed in a raceway". Do you mean all conductors in one raceway or each set in it's own raceway?

The 600amp circuit will have 2 sets of wires and 2 raceways, each set being in their own raceway for a parallel run.

 

[LarryFine]

The neutral counts in conduit fill but not as a CCC.

 

[Tajon]

The neutral counts in conduit fill but not as a CCC.

Thank you. I think I get it. The neutral isn't considered a CCC but still needs to be the same size as the CCCs

 

[Carultch]

Thank you. I think I get it. The neutral isn't considered a CCC but still needs to be the same size as the CCCs

Not necessarily. If there is a good chance that the neutral will carry as much current as any one of the CCC's, then it would need to be as large as the CCC's. Otherwise, it needs to be sized for the calculated current it carries, and no smaller than the EGC, if it carries a lot less current.

 

[electrofelon]

Not necessarily. If there is a good chance that the neutral will carry as much current as any one of the CCC's, then it would need to be as large as the CCC's. Otherwise, it needs to be sized for the calculated current it carries, and no smaller than the EGC, if it carries a lot less current.

Not disagreeing that the neutral can be reduced in size, but in my experience it is common practice to provide a full sized neutral for a wye system. I don't recall ever providing a reduced neuter.

 

[petersonra]

Is there a special calculation for 3 phase feeders? 400 kcmil for a 250A panel and 2 sets of 500 kcmil in parallel for a 600A panel seems like over kill. Am I missing something?

I used the 75 degree column from 310.15(B)(16) and applied the adjustment factor from 310.15(B)(3)(A)

Does anyone know of anything else that is applied to 3 phase feeders? Any help is greatly appreciated!

-Jose

sounds about right if using aluminum wire and it is a long enough run that VD is a consideration.