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Old 09-11-2012, 02:14 PM   #1
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industrial motor circuit


Hey guys, i'm trying to make sure my companies air compressor circuit is up to code. The NEC is a little confusing to me on this topic. we currently have a 300hp compressor supplied by a 600amp molded case circuit breaker. line of sight is an issue. my question is, according to NEC, do i need a CB at start of circuit sized to protect feeder conductors, a CB sized for motor FLA (for branch-circuit and short-circuit protection) and then a Disconnect at the motor for line of sight? also does the disconnect have to be fused or not? thanks for the help.

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Old 09-11-2012, 03:54 PM   #2
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What is the voltage? Or just post all the nameplate data on the motor.

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Old 09-11-2012, 04:01 PM   #3
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480v, 3ph, FLA 349amps
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Old 09-13-2012, 12:52 PM   #4
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Scott, Sorry I forgot you and your post.

1) Breaker - Motor FLA x 2.50 = 872 amps. Table 430.50 This is the maximum allowable breaker current for a squirrel cage motor using a regular inverse time breaker. The existing breaker is okay.
2) Conductors - Motor FLA x 1.25 = 436 amp. Use article 310.15B to find conductor size. What size wire do you have?

Yes, you need a breaker sized as above to protect the motor and a non-fused disconnect at the motor if it is not in sight of the breaker. 430.107
This is one instance where the breaker size can exceed the wire amp rating.

Note: These calculations are not engineered to your application. This is for exercise and nothing more. Consult the NEC and the engineering dept for specifics.

Last edited by J. V.; 09-13-2012 at 12:54 PM.
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Old 09-13-2012, 08:35 PM   #5
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Quote:
Originally Posted by J. V. View Post
Scott, Sorry I forgot you and your post.

1) Breaker - Motor FLA x 2.50 = 872 amps. Table 430.50 This is the maximum allowable breaker current for a squirrel cage motor using a regular inverse time breaker. The existing breaker is okay.
2) Conductors - Motor FLA x 1.25 = 436 amp. Use article 310.15B to find conductor size. What size wire do you have?

Yes, you need a breaker sized as above to protect the motor and a non-fused disconnect at the motor if it is not in sight of the breaker. 430.107
This is one instance where the breaker size can exceed the wire amp rating.

Note: These calculations are not engineered to your application. This is for exercise and nothing more. Consult the NEC and the engineering dept for specifics.
According to table 430.250 the motor FLA is 361 amps.

361 x 125% = 451.25A.

The ITCB is 361 x 250% = 902.5A

I like your note.
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Old 09-13-2012, 08:38 PM   #6
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Quote:
Originally Posted by scott.cameron View Post
Hey guys, i'm trying to make sure my companies air compressor circuit is up to code. The NEC is a little confusing to me on this topic. we currently have a 300hp compressor supplied by a 600amp molded case circuit breaker. line of sight is an issue. my question is, according to NEC, do i need a CB at start of circuit sized to protect feeder conductors, a CB sized for motor FLA (for branch-circuit and short-circuit protection) and then a Disconnect at the motor for line of sight? also does the disconnect have to be fused or not? thanks for the help.
Use a non-fused disconnect otherwise the conductors from the breaker to the disco are considered feeders and need to be sized as such per 430.62.
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Old 09-14-2012, 10:25 AM   #7
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Nameplate amps are 349. That is what I used. Must I use the NEC current?
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Old 09-14-2012, 10:26 AM   #8
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Quote:
Originally Posted by J. V.
Nameplate amps are 349. That is what I used. Must I use the NEC current?
Yes per 430.6.
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Old 09-14-2012, 11:05 PM   #9
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You need only one circuit protection. It can be a breaker or fuses. You can have more than one, but it's dumb.

430.28 covers feeder taps and location of the protection.

Basically it says that you can have conductors tapped from a larger source if they land in a protective device under two conditions.

1) If the source is 1000% or less than the rating of the tapped conductors, they can be not longer than 10' from the tap to the protective device.

2) If the source is 300% or less than the rating of the tapped conductors, they can be not longer than 25'.

There's an exception about buildings 35' high or higher, but I doubt if it applies here.

In my experience, a 600 amp breaker is ok for a 300HP motor operating at 480 volts. If the breaker is in in compliance with 1) or 2) above, then the motor feeders are protected.

If needed, a non-fused disconnect can be installed closer to the motor. Or a fused one, but there's no reason for the fuses.

Rob
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Old 09-17-2012, 02:14 PM   #10
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Thanks guys, the info helped. Appreciate your responses.
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Old 09-17-2012, 03:01 PM   #11
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6ea 4/0 THHN CONDUCTORS
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Old 09-17-2012, 08:06 PM   #12
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Per the NEC, two parallel (6 wires) of 3/0 copper would be the minimum if there are 3 wires in two conduits. If all 6 are in the same conduit, 4/0 copper is the minimum.

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Old 09-18-2012, 12:09 AM   #13
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300HP? And your starting this across the line without a VFD or softstart?

That is a lot of HP to run just off a contactor....
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Old 09-18-2012, 09:27 AM   #14
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Quote:
Originally Posted by micromind View Post
Per the NEC, two parallel (6 wires) of 3/0 copper would be the minimum if there are 3 wires in two conduits. If all 6 are in the same conduit, 4/0 copper is the minimum.

Rob
Rob,

I disagree. The motor brach circuit conductors have to be sized at 125% of the motor FLA using Table 430.250.

361 x 125% = 451.25A

700 KCMIL THHN CU (rated 460A) per conductor if three are used.

250 KCMIL THHN CU x 2 per phase if two sets of paralleled (6 conductors in one conduit).

250 KCMIL THHN CU are rated 290A x 2 = 580A x 80% = 464A
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Old 09-18-2012, 09:31 AM   #15
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Quote:
Originally Posted by ddawg16 View Post
300HP? And your starting this across the line without a VFD or softstart?

That is a lot of HP to run just off a contactor....
The local utility probabely has restrictions on the size of motor started by across the line starters. I think around here it is 150HP.

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