Compressor Feed

[arnettda]

I have a dual motor compressor that I am wiring 480 volt 15 Hp 3phase. The control box has two separate contactor/overloads installed in. I have sized both motor disconnects , fuses and wiring size as per the NEC. I am feeding both disconnects with a single 100 amp feeder due to not being able to get any other breaker in time frame we have.
My question- Is this is the correct way to do this? I keep thinking that since it is a single unit with a single control panel I should be able to feed it with a single disconnect but I do not find where I can do this? Thank you

 

[augie47]

No nameplate information on the control panel ??

 

[arnettda]

No nameplate information on the control panel ??

There is, it says 65 amps but also two 20 HP motors but they are only 15 Hp motors.

 

[jap]

I would size the feeder for the whole unit, install 1 disconnect to a distribution block, then, to an appropriate size fuseblock and set of fuses for each motor.

But that's just me.

JAP>

 

[arnettda]

I would size the feeder for the whole unit, install 1 disconnect to a distribution block, then, to an appropriate size fuseblock and set of fuses for each motor.

But that's just me.

JAP>

Thank you, So two fused disconnect at the compressor would be legal just not your preferred installation method. Each motor still needs its own fuses besides the Feeder protection.

 

[augie47]

We are assuming that both motors can operate at one time. Both your method of japs would work

 

[arnettda]

We are assuming that both motors can operate at one time. Both your method of japs would work

They are a Lead and Lag but both can work at same time. I also assume they alternate running but have not exactly figured that out from the wiring diagram. Is it something with the lead lag switches? Thank you.

 

[retirede]

They are a Lead and Lag but both can work at same time. I also assume they alternate running but have not exactly figured that out from the wiring diagram. Is it something with the lead lag switches? Thank you.

In the old days, there were 2 pressure switches, one set at a slightly lower pressure than the other. If the system pressure dropped to setting of the lower one, it means that 1 unit can’t keep up and the second one is started. The lead/
lag switch simply switches which compressor is controlled by which pressure switch in order to equalize run time. It can be automatic or manual. Nowadays, it’s controlled by the electronics.