Motor Feeder Breaker Size

[dickricker]

Hi,

Following NEC 2011, I have a question concerning motor feeder circuit breaker size.

Motor 1: 100 HP, 460 VAC, soft-starter controlled, NEC FLC = 124 A (Table 430.250)
Motor 2: 500 HP, 460 VAC, soft-starter controlled, NEC FLC = 590 A (Table 430.250)

Motor branch-circuit conductor ampacity not less than 125% NEC FLC (430.22)
Motor 1: 124 A x 1.25 = 155 A minimum ampacity
Motor 2: 590 A x 1.25 = 738 A minimum ampacity

Motor branch-circuit IT circuit breaker maximum 250% NEC FLC (Table 430.52)
Next standard size up if not exact
Motor 1: 124 A x 2.50 = 310 A, use 350 A
Motor 2: 590 A x 2.50 = 1475 A, use 1600 A

Motor feeder-circuit conductor ampacity not less than 125% NEC FLC of largest motor + 100% NEC FLC of other motors (430.24)
(590 A x 1.25) + 124 A = 862 minimum ampacity

Motor feeder-circuit IT circuit breaker not greater than largest motor breaker plus 100% NEC FLC of other motors (430.62)
Next standard size down if not exact
1600 A + 124 A = 1724 A, use 1600 A

So, now we have a 1600 A breaker ahead of a set of 1600 A + 350 A breakers.
This doesn't seem right.
Am I misreading something?
Is this a normal setup with a large motor and a small motor on the same feeder circuit?

Thank You!

 

[Gaffen99]

Why 2 sets of breakers on a single circuit?

 

[texie]

Hi,

Following NEC 2011, I have a question concerning motor feeder circuit breaker size.

Motor 1: 100 HP, 460 VAC, soft-starter controlled, NEC FLC = 124 A (Table 430.250)
Motor 2: 500 HP, 460 VAC, soft-starter controlled, NEC FLC = 590 A (Table 430.250)

Motor branch-circuit conductor ampacity not less than 125% NEC FLC (430.22)
Motor 1: 124 A x 1.25 = 155 A minimum ampacity
Motor 2: 590 A x 1.25 = 738 A minimum ampacity

Motor branch-circuit IT circuit breaker maximum 250% NEC FLC (Table 430.52)
Next standard size up if not exact
Motor 1: 124 A x 2.50 = 310 A, use 350 A
Motor 2: 590 A x 2.50 = 1475 A, use 1600 A

Motor feeder-circuit conductor ampacity not less than 125% NEC FLC of largest motor + 100% NEC FLC of other motors (430.24)
(590 A x 1.25) + 124 A = 862 minimum ampacity

Motor feeder-circuit IT circuit breaker not greater than largest motor breaker plus 100% NEC FLC of other motors (430.62)
Next standard size down if not exact
1600 A + 124 A = 1724 A, use 1600 A

So, now we have a 1600 A breaker ahead of a set of 1600 A + 350 A breakers.
This doesn't seem right.
Am I misreading something?
Is this a normal setup with a large motor and a small motor on the same feeder circuit?

Thank You!

Good example of odd combinations that can arise from the Art. 430 calculations. At first glance your methodolgy and math looks correct. Lets see what the other gurus have to say. This type of question is similar to some you might find on some of the more difficult state master exams.

 

[kwired]

Hi,

Following NEC 2011, I have a question concerning motor feeder circuit breaker size.

Motor 1: 100 HP, 460 VAC, soft-starter controlled, NEC FLC = 124 A (Table 430.250)
Motor 2: 500 HP, 460 VAC, soft-starter controlled, NEC FLC = 590 A (Table 430.250)

Motor branch-circuit conductor ampacity not less than 125% NEC FLC (430.22)
Motor 1: 124 A x 1.25 = 155 A minimum ampacity
Motor 2: 590 A x 1.25 = 738 A minimum ampacity

Motor branch-circuit IT circuit breaker maximum 250% NEC FLC (Table 430.52)
Next standard size up if not exact
Motor 1: 124 A x 2.50 = 310 A, use 350 A
Motor 2: 590 A x 2.50 = 1475 A, use 1600 A

Motor feeder-circuit conductor ampacity not less than 125% NEC FLC of largest motor + 100% NEC FLC of other motors (430.24)
(590 A x 1.25) + 124 A = 862 minimum ampacity

Motor feeder-circuit IT circuit breaker not greater than largest motor breaker plus 100% NEC FLC of other motors (430.62)
Next standard size down if not exact
1600 A + 124 A = 1724 A, use 1600 A

So, now we have a 1600 A breaker ahead of a set of 1600 A + 350 A breakers.
This doesn't seem right.
Am I misreading something?
Is this a normal setup with a large motor and a small motor on the same feeder circuit?

