Sizing for Sub Panel

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Strahan

Strahan

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Watsontown, PA
I was recently asked to size conductors and OCPD for a 125amp 120/208 sub panel. Using table 310.16 75deg column I would use 1AWG correct? Would the neutral also be required to be sized the same? I'm planning on using the rigid conduit as my bonding means between panels with proper bonding bushings at each end. Thanks guys it has been awhile for me just need to make sure I'm on the right path.
 
Strahan said:
I was recently asked to size conductors and OCPD for a 125amp 120/208 sub panel. Using table 310.16 75deg column I would use 1AWG correct? Would the neutral also be required to be sized the same? I'm planning on using the rigid conduit as my bonding means between panels with proper bonding bushings at each end. Thanks guys it has been awhile for me just need to make sure I'm on the right path.
Click to expand...
Well the proper way to size the feeder and its ocpd is to do a Article 220 load calc. Until you do this, how do you know the sub panel isn't overloaded before you install it???

Otherwise, yes you can size the feeder to the rating of the panel if you know for a fact it won't be overloaded and will likely have loads added in the future.

The neutral only needs to be sized to handle the maximum unbalanced neutral current but no smaller than stipulated in 215.2(A), which typically is no smaller than the required minimum EGC size... but if you foresee future loads being added, you may want to upsize it from the minimum.
 
Smart $ said:
Well the proper way to size the feeder and its ocpd is to do a Article 220 load calc. Until you do this, how do you know the sub panel isn't overloaded before you install it???

Otherwise, yes you can size the feeder to the rating of the panel if you know for a fact it won't be overloaded and will likely have loads added in the future.

The neutral only needs to be sized to handle the maximum unbalanced neutral current but no smaller than stipulated in 215.2(A), which typically is no smaller than the required minimum EGC size... but if you foresee future loads being added, you may want to upsize it from the minimum.
Click to expand...

Load calc was done and this panel size was determined. No non-linear loads so downsizing the neutral (grounded conductor) should be fine. From your article reference I come up with a 6AWG for my neutral. Now as you stated this is the required size of the equipment grounding conductor which in my case will be the rigid conduit. Does this sound ok?
 
The rigid conduit is fine as the EGC. The bonding bushings are not required but you can use them if you like.
 
Strahan said:
Load calc was done and this panel size was determined. No non-linear loads so downsizing the neutral (grounded conductor) should be fine. From your article reference I come up with a 6AWG for my neutral. Now as you stated this is the required size of the equipment grounding conductor which in my case will be the rigid conduit. Does this sound ok?
Click to expand...

Just to make sure... your maximum unbalanced current on the neutral is as if you turn on the 1? loads only one leg at a time, e.g. all the 1? loads on Line A only, then Line B only, and finally Line C only. The leg with the highest load current draw is the maximum unbalanced current.
 
Thanks guys for your help!! In the last couple of years I have been doing more PLC work and troubleshooting and less design so I'm a little rusty:D
 
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