Feeder Question / Advice

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Javi_M

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San Antonio, Texas
Just hoping for some quick help understanding if a contractor actually specify correctly the electrical hardware for a job I require.

Rated Amperage: 500 A
Run Length: 200 Feet
Line Voltage: 480 Vac 3-Phase
Load: 360 FLA
Conductors: 2-per phase plus 1 ground

I'm looking to understand if this feeder requires a 2 EMT conduit runs and if a 4/0 wire gauge is adequate to carry the load or did they under rated the wire.

Regards,

Javi
 
4/0 copper is rated for 230 amps. Parallel runs would be 460 amps, but only if they are in separate conduits. 1 conduit and you would have six current carrying conductors, which derates to 80% ampacity, or 368 amps.

You can use a 500 amp breaker with 460 amps of conductors, provided the calculated load is less than 460 amps.

You cannot use parallel runs of 4/0 in 1 conduit with a 500 amp breaker, even though the wire ampacity exceeds your full load amps. Unless it's an HVAC load.

Edited to add... I did not even consider voltage drop, which is going to be significant at that current.

What is your load?

Welcome to The Forum.
 
Actually, I think I derated from the wrong ampacity. That should be 80% of the 90-degree column, 260 amps each, so 80% of 520 is 416 amps.

Been a long day, it's late, and I'm going to bed. You can look up the voltage drop by Googling voltage drop calculator, and in putting your values into it.
 
According to my Southwire App parallel #3/0 Cu conductors would give you a VD of 1.14%, at 400 amps of conductor ampacity that would still too small for a 500 amp OCPD at the beginning of the feeder.

Parameters:
200'
480 volts
Conduit
360 amps
2 sets
 
Thank so much everyone for your input. This load is a 300HP induction motor with an FLA of 360Amps.

They put six current carrying conductors plus a ground wire into a 4 inch EMT conduit. I've being suspicious that this is wrong but I wanted some expert advice.
 
I forgot to mention that this motor is driving a 40K PSI HP prump in case if this fact is significant for the calculations. So we could say that the consensus is that this installation requires a 2 runs EMT conduits to prevent derating on our conductors ampacity.

Base on this fact,what would be the recommended wire gauge to properly handle this load?
 
Javi_M said:
Thank so much everyone for your input. This load is a 300HP induction motor with an FLA of 360Amps.

They put six current carrying conductors plus a ground wire into a 4 inch EMT conduit. I've being suspicious that this is wrong but I wanted some expert advice.
Click to expand...

You didn't say the load was a motor. That changes things. Let's assume a power factor of 90%. 361 value from table 430.250 times 1.1 for a power factor of 900% = 397 amps time 125% per 496 amps. In this case, 4/0 wire would not be large enough. If there was a 100% power factor then the amperage would be 451 amps and in that case parallel 4/0 would be adequate. However, why would you ever feed a 300 hp motor from a 500 amp breaker? Table 430.52 allows a 750A breaker for this motor. 500A seems awful small.
 
This equipment it already comes with a 500 Amp OCPD in it's control panel. Their electrical designer gave me this specifications when I started negotiating the purchasing of this equipment.

{Full Load Amps 361, 125% is 452 and the soft start manufacturer requires a 500 amp circuit breaker}

The motor placard says that the Power Factor is 86 (0.86).
 
Javi_M said:
This equipment it already comes with a 500 Amp OCPD in it's control panel. Their electrical designer gave me this specifications when I started negotiating the purchasing of this equipment.

{Full Load Amps 361, 125% is 452 and the soft start manufacturer requires a 500 amp circuit breaker}
Click to expand...

OK for the breaker size and if the EE is OK with using 452 amps which assumes a 100% power factor then fine. Let it lie.
 
Strathead said:
You didn't say the load was a motor. That changes things. Let's assume a power factor of 90%. 361 value from table 430.250 times 1.1 for a power factor of 900% = 397 amps time 125% per 496 amps. In this case, 4/0 wire would not be large enough. If there was a 100% power factor then the amperage would be 451 amps and in that case parallel 4/0 would be adequate. However, why would you ever feed a 300 hp motor from a 500 amp breaker? Table 430.52 allows a 750A breaker for this motor. 500A seems awful small.
Click to expand...

361A * 125% = 451A.

2 sets of #4/0 = 460A. The 4/0 conductor is large enough.
 
Javi_M said:
David,

but only if you put them in two separate conduits, correct.
Click to expand...

Yes. But (6) #4/0 conductors won't fit in a single 2" conduit, so I'd have to assume it's two separate 2" conduits.

Edit: Sorry, I read your original post as being 2" EMT runs. In a single 4" conduit, it would not work.
 
Javi_M said:
They currently ran a 4 inch EMT conduit and pulled all 6 conductors plus the ground. They will have to run another conduit so the conductors can be separated, correct?
Click to expand...

Unless it is an Other Than Continuous Duty application, I'd say they either need the 2 sets of 4/0 in separate conduits, or 2 sets of 250mcm in a single conduit.
 
If they go with two conduits, don't the paralleling rules require the second conduit also be 4"?
 
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