Replacing a 480v RTU

[Sparky202]

We currently have a 480V 3-Phase RTU with a 132amp MCA and 150amp MOP that needs to be replaced. The new RTU has a higher amperage, with a 147amp MCA and 175amp MOP. The question is whether we can continue to use the existing (3) 1/0 THHN feeder on a 150amp breaker for the new RTU. The feeder is 92ft long. The building engineer is confident that the existing feeder and breaker will suffice, but the building inspector advises against it. Any guidance on this matter would be greatly appreciated. Thank you.

 

[augie47]

The 1/0 will be compliant.
The breaker might cause some nuisance tripping

 

[Sparky202]

so if we change the breaker to 175amp and that’s OK using 1/0 THHN wire? I just wanna make sure we’re all on the same page.

 

[infinity]

so if we change the breaker to 175amp and that’s OK using 1/0 THHN wire? I just wanna make sure we’re all on the same page.

Yes you'll need a 147 amp or larger conductor so #1/0 copper is good. The 150 amp OCPD will probably work but you can change it to a 175 if needed I would try the 150 first.

 

[hillbilly1]

Usually the nameplate will give a minimum wire size, and a maximum ocp.

 

[MyCleveland]

so if we change the breaker to 175amp and that’s OK using 1/0 THHN wire? I just wanna make sure we’re all on the same page.

Don't forget the required AFC label.