Compressor Feeder Calculation

Status
Not open for further replies.
S

sasser12

Member
Location
Fort Walton Beach
How would you size the feeder to two air compressor packages if this the only information provided on the manufactures label.

221HP, 165KW, 460volt 3PH.

The documents that came with the compressors list parallel runs of 3/0 as supply conductors and parallel 200 amp fuses (6 ea.).
When I requested more info from the manufacture, such as the minimum supply circuit conductor ampacity and the maximum ampere rating of the short circuit/ground fault device (430.7(D) (1) or what is the calculated FLC for the package, the emailed reply was:
Parallel 250MCM
6 200 amp fuse or 3 325 amp fuse class K5

I would appreciate any help on this.
 
sasser12 said:
The documents that came with the compressors list parallel runs of 3/0 as supply conductors and parallel 200 amp fuses (6 ea.).
Click to expand...

I was very tempted to tell you to parallel 3/0 and parallel 3 200 amp fuses but I won't.:grin:

I would not pull a single wire until the unit was there and I got the info I needed. Too often the literature is incorrect.
 
The documents that came with the compressors list parallel runs of 3/0 as supply conductors and parallel 200 amp fuses (6 ea.).
Click to expand...
You can't parallel fuses(unless it is a factory assembly). The extrapolated current from Table 430.250 for the 221 hp motor would be 266 amps and the minimum conductor ampacity would be 332 amps. You could use a 300 amp dual element fuse with parallel 2/0s and be in compliance with the NEC.
Don
 
This compressor is driven by an electric motor, and is thus governed by article 430 of the NEC. The reason I say this is because wire size, overcurrent and short-circuit protection are all chosen separately based on the motor in question.

Wire size is based on the motor full-load amps, plus any other loads associated with the equipment. Like control power transformers, sump heaters, etc.

Overcurrent is provided for by the motor overloads. This can be built into the motor, or more likely, a separate overload device. Like the heater part of a starter.

Short circuit protection must be provided by the installer, and is usually a circuit breaker or a set of fuses in some sort of distribution panel.

If I were installing this compressor, I'd interpolate the HP/ amps, and size the wire accordingly. Overcurrent protection is already done by the motor overload relay. As a general rule of thumb, for a 480v 3ph motor, I double the HP to get the fuse/ breaker size. If this motor starts across-the-lines (unlikely), I'd choose 450 or 500 amp fuses or breaker. If it has some sort of 'reduced voltage' starting system, 400's would do fine.

Notice that the wire size is too small for the fuse/ breaker size. That's ok, because we're dealing with a motor here. The maximum continuous current is limited by the motor overload device.
 
Nevertheless, Atlas should get their act together.

The code also requires the equipment manufacturer to put a nameplate on the the equipment listing the MCA and maximum SCP. They have that info and there's no way for you to "guess" it correctly unless you have a complete list of all the loads served from their panel. That could include electric heaters, fans, control power, small pumps - all kinds of things in addition to the main compressor motor.

I've run into the same thing and they are in violation of the NEC if they don't have that nameplate on the compressor when it arrives. They can't possibly size the wire for you since they don't know what kind of wire your pulling, what kind of conduit or tray your using, what the ambient temperature will be etc etc. They are way off the mark in their response.
 
micromind said:
I'd interpolate the HP/ amps, and size the wire accordingly.
Click to expand...

Watch out, you need to comply with 430-22(A) and 430-6(A) meaning you shall size supply conductors no less than 125% of the FLA as listed in table 430-250. The overloads are the only item sized off the actual nameplate FLA.
 
micromind said:
As a general rule of thumb, for a 480v 3ph motor, I double the HP to get the fuse/ breaker size.
Click to expand...

Watch out again, you?re correct in knowing the motor is protected against overload with the additional overload protection as well as the conductors. But you still need to comply with 430-52 and with this you cannot exceed the values given in table 430-52.
Less is okay but why would less be considered? At maximum it is still protecting against short-circuits and ground faults, at least if it is sized according to the table one will avoid nuisance tripping.
Sorry to sound like I?m ?bird-dogging? you I just couldn?t pass this up :)
 
Confused About UR layout

Confused About UR layout

Ok,

I'm Confused here,,,the label Nameplate you have
221 Hp
165Kw
460v 3 phase
Is this For 1 compressor with 2 pumps & 2 Motors?
(Will the compressor come with a CONTROL BOX, with 2 starters etc...)

Or is it

1 pump/motor @
221 hp
165Kw
460v 3 phase

and your going to use 2 of these....???? Please clearify, cause im going to take a crack at this and if anyone see's something wrong please elaborate with us all.



Another thing,,,
165Kw=359amps


P/E = amps

(165,000 / 460 = 359amps)
So im assuming this is FL. amps on the compressor with 2 motors.
Right?
 
WPiper,
The code also requires the equipment manufacturer to put a nameplate on the the equipment listing the MCA and maximum SCP.
Click to expand...
I don't think so. Can you cite the code section?
Don
 
221hp x 746w/hp ~ 165kw
This implys the entire cpmpressor package is 165kw with an equivelant hp load of 221. Which implys there are multiple motors, control panels, heaters, etc fed from the single supply (Otherwise the one motor would be a theoretically perfect motor with no losses p.f.=1, eff = 1)

165000 / (460 * 1.732 ) = 207 Amp load

27 *1.25 = 259 amps

using 60 degree termination lug ratings for compressor (most conservative approach)
350 kcmil cable - 300a fuse
 
Status
Not open for further replies.
Top