250 HP Air Compressor Wiring

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jbruinsma

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I have an existing 250Hp 480V 3ph air compressor(337 package amps) that is outdoors and is fed from inside a Building next to it by a 350 Amp breaker. Because the breaker is undersized I need to change it to a 600A as per recommendations by the compressor manufacturer. My question is: Can I reuse the existing 500MCM(kcmil) conductors feeding it, and if not, what size conductor do I need? :confused:
 
Re: 250 HP Air Compressor Wiring

500 kcmil @ 75 C is rated 380 amps.

You said "337 package amps" so I imagine you have a name plate that says minimum circuit ampacity 337 amps.

If this is true the 500 kcmils will be fine.

If there a maximum fuse or breaker rating on this unit?

Or will the compressor manufacturer provide something in writing that the 600 amp breaker is the required breaker?
 
Re: 250 HP Air Compressor Wiring

The 600 Amps is just a recommendation to allow for the starting current. I will check the nameplate and get the full data. Some times I transpose numbers.
Thanks for the reply.
 
Re: 250 HP Air Compressor Wiring

Per 430.22, wouldn't the conductors have to be sized for 125% of the full load current? This would 337 * 1.25 = 421 amps. After all, an air compressor could run continuously if there was an open valve or something that could let the air back out. The handbook commentary for 430.22(E) says "A motor is considered to be continuous duty unless the nature of the apparatus it drives is such that the motor cannot operate continuously with load under any condition of use."

Steve
 
Re: 250 HP Air Compressor Wiring

Steve I am guessing here but I do not believe we have just a motor here, I believe we have a packaged unit, just like an HVAC packaged unit so the name plate min circuit ampacity would be used.

The NEC says a 250 HP motor @ 480 volts only draws 302 amps.

EDIT for code reference 430.7(D) Multimotor and Combination-Load Equipment.

[ January 15, 2004, 05:22 PM: Message edited by: iwire ]
 
Re: 250 HP Air Compressor Wiring

I think I get it, the 337 amps listed on the unit may already include the 1.25. Seems like every post I see lately about sizing wire or breakers, I'm not sure if the 1.25 applies or not.

Steve
 
Re: 250 HP Air Compressor Wiring

Here is the information from the Air Compressor:

Manufacturer: Ingersol-rand
Drive motor 250 hp
Fan motor 15 hp
Total package amps: 337
Voltage: 460V 3ph 60 Hz

This is a rotary screw compressor, so the motor runs continuously, just loaded or unloaded. The fan also run continuously for cooling.
 
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