Conductor Sizing

[CopperTone]

I want to double check this job I'm putting together. It consists of installing power to a packaged chiller unit on a roof.

the specs say this. Trane RTAA 125

max main breaker size 350amps
min. circuit ampacity 259 amps
460 V 3 phase
use a fused disconnect
time delay or dual element (RDE) fuse 300amp


so if I understand this correctly - I can change/adjust existing switch gear trip to 350amps for the breaker , conductor size from switch gear to disconnect on roof 100' away will be 3 - #300mcm thhn copper with a #3 copper grounding conductor.

#300 kcmil is good for 285amps from 75 degree C column of 310.16 table - is that correct?

then obviously a nema 3R rated 480V 3 phase fused disconnect on the roof at the unit

use 300amp fuses in this disconnect?


then use 3 - # 300mcm & 1 # 3 grounding wires from the disconnect on the roof to the wiring compartment in the unit?

I get confused with the fact that there is a 350amp breaker, 285amp rated wire, and a 300amp fuse?

Is this correct? thanks

 

[Dennis Alwon]

I didn't check your numbers but yes you can have a conductor that is rated less than the OCPD because the unit itself has overload protection built in.

 

[augie47]

It would be acceptable here, but, we look at that fusible disconnect as a "disconnect" in this case and not the branch circuit protective device.
An argument can be made that the wiring from the gear to the disconnect is a feeder and then it would be questionable if you could exceed a 300 amp limit on a 285 amp rated wire.
we'll see what others say.

 

[Dennis Alwon]

An argument can be made that the wiring from the gear to the disconnect is a feeder and then it would be questionable if you could exceed a 300 amp limit on a 285 amp rated wire.
we'll see what others say.

Wouldn't 240.4(B) kick in here

 

[augie47]

yes. but unless I'm mistaken his wire ampacity is 285 and he wants to go to 350 skpping the 300 "standard".
Again, I have no problem, but I've seen other inspectors dis-allow it.