Thank You!

430.52 gives us the maximum breaker size. If the unit will start with a lower setting there is nothing that says you can't use a lower setting.

Since these are on soft starters they very well may start with no trouble on a 1000 amp breaker, or even less if full load rating is never reached.

 

[weressl]

430.52 gives us the maximum breaker size. If the unit will start with a lower setting there is nothing that says you can't use a lower setting.

Since these are on soft starters they very well may start with no trouble on a 1000 amp breaker, or even less if full load rating is never reached.

Breakers of that size - 1600A - are readily available with adjustable trips not only in the long time, short time and instantaneous current setting, but also the time delays and I²t settings, so the two 1600A frame breakers can be coordinated.

 

[kwired]

Breakers of that size - 1600A - are readily available with adjustable trips not only in the long time, short time and instantaneous current setting, but also the time delays and I²t settings, so the two 1600A frame breakers can be coordinated.

I realize that, same would likely apply to a 1000 amp breaker. My point was that according to 430.52 the 1600 amp was the Max size and something smaller is not a code violation.

 

[weressl]

I realize that, same would likely apply to a 1000 amp breaker. My point was that according to 430.52 the 1600 amp was the Max size and something smaller is not a code violation.

Yes, yes. My post did not try to imply any disagreement with yours, just to emphasise that if you have two 1600A frame breakers you can still coordinate those.:happyyes:

 

[dickricker]

Thank you all for your input.

In answer:

Why 2 sets of breakers on a single circuit?

We have a feeder running out to the panel which contains the motor branch circuit breakers near the machine. We want a circuit breaker / disconnect on the building where the feeder originates.


1600 A rather than 1000 A?

I realize that we could certainly use a smaller than 1600 A motor branch circuit breaker for the 500 HP motor. But does 430.62 allow me to keep the 1600 A motor feeder circuit breaker if I reduce the size of the largest motor branch circuit breaker? I think that this is an interpretation of 430.62 issue. I will study it more fully, and report back.

Thanks Again!

 

[dickricker]

NEC 2011 430.62 (A) says:
Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group.

Can this be interpreted to mean that the maximum motor feeder breaker size is "based on the maximum permitted value" for the largest motor branch-circuit breaker, rather than on the actual value for the largest motor branch-circuit breaker? The key question here is "maximum permitted" vs "actual". Does anyone know which one NEC intends?

One would think that if the calculations allowed a 1600 A motor feeder breaker, then any size smaller than that downstream would be OK.

If it turns out that a smaller motor branch breaker, say 1200 A, requires a re-calculation of the motor feeder breaker size, and I end up with a 1200 A feeder breaker, then I do like the plan:

Originally Posted by weressl

Breakers of that size - 1600A - are readily available with adjustable trips not only in the long time, short time and instantaneous current setting, but also the time delays and I2t settings, so the two 1600A frame breakers can be coordinated.

 

[texie]

NEC 2011 430.62 (A) says:
Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group.

Can this be interpreted to mean that the maximum motor feeder breaker size is "based on the maximum permitted value" for the largest motor branch-circuit breaker, rather than on the actual value for the largest motor branch-circuit breaker? The key question here is "maximum permitted" vs "actual". Does anyone know which one NEC intends?

One would think that if the calculations allowed a 1600 A motor feeder breaker, then any size smaller than that downstream would be OK.

If it turns out that a smaller motor branch breaker, say 1200 A, requires a re-calculation of the motor feeder breaker size, and I end up with a 1200 A feeder breaker, then I do like the plan:

Something to consider: Max OCPD based on 430.62 only applies if your feeder conductors are based on 430.24. If you size the feeder conductors to match the feeder breaker you can use any size feeder/breaker you wish.

 

[dickricker]

Something to consider: Max OCPD based on 430.62 only applies if your feeder conductors are based on 430.24. If you size the feeder conductors to match the feeder breaker you can use any size feeder/breaker you wish.

Good Point. Thank You!

However, we do want to base our feeder conductor ampacity on 430.24 --- that would be 862 Amps. Feeder conductor size to match the feeder breaker would require 1600 Amp ampacity. That is almost twice the number/size of conductors (5 x 400 kcmil vs 3 x 350 kcmil). Quite a difference in expense for a 75 ft - 100 ft run.

 

[kwired]

Good Point. Thank You!

However, we do want to base our feeder conductor ampacity on 430.24 --- that would be 862 Amps. Feeder conductor size to match the feeder breaker would require 1600 Amp ampacity. That is almost twice the number/size of conductors (5 x 400 kcmil vs 3 x 350 kcmil). Quite a difference in expense for a 75 ft - 100 ft run.

