MCC breaker size

[m sleem]

E.g. having several motors supplied by one mcc, the largest one is "a" & the rest are "b", which scenario of the following two is right where all of the motors continuously working,

1. Ocpd=next down size of 2.5 x a+1.25 x b
2. Ocpd=next higher size of 1.25 x (a+b)+0.25 x a

Thanks in advance

 

[m sleem]

Appreciate your feedback.

 

[petersonra]

C-none of the above

 

[m sleem]

C-none of the above

 

[Smart $]

Neither are correct. How confusing is that?

 

[don_resqcapt19]

430.62 Rating or Setting ? Motor Load
(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 4 30.24 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.

 

[m sleem]

If you mean that ocpd=2.5xa+1xb, so what about the continuity rule.
& The second scenario i found hints in NEC examples.

 

[m sleem]

I learned from you that getting ocpd size will be after applying the demand factor & the demand load of several motors =1.25x max + others.

 

[Smart $]

If you mean that ocpd=2.5xa+1xb, so what about the continuity rule.
& The second scenario i found hints in NEC examples.

I learned from you that getting ocpd size will be after applying the demand factor & the demand load of several motors =1.25x max + others.

What continuity rule?

What second scenario?

There is no demand factor, demand load for motors. Only allowance is for motors unlikely to be operating concurrently.

 

[augie47]

You have to keep in mind that each and every motor must have GFSC protection as specified in 430.52.
The rule you are stating (2.5 X a + b) is basically the feeder protection rule, normally supplying individual GFSC devices for each motor.
It does not work often that a singular GFSC device will satisfy as the lone protection for more than one motor

 

[m sleem]

What continuity rule

1.25x continuous load+1x non-continous load.

What second scenario?

Mentioned in first post.

There is no demand factor, demand load for motors. Only allowance is for motors unlikely to be operating concurrently.

I mean this section 220.18(A).

 

[Smart $]

1.25x continuous load+1x non-continous load.

125% factoring for continuous loads under 210.19(A) or 215.2(A) does not apply to motor branch and feeder circuits. The 125% is already factored into the calculated load under under Article 220 through Articles 430 and 440 requirements.

Mentioned in first post.

I mean this section 220.18(A).

Your mixing calculated load, conductor, and OCPD sizing all together.

You have to consider conductor sizing and OCPD ratings separately regarding motors.

To tell you the truth, I have no idea why 220.18 is even in the Code. Stipulations of (A) and (C) are already covered by other requirements, and (B) should be in 220.12, perhaps 220.14(D), (F), and (G) too.

 

[kwired]

Your mixing calculated load, conductor, and OCPD sizing all together.

You have to consider conductor sizing and OCPD ratings separately regarding motors.

I think I agree with that.

Minimum conductor ampacity to the MCC needs to be 125% of largest motor full load current plus 100% of all other loads.
Maximum overcurrent protection (of the minimum ampacity conductor) using inverse time breaker would be 250% of largest motor full load current plus 100% of all other loads.
It may be allowed to be increased if it doesn't hold for starting.

Run a larger feeder then minimum necessary and it gets harder to understand. Say you only need approximately a 200 amp supply conductor and choose to run 400 amp supply conductors for possible future load.... If you decide to protect them above 400 amps (2.5 times the largest motor could put you above 400) you may be justified to do so, but you may want to give those details to your AHJ or he may reject the installation.

 

[GoldDigger]

Run a larger feeder then minimum necessary and it gets harder to understand. Say you only need approximately a 200 amp supply conductor and choose to run 400 amp supply conductors for possible future load.... If you decide to protect them above 400 amps (2.5 times the largest motor could put you above 400) you may be justified to do so, but you may want to give those details to your AHJ or he may reject the installation.

Would that also trigger the upsized EGC provision? Larger wire for future loads is not quite the same as larger wire for voltage drop, but the practical effect on EGC current and voltage is the same.

 

[kwired]

Would that also trigger the upsized EGC provision? Larger wire for future loads is not quite the same as larger wire for voltage drop, but the practical effect on EGC current and voltage is the same.

Very good question, this is one aspect I don't like about how they have worded this for EGC selection. And when dealing with motors and higher then otherwise usual overcurrent protection being allowed it just gets more complicated. You run into a lot of why is EGC "A" not permitted to protect same size conductors as same sized EGC "B", all because the overcurrent device is allowed to be different for a motor circuit, or even feeders supplying motor(s).

 

[Smart $]

Would that also trigger the upsized EGC provision? Larger wire for future loads is not quite the same as larger wire for voltage drop, but the practical effect on EGC current and voltage is the same.

Good question, and I believe the parameters make a difference.

If you upsize the OCPD rating beyond permitted for a feeder (not permitted for branch circuits), you have to upsize the conductor size (ampacity). Code is not quite clear to treat this, but you may have to upsize the EGC just because the OCPD rating is greater.

If you upsize only the conductor, upsizing the EGC under 250.122 kicks in.

This exemplifies the conundrum brought up countless times, where you can increase both OCPD rating and conductor size and use a smaller EGC than if you just upsized the circuit conductor and 250.122(B) kicks in.