three phase sub panel wire sizing
- Thread starter ron b
- Start date
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bphgravity
Senior Member
- Location
- Florida
Re: three phase sub panel wire sizing
Determine the total load that the feeder will serve, use 310.16 (75? column) to size the conductors, provide proper overcurrent protection. Remember to add 25% to any continuous loads. The grounded conductor may be reduced to carry only the unbalanced load but may not be less than the required equipment grounding (bonding) conductors as sized by Table 250.122.
Determine the total load that the feeder will serve, use 310.16 (75? column) to size the conductors, provide proper overcurrent protection. Remember to add 25% to any continuous loads. The grounded conductor may be reduced to carry only the unbalanced load but may not be less than the required equipment grounding (bonding) conductors as sized by Table 250.122.
- Location
- Massachusetts
Re: three phase sub panel wire sizing
Bryan, this is a real question.
Why would I not simply choose conductors rated 200 amps or more and forget about the calculations?
Assuming this sub panel is protected by 200 amp OCP the smallest conductor allowed regardless of calculations will be 3/0 and the largest required conductor would be 3/0.
Edit - 3/0 CU
Am I missing something?
[ June 25, 2005, 09:52 AM: Message edited by: iwire ]
Bryan, this is a real question.
Why would I not simply choose conductors rated 200 amps or more and forget about the calculations?
Assuming this sub panel is protected by 200 amp OCP the smallest conductor allowed regardless of calculations will be 3/0 and the largest required conductor would be 3/0.
Edit - 3/0 CU
Am I missing something?
[ June 25, 2005, 09:52 AM: Message edited by: iwire ]
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: three phase sub panel wire sizing
However, I recently reviewed a design in which all of the sub-panels (large building, there were a bunch of panels) were 225 amp, regardless of the load. There might be a 50 amp OCPD and 50 amp feeder conductors, but the panel was shown as 225 amps. If this had been an early stage in the design, I might have guessed that this was just a way of showing that a panel is needed, with the actual ratings to be determined later. But it was a final design. My review comments included this issue, of course, and I can only suspect that someone?s brother-in-law has a warehouse full of 225 amp panels he wanted to offload.
The point is that you only need to size the feeder for the calculated load. Anything beyond that is a design choice.
You are not. I agree with you.
However, I recently reviewed a design in which all of the sub-panels (large building, there were a bunch of panels) were 225 amp, regardless of the load. There might be a 50 amp OCPD and 50 amp feeder conductors, but the panel was shown as 225 amps. If this had been an early stage in the design, I might have guessed that this was just a way of showing that a panel is needed, with the actual ratings to be determined later. But it was a final design. My review comments included this issue, of course, and I can only suspect that someone?s brother-in-law has a warehouse full of 225 amp panels he wanted to offload.
The point is that you only need to size the feeder for the calculated load. Anything beyond that is a design choice.
- Location
- Massachusetts
Re: three phase sub panel wire sizing
IMO it would take unusual circumstances to justify a panel at all with only a 50 amp OCP. Run the circuits to another panel location.
What a waste of labor and material. :roll:
That would drive me nuts. :roll:
IMO it would take unusual circumstances to justify a panel at all with only a 50 amp OCP. Run the circuits to another panel location.
What a waste of labor and material. :roll:
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: three phase sub panel wire sizing
The neutral would only be a current carrying conductor if a major portion of the load was non-linear. At that point derating would apply. Otherwise the 3/0 that iwire suggested would be fine.
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