You mentioned in the OP that both motors are soft started. Good chance you would never have any trouble tripping 1000 or 1200 amp breaker because you are limiting the starting current which is the reason you are allowed to have breaker set at a higher point than the conductor ampacity in the first place.

 

[texie]

Good Point. Thank You!

However, we do want to base our feeder conductor ampacity on 430.24 --- that would be 862 Amps. Feeder conductor size to match the feeder breaker would require 1600 Amp ampacity. That is almost twice the number/size of conductors (5 x 400 kcmil vs 3 x 350 kcmil). Quite a difference in expense for a 75 ft - 100 ft run.

I didn't think you would want to do his and this illustrates why the 430.24 rule can be such a help. I was just mentioning it as a clarification.

 

[dickricker]

You mentioned in the OP that both motors are soft started. Good chance you would never have any trouble tripping 1000 or 1200 amp breaker because you are limiting the starting current which is the reason you are allowed to have breaker set at a higher point than the conductor ampacity in the first place.

Yes. Yes. I agree 100%

However, the 430.62 question remains. Using 430.24 feeder conductor ampacity, following 430.62, using a 1600 Amp (or 1200 Amp) motor branch circuit breaker to calculate maximum feeder circuit breaker size and coming up with 1600 Amps (or 1200 Amps), can I then reduce the size of the motor branch circuit breaker without recalculating and reducing the size of the feeder circuit breaker?

What I would like to do is use a 1000 Amp or 1200 Amp motor branch circuit breaker and a 1400 or 1600 Amp feeder circuit breaker, and still use feeder conductor ampacity based on 430.24

I this in line with NEC 2011?

Or, will I need to use the same size breakers in both locations and adjust their trip points to achieve coordination?

 

[dickricker]

I didn't think you would want to do his and this illustrates why the 430.24 rule can be such a help. I was just mentioning it as a clarification.

Yes. Thank You. The 430.24 rule can be quite a money and work saver.

 

[augie47]

Yes. Yes. I agree 100%

However, the 430.62 question remains. Using 430.24 feeder conductor ampacity, following 430.62, using a 1600 Amp (or 1200 Amp) motor branch circuit breaker to calculate maximum feeder circuit breaker size and coming up with 1600 Amps (or 1200 Amps), can I then reduce the size of the motor branch circuit breaker without recalculating and reducing the size of the feeder circuit breaker?

What I would like to do is use a 1000 Amp or 1200 Amp motor branch circuit breaker and a 1400 or 1600 Amp feeder circuit breaker, and still use feeder conductor ampacity based on 430.24

I this in line with NEC 2011?

Or, will I need to use the same size breakers in both locations and adjust their trip points to achieve coordination?

Since 240.6 lists the next standard size breaker above 1200 as 1600, you could use a 1200 branch and a 1600 mp feeder OCP device.

 

[kwired]

Yes. Yes. I agree 100%

However, the 430.62 question remains. Using 430.24 feeder conductor ampacity, following 430.62, using a 1600 Amp (or 1200 Amp) motor branch circuit breaker to calculate maximum feeder circuit breaker size and coming up with 1600 Amps (or 1200 Amps), can I then reduce the size of the motor branch circuit breaker without recalculating and reducing the size of the feeder circuit breaker?

What I would like to do is use a 1000 Amp or 1200 Amp motor branch circuit breaker and a 1400 or 1600 Amp feeder circuit breaker, and still use feeder conductor ampacity based on 430.24

I this in line with NEC 2011?

Or, will I need to use the same size breakers in both locations and adjust their trip points to achieve coordination?

430.24 is conductor minimum ampacity and you must have conductor with ampacity no less than what comes from 430.24. Anything larger is acceptable.

430.62 is maximum short circuit and ground fault protection.
Anything smaller is acceptable.


A feeder overcurrent device does not have to be higher setting than a branch circuit it supplies, even though it is typical to see that.

 

[dickricker]

Since 240.6 lists the next standard size breaker above 1200 as 1600, you could use a 1200 branch and a 1600 mp feeder OCP device.

Thank You, Augie47!

Just to be 100% clear:

Do you mean that I could use a 1200 Amp branch and a 1600 Amp feeder OCP device with feeder conductor ampacity following 430.24?

Do you interpret 430.62 to mean that the maximum motor feeder breaker size is calculated using the maximum permitted branch-circuit breaker value for the largest motor, rather than the actual branch-circuit breaker value for the largest motor?

 

[dickricker]

A feeder overcurrent device does not have to be higher setting than a branch circuit it supplies, even though it is typical to see that.

Good Point! Thank You!