Motor feeder calculation

[texie]

I'm struggling for the correct MCC bus/MCC main breaker size for the load below:

2 each 100 HP 480 motors. Table value of 124 amps each. There is also 53 amps of other misc. calculated load fed by the MCC.
Each motor is VFD with a nameplate input of 156 amps. Each VFD has a 200 amp inverse time breaker in the MCC.

Method 1: 124 amps + 200 amps = 324 + 53 = 377 (400 amp breaker/bus).

Method 2: 156 amps + 200 amps = 356 + 53 = 409 (600 amp bus/450 or larger main breaker).

The real issue here is whether method 1 is NEC compliant as if not the the next standard MCC bus sizing is 600 and adds a great deal of cost.

 

[Jraef]

I'm struggling for the correct MCC bus/MCC main breaker size for the load below:

2 each 100 HP 480 motors. Table value of 124 amps each. There is also 53 amps of other misc. calculated load fed by the MCC.
Each motor is VFD with a nameplate input of 156 amps. Each VFD has a 200 amp inverse time breaker in the MCC.

Method 1: 124 amps + 200 amps = 324 + 53 = 377 (400 amp breaker/bus).

Method 2: 156 amps + 200 amps = 356 + 53 = 409 (600 amp bus/450 or larger main breaker).

The real issue here is whether method 1 is NEC compliant as if not the the next standard MCC bus sizing is 600 and adds a great deal of cost.

You have an MCC that offers 400A bus? Most I know of start at 600A.

Where is the 200A coming from, the size of the breakers feeding the drives? There are no strict rules on sizing the MCC bus, but the safe bet is just the total connected FLA, you don't need to size it for the size of the breakers feeding individual loads. If you know for sure that some things will not run when other things are running though, you can even down size the bus for the maximum LOAD it can see. But a good practice is to not skip on bus sizing if there is any chance that someone may want to add loads later. You can't change the bus bars after the fact.

The MCC main breaker only needs to protect the bus size you use in the MCC. It can be smaller if you want, just not larger. I'd just go for 600A bus and breaker.

 

[dkidd]

You probably need 2 vertical sections. Vertical bus comes in 300 amp.

Rockwell

Line power is distributed throughout the MCC via an isolated bus work
structure. The main horizontal bus is in the center of each section. Standard,
center-fed, 300 A rated vertical bus supplies power to the individual units
above and below the horizontal bus for an effective 600 A capacity, providing
virtually unrestricted unit arrangement. An optional 600 A vertical bus
provides 1200 A effective rating.

 

[kwired]

I'm struggling for the correct MCC bus/MCC main breaker size for the load below:

2 each 100 HP 480 motors. Table value of 124 amps each. There is also 53 amps of other misc. calculated load fed by the MCC.
Each motor is VFD with a nameplate input of 156 amps. Each VFD has a 200 amp inverse time breaker in the MCC.

Method 1: 124 amps + 200 amps = 324 + 53 = 377 (400 amp breaker/bus).

Method 2: 156 amps + 200 amps = 356 + 53 = 409 (600 amp bus/450 or larger main breaker).

The real issue here is whether method 1 is NEC compliant as if not the the next standard MCC bus sizing is 600 and adds a great deal of cost.

Minimum feeder size would be 125% of largest motor plus all other loads 155 + 124 + 53 = 332.

The fact you have VFD's moves those two motors to 156 amps each, so 195 + 156 + 53 = 404. (If I knew no other load were to be added and that the 100 hp motors seldom if ever were going to run at full load I might put it on a 400 amp feeder depending on the situation, though code will say it needs minimum 404. If it were load that runs 24/7 though it may be well worth going with at least 600 amp feeder and gear anyway.

There sort of is no minimum overcurrent protection requirement so 400 amp overcurrent device is probably fine if it holds, and it probably will.

 

[david luchini]

Minimum feeder size would be 125% of largest motor plus all other loads 155 + 124 + 53 = 332.

The fact you have VFD's moves those two motors to 156 amps each, so 195 + 156 + 53 = 404. (If I knew no other load were to be added and that the 100 hp motors seldom if ever were going to run at full load I might put it on a 400 amp feeder depending on the situation, though code will say it needs minimum 404. If it were load that runs 24/7 though it may be well worth going with at least 600 amp feeder and gear anyway.

There sort of is no minimum overcurrent protection requirement so 400 amp overcurrent device is probably fine if it holds, and it probably will.

The way I read things, the MCC bus would have to have an ampacity of not less than 155 + 124 + 53 = 332A, per 430.24,
and not less than (156 + 156) * 1.25 = 390A, per 430.122(A).

 

[Smart $]

The way I read things, the MCC bus would have to have an ampacity of not less than 155 + 124 + 53 = 332A, per 430.24,
and not less than (156 + 156) * 1.25 = 390A, per 430.122(A).

+1

 

[Jraef]

+1

430.122 would not apply, that has only to do with the BRANCH circuit conductors feeding the VFD. Most of the time, people use 430.26 as the basis, meaning they use demand factors, and AHJs generally don't question it, assuming that in an industrial environment, people know more about their operating plan than the AHJ will. But if you want to go by 430.24, the 332A value is the MIMINUM conductor size (albeit not the optimum, because leaving room for growth is prudent).

As for the Main Breaker, 430.92 is specific to MCCs and just says that the Main must protect the bus size.

