Reduce neutral for non-linear loads

[freddie]

If you have 3ph sub-panel box which is fed from a main dis. panel (wire size 3 4/0 phase wire, 2/0 neutral, #4 epuipment ground). This sub panel overcurrent protection is 225amps and is feeding non-linear loads such as computers servers etc.Is it true that the 3rd harmonic current and unbalanced current would not exceed the of a 2/0 copper conductor in the future? Also will the reduce size in neutral wire (2/o)be suitable for that panel box?

 

[petersonra]

Re: Reduce neutral for non-linear loads

Originally posted by freddie:
If you have 3ph sub-panel box which is fed from a main dis. panel (wire size 3 4/0 phase wire, 2/0 neutral, #4 epuipment ground). This sub panel overcurrent protection is 225amps and is feeding non-linear loads such as computers servers etc.Is it true that the 3rd harmonic current and unbalanced current would not exceed the of a 2/0 copper conductor in the future? Also will the reduce size in neutral wire (2/o)be suitable for that panel box?

Good question. I would not rely on it being adequate. Personally I think neutrals should be full size. Thre is no good way to tell just what the neutral current will end up being especially in the situation you describe.

 

[iwire]

Re: Reduce neutral for non-linear loads

You should take a look at 220.22 Feeder or Service Neutral Load.

Here is part of it.

There shall be no reduction of the neutral capacity for that portion of the load that consists of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system.

A good portion of our jobs are in buildings with many non-linear loads, I have never seen an engineer reduce the neutral size on anything.

About half the time the neutral is doubled in size and the panel has a 200% rated neutral bar.

A 225 amp panel would have one 4/0 for A, B & C and two 4/0s for the Neutral.

I believe this is one of the reasons for the exception to 408.21

408.21 Grounded Conductor Terminations.
Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.

Exception: Grounded conductors of circuits with parallel conductors shall be permitted to terminate in a single terminal if the terminal is identified for connection of more than one conductor.

[ September 18, 2004, 06:55 PM: Message edited by: iwire ]

 

[ron]

Re: Reduce neutral for non-linear loads

You have had two good responses. Especially with strictly non-linear loads on the panel, the neutral current may exceed the phase current.