mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
Is there anything in the code that allows for an OCPD to be larger than the conductors feeding an MCC?
MCC Feeder OCPD
- Thread starter mbrooke
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Greentagger
Senior Member
- Location
- Texas
- Occupation
- Master Electrician, Electrical Inspector
Would 240.4(B)(1)-(3) fit your installation?
- Location
- Illinois
- Occupation
- retired electrician
Take a look at Part V of Article 430, but it appears to me that the application of 430.62(A) will actually require conductors with an ampacity greater than the rating of the OCPD for many MCCs. It will change based on the actual combination of motors fed from the MCC.
All of the ones I installed have just had the conductors sized like any other feeder, that is the conductor ampacity is equal to or greater than the OCPD rating.
All of the ones I installed have just had the conductors sized like any other feeder, that is the conductor ampacity is equal to or greater than the OCPD rating.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
Its over 800 amps, but good reply.
mbrooke
Batteries Included
- Location
- United States
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- Technician
I'm going to have to re-read this one a few times.
FWIW, all the motors are continuously run for more than 3 hours many 24/7.
- Location
- Illinois
- Occupation
- retired electrician
I don't think that makes any difference for most motor loads...the conductors are all required to be sized at 125% of the motor full load current as found in the Article 430 tables.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
Right, but the NEC doesn't let the MCC feeder OCPD be sized at 250%.
- Location
- Illinois
- Occupation
- retired electrician
It is sized at 250% of the largest motor plus the sum of the full load currents of all of the other motors.
Why would you need this?
Let’s say nuisance tripping is an issue. The breaker is there to protect the feeder and if it’s MLO the MCC. So if you had say only one bucket then nuisance tripping would be an issue if you sized for 125% of the motor FLA. The bucket has its own breaker. But if you sized the feeder to 200% of FLA (250% x 80%) then you get a breaker at 250%. Of course you’d want larger for coordination reasons. Anything else is less severe due to diversity...this is worst case.
The one and only issue with this is if the feeder breaker doesn’t protect the MCC busbar. But you still can use an MCC with a main breaker which is governed by UL, not NEC, and most likely would be coordinated via either dynamic resistance or just TCCs. It would have either a slow or no instantaneous for instance.
Typically with feeders to MCCs we do not just take 125% of the load FLAs for two reasons. One is avoiding LRC. Second is it’s an MCC...you WILL swap out components from time to time. Often they are specified with at least 25% extra capacity. An 800 A horizontal bus with 300 A verticals is common. No bucket will ever reach 800 A by definition.
xptpcrewx
Power System Engineer
- Location
- Las Vegas, Nevada, USA
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- Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Yes and no.
430.62(A) may provide a larger OCPD rating/setting than the ampacity of the conductors; however, if this OCPD is also protecting the MCC, then 430.94 requires the rating/setting be limited to the rating of the common power bus.
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