 

[Smart $]

430.122 would not apply, that has only to do with the BRANCH circuit conductors feeding the VFD. Most of the time, people use 430.26 as the basis, meaning they use demand factors, and AHJs generally don't question it, assuming that in an industrial environment, people know more about their operating plan than the AHJ will. But if you want to go by 430.24, the 332A value is the MIMINUM conductor size (albeit not the optimum, because leaving room for growth is prudent).

As for the Main Breaker, 430.92 is specific to MCCs and just says that the Main must protect the bus size.

Almost a +1. Only part I am in disagreement with is your first sentence. The title of 430.122(A): Branch/Feeder Circuit Conductors. The supply conductors (unless you are supplying the MCC with a service) and bus ampacity must be no less than 125% of the VFD(s) rated input current.

 

[Jraef]

Almost a +1. Only part I am in disagreement with is your first sentence. The title of 430.122(A): Branch/Feeder Circuit Conductors. The supply conductors (unless you are supplying the MCC with a service) and bus ampacity must be no less than 125% of the VFD(s) rated input current.

Sure, but you don't need to use the 125% of the VFD rating twice, the 125% of the VFD rating applies to EACH of the VFD circuits, not to the multi-motor circuit. If you have main "conductors" (busbars) rated for 2 identical VFDs, it's automatically going to be sized at more than 125% of the one VFD...

I suppose if you had a 100HP VFD and a 1HP VFD, the combined motor FLA would not exceed the 125% of the 100HP VFD rated amps. So by that reasoning, then yes, my wording is wrong as a general rule. But specific to THIS project, it holds up. I suppose it could be said that the 430.122 rule could be a cross-check issue, in that if the 430.24 total does NOT exceed the 430.122 rule for the largest VFD, then 430.122 would kick in. Highly unlikely though.

 

[david luchini]

Sure, but you don't need to use the 125% of the VFD rating twice, the 125% of the VFD rating applies to EACH of the VFD circuits, not to the multi-motor circuit. If you have main "conductors" (busbars) rated for 2 identical VFDs, it's automatically going to be sized at more than 125% of the one VFD...

There's no language in 430.122 to suggest that the 125% applies only to the largest VFD.

 

[Jraef]

There's no language in 430.122 to suggest that the 125% applies only to the largest VFD.

BranchlFeeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input current to
the power conversion equipment

Singluar.

 

[david luchini]

Singluar.

The word "an" does not suggest that the 125% applies only to the largest vfd.

In this example, motor 1 has power conversion equipment included as part of AN adjustable speed drive system, AND motor 2 has power conversion equipment included as part of AN adjustable speed drive system.

The FEEDER circuit conductors supply power conversion equipment with rated input current of 312A.

 

[Smart $]

Singluar.

Also says system. Two motors, two vfd's, one feeder, one system.

PS: The word equipment can be either singular or plural.

 

[Ingenieur]

I interpret it that it applies to 1 piece of equipment
multiple would be handled like motors
125% x largest + sum of the rest
but it is not clear

it says sections 430 parts I thru IX apply unless modified in part X
there is no multiple motor section in part X so imo the multiple motor rule applies

 

[kwired]

The way I read things, the MCC bus would have to have an ampacity of not less than 155 + 124 + 53 = 332A, per 430.24,
and not less than (156 + 156) * 1.25 = 390A, per 430.122(A).

What about the additional 53 amps of "other misc" for the second calculation?

 

[Smart $]

I interpret it that it applies to 1 piece of equipment
multiple would be handled like motors
125% x largest + sum of the rest
but it is not clear

it says sections 430 parts I thru IX apply unless modified in part X
there is no multiple motor section in part X so imo the multiple motor rule applies

1. The fact the title mentions feeder implies one or more motors controlled by an adjustable-speed drive, possibly one or more motors not controlled by an adjustable-speed drive, and other non-motor loads may be supplied by the circuit conductors mentioned therein.
2. Parts I through IX apply except as modified or supplemented by Part X... and 430.122 modifies the minimum feeder conductor ampacity as determined per 430.24.

 

[Ingenieur]

1. The fact the title mentions feeder implies one or more motors controlled by an adjustable-speed drive, possibly one or more motors not controlled by an adjustable-speed drive, and other non-motor loads may be supplied by the circuit conductors mentioned therein.
2. Parts I through IX apply except as modified or supplemented by Part X... and 430.122 modifies the minimum feeder conductor ampacity as determined per 430.24.

1 no it does not
2 no it does not

there is nothing in X that addresses feeders for mult drives (only ol's)
there I thru IX dertimine mult drive applications

this is my interpretation
you have yours
this should be submitted and clarified

 

[Ingenieur]

http://www.neca-neis.org/code-question-of-the-day/code-question/cqd-for-3-10-2011

excerpt
Where multi-motor installations have individual drives, each drive should be sized for the individual motor. The branch circuit conductors supplying the power conversion equipment for the multi-motor power conversion equipment should be sized according to 430.24.


clear as mud lol

 

[Smart $]

1 no it does not
...

Hmmm. 1 is not debatable from my perspective. What in 1 are you in disagreement with?

 

[Ingenieur]

Hmmm. 1 is not debatable from my perspective. What in 1 are you in disagreement with?

It does not imply more than 1 motor
no more than 'feeder' does in fig 430.1
which references part II 430.22

I would size it 1.25 x 156 + 156 + 1.25 x 53 = 418
in reality based on likelyhood of expansion 600 to a 600 mcc

this would be a good one to submit to the nec for written clarification