MCC Sizing

[davidb123]

Gentlemen,

MCC BUS

I am wondering if the way I think about how to size MCC, Cable et switchboard breaker is right. Lets say I have a 480V MCC with twelve 50hp motors and a 100hp motor. So, by the values in table 430.250 and by article 430.24 , the bus rating shall not be less than :12x65A + 1x124x1,25 = 935A . I consider that the bus is "the conductor that supply several motors..."

For the overcurrent protection device (based on 430.62) the main breaker of the MCC shall not be greater than the OCPD of the biggest motor + FLA of other motors. Let's assume that I've used fast acting fuses on the 100hp, 175% of 124A = 217A + all other FLA : 217A + 12x65A = 997A

So, I could chose a 1200A bus with main breaker of 1000A. Make sense ? What if I plan to add load in the future and go for a 1600A bus. I would not have the right to put a main breaker of 1600A (cause I know I will need it) even if every bucket in my MCC has its own OCPD... ?

Cable Size from switchboard to MCC

Could the ampacity of the cable that feeds the MB of the MCC be rated according to the connected load ? or HAS (make sense if I plan to add load) to be rated according to the bus size ? Let's say I go with 1200A, as I consider my facitlity as a Supervised Industrial Installation, 240.91 says that feeder cable shall be rated not less than 95% of the "load" so I could go with 3x3c500MCM for 1140A (which is exactly 95% of 1200)... Right ?

Any help will be really apprecitated,

Thanks guys

 

[Smart $]

I am wondering if the way I think about how to size MCC, Cable et switchboard breaker is right. Lets say I have a 480V MCC with twelve 50hp motors and a 100hp motor. So, by the values in table 430.250 and by article 430.24 , the bus rating shall not be less than :12x65A + 1x124x1,25 = 935A . I consider that the bus is "the conductor that supply several motors..."

Good by me.

For the overcurrent protection device (based on 430.62) the main breaker of the MCC shall not be greater than the OCPD of the biggest motor + FLA of other motors. Let's assume that I've used fast acting fuses on the 100hp, 175% of 124A = 217A + all other FLA : 217A + 12x65A = 997A

I believe you have to use the same type of protective device rating for the largest motor as you will be using for the MCC. Say your MCC main is a breaker, then the rating is based on 250% × 124 + 12 × 65 = 1090A.

BTW, 175% is the rating for delay fuses. For fast acting it's 300%.

So, I could chose a 1200A bus with main breaker of 1000A. Make sense ? What if I plan to add load in the future and go for a 1600A bus. I would not have the right to put a main breaker of 1600A (cause I know I will need it) even if every bucket in my MCC has its own OCPD... ?

If you choose a 1200A bus, you can use a 1200A breaker. 1200A is greater than your 430.24 value (935A per your calc). See 430.62(B).

Cable Size from switchboard to MCC

Could the ampacity of the cable that feeds the MB of the MCC be rated according to the connected load ? or HAS (make sense if I plan to add load) to be rated according to the bus size ? Let's say I go with 1200A, as I consider my facitlity as a Supervised Industrial Installation, 240.91 says that feeder cable shall be rated not less than 95% of the "load" so I could go with 3x3c500MCM for 1140A (which is exactly 95% of 1200)... Right ?

If your switchboard breaker is rated 1200A, you can go with a feeder having an ampacity of 95% of 1200A, or 1140A as you have noted... but it is not based on the load; it is based on the 1200A breaker rating.

 

[Smart $]

PS: Welcome to the forum. :thumbsup:

 

[davidb123]

Thanks for the answer.

Well for the fuses you're right I wasn't meaning fast acting but time delay of course.

What I don't get is you said : If you choose a 1200A bus, you can use a 1200A breaker. 1200A is greater than your 430.24 value (935A per your calc). See 430.62(B).

But 430.62 tells me that the main breaker shall not be higher than the 1090A you got by adding the OCPD of the largest motor + FLA of other motor. Having a 1200A main is violating that article, ain't it ?

 

[davidb123]

Oh nevermind, I just got it by reading again 430.62(B)!

Thank you very much!

 

[Smart $]

 

[Ingenieur]

Sizing the main 1.25 x largest + sum (rest)
does that apply if each motor ckt has its own cb?
or is it sized for the bus?

 

[Smart $]

Sizing the main 1.25 x largest + sum (rest)
does that apply if each motor ckt has its own cb?
or is it sized for the bus?

Not really sure what you are asking...???

Code doesn't really have any provisions for sizing an MCC. It is a general assumption the MCC [main bus] rating must be not less than the minimum ampacity required for its feeder circuit.

 

[petersonra]

Not really sure what you are asking...???

Code doesn't really have any provisions for sizing an MCC. It is a general assumption the MCC [main bus] rating must be not less than the minimum ampacity required for its feeder circuit.

Motor control centers in general are governed by UL standards for motor control centers. In general you have to protect the main bus in some way from over current. That can be either a main on the motor control center itself or it can be protected on the feeder motor control center can be mlo.

 

[Smart $]

Motor control centers in general are governed by UL standards for motor control centers. In general you have to protect the main bus in some way from over current. That can be either a main on the motor control center itself or it can be protected on the feeder motor control center can be mlo.

UL std's have no bearing on the minimum MCC rating required for any one particular use under NEC purview.

 

[Jraef]

Read the exceptions, it says that if the loads are fed from an MCC, use 430.94, which says essentially what Smart $ just said, but clears you of the main OCPD limitations in 430.62b. You can have an MCC with 3000A bus feeding nothing but a single 10HP motor if you like, as long as the feeder circuit TO that MCC is properly sized for a 3000A load and the OCPD matches. You dont even need to have a main breaker IN the MCC itself in fact.

Actually, you could have a 3000A MCC fed by a 100A feeder if you like to waste money...