Confirmation of new load in MCC Feeder

[goodoboy]

Hello,

Excuse my ignorance, I am learning as I go.

I have an existing MCC feeder that feeds about 7 loads. There is a spare bucket I want to use. It's a size 5 starter. The load I want to add is a 480V, 3phase, 150 amp.


What study or calculation I need to perform to adjust the feeder breaker setting to account for this load? One the electrician mention a new arc study is needed.

Thank you,

 

[augie47]

What is the nature of the new load (mortor ? continuous load ? )
\
Without added details it would seem you might not need the starter at all and will only be using the bucket as a feeder breaker.
Basic steps: calculate your load (adding growth factor if desired); select your conductor; select your overcurrent protection.
In this case a couple of potential "pit-falls". (a) does your new load involve a neutral load and does your MCC have a neutral supply and (b) what type of overcurrent device is present ? some MCCs have instantaneous trip breakers designed only for your motor load.

 

[Jraef]

Hopefully you are REPLACING that Size 5 starter with a feeder breaker, because you CANNOT use a starter as a feeder, and you CANNOT use the breaker in the starter as a feeder if it is an "MCP" (mag-only trip) breaker, as is the case in 99% of MCC starter buckets.

 

[goodoboy]

What is the nature of the new load (mortor ? continuous load ? )
\
Without added details it would seem you might not need the starter at all and will only be using the bucket as a feeder breaker.
Basic steps: calculate your load (adding growth factor if desired); select your conductor; select your overcurrent protection.
In this case a couple of potential "pit-falls". (a) does your new load involve a neutral load and does your MCC have a neutral supply and (b) what type of overcurrent device is present ? some MCCs have instantaneous trip breakers designed only for your motor load.

Thank you for replying.

The new load is a pump.

Let start over cause I am still a bit new.

We have something http://prntscr.com/ahq3kv like this. I am installing a new load (pump 480V, 3phase, 150 amp) in one of the bucket with NEMA 5 starter.

To avoid not tripping the entire MCC, i need to set the breaker to the entire MCC to adjust for the new load.

 

[augie47]

150 amp would likely be a 125HP motor which matches your NEMA 5.

Jraef has joined the thread and I will await his far more knowledgeable input

 

[petersonra]

Thank you for replying.

The new load is a pump.

Let start over cause I am still a bit new.

We have something http://prntscr.com/ahq3kv like this. I am installing a new load (pump 480V, 3phase, 150 amp) in one of the bucket with NEMA 5 starter.

To avoid not tripping the entire MCC, i need to set the breaker to the entire MCC to adjust for the new load.

First, you need to do a load calculation for the feeder conductors feeding the MCC. Article 220 will tell you how to do this. If it is all motor loads the calculation is basically to add up the full load currents of all the motors (use the tables in the code and not the nameplates on the motors).

Then you need to determine the size of the feeder coming into the MCC. The ampacity of those conductors will be the calculated load plus 125% of the FLC of the largest motor. That will tell you if the conductors feeding the MCC are large enough.

You can set the main breaker on the MCC to whatever you want as long as it does not exceed the rating of the MCC bus or the breaker feeding the MCC protects the MCC bus. In fact if the breaker or other OCPD that feeds the conductors that feed the MCC protects the MCC bus, you don't even need a main on the MCC.

 

[Jraef]

Bob is right, and that's where to start.

If we assume all of that has been done already, and all you are asking for is advice on the instantaneous trip setting with the new load added, then it's something you are going to have to determine based on your load profile. Basically you need to look at the total possible load that can be running when that large motor starts, and add the locked rotor current of that motor. So let's pretend that your 7 existing loads can all be running when the new 150HP motor is turned on and those 7 loads add up to 480A total (based on FLA). The 150HP motor is going to be 180A from the NEC table, so assume the LRC is 1080A. The instantaneous trip setting of the breaker feeding the MCC (again, ASSuming no main in the MCC) will need to be set at no LESS than 480+1080= 1560A.

You total load for this new MCC lineup would be 660A. Now, ASSuming you had an 800A feeder (and the MCC bus is rated 800A), most 800A Thermal-Mag breakers will have an instantaneous trip setting ranging from 5-10X the breaker rating, so a range of adjustment from 4000 - 8000A. So basically if you have chosen your breaker correctly for the load as Bob had said, setting it to not trip on starting that motor should not really be an issue.

If however you have a situation wherein after a power failure, ALL 8 loads can come on at once, then you have to look at the nature of the loads. If they are all AC motors, your potential starting current can be close to 4,000A and if your feeder is set that low, it may trip. Only you can determine all of this.

 

[kwired]

If you had a size 5 starter to begin with, you may have slight difference in motor size from what was originally designed for, but should still be in the same ballpark if it was ever done right to begin with.

If it holds during starting you are probably fine. If not you either need to change the feeder device or maybe utilize reduced voltage starting/soft starter methods.

 

[dkidd]

To avoid not tripping the entire MCC, i need to set the breaker to the entire MCC to adjust for the new load.

That requires a coordination study. They can be done manually by overlaying protective device trip curves, or with commercial calculation software.