Derating Neutral

[zinsco]

Installing a 100 amp sub panel that will feed 3-240v 30 amp A/C units only. Main panel is 200 amp.

#1 - Does the sub panel require a neutral even though there will be no loads using a neutral?

#2 - If I do run a neutral, to leave the capability of taking a few 120v loads out, can I derate the neutral and if so to what size?

#3 - Planning on running #2's for my feeder wires, but I was surprised to read somewhere that you could go #4 on the feeder wires for 100 amps to a sub panel. This can't be right. Am I wrong? I would like running #4's if legal.

 

[jumper]

Installing a 100 amp sub panel that will feed 3-240v 30 amp A/C units only. Main panel is 200 amp.

#1 - Does the sub panel require a neutral even though there will be no loads using a neutral?

#2 - If I do run a neutral, to leave the capability of taking a few 120v loads out, can I derate the neutral and if so to what size?

#3 - Planning on running #2's for my feeder wires, but I was surprised to read somewhere that you could go #4 on the feeder wires for 100 amps to a sub panel. This can't be right. Am I wrong? I would like running #4's if legal.

1. No

2. If a neutral is run, it could be as small as the EGC. Depends what future loads are.

3. Assuming CU, #3 is sufficient but #2 is fine. #4 can only be used under certain conditions and does not apply here.

 

[kwired]

1. No

2. If a neutral is run, it could be as small as the EGC. Depends what future loads are.

3. Assuming CU, #3 is sufficient but #2 is fine. #4 can only be used under certain conditions and does not apply here.

Since the load supplied is motors/hermetic refrigerant compressors, it is possible one could protect smaller then 3 AWG with a 100 amp OCPD. The feeder conductor only needs to be 125% of largest motor/compressor plus all other loads, but could have even higher OCPD to allow for motor starting.

As far as a neutral - no one is not needed or required, but depending on the installation details you likely are required to have a 120 volt receptacle(s) near the supplied AC units and it may be easy to supply them from this panel - if so you would need a neutral for those.

 

[kwired]

I recently ran a 600 amp feeder to a location that supplied several compressor units, all 208 volts with no neutral. I did run a neutral because in same area we had required 120 volt receptacle for servicing the compressor units and some lighting for the compressor area. The ampacity required by the compressor units was more then 2/3 of the feeder ampacity - the calculated neutral load was 20 amps or less, but still needed at least a 250.122 sized conductor for the feeder neutral, and since 1/0 is smallest size that can be paralleled, I still ended up with about 150 amps of neutral conductor though I only needed about 20 for the actual load supplied.