3 75 HP Motors added to MCC

[tmag28]

Hi everyone,

I need a lot of help here. So I'm interning at this place as an EE...as the only EE working here since we don't have anyone.

So we are installing 3 75 HP motors to a system that has the following

Transformer 12470 - 480Y/277V

Braker 1600AF/16000A

4 600AF/600A breakers that go to an MCC column each

MCC1 load is 405 HP
MCC2 load is 413 HP
MCC3 load is 188.5 HP
MCC4 load is 374 HP

for a total of 1380 HP...

we are looking to add these 3 75 hp motors to MCC3

These motors are 3 phase, 460V, 96FLA

Can anyone help me how to do the load calculations for these to check if we will be ok?

 

[augie47]

Take a look at 430.24 to verify your conductor size and 430.62 for the SCGF protection.
Both basically look at sizing for your largest motor and adding the sum of the Code FLA of other motors.

 

[don_resqcapt19]

If they are all running at the same time and near full load, you will be close to 1900 amps. I would expect that they are not all at full load and not all running at the same time. Most industrial MCCs have connected loads that exceed what is permitted by the rules in the NEC.

 

[kaichosan]

Hi everyone,

I need a lot of help here. So I'm interning at this place as an EE...as the only EE working here since we don't have anyone.

So we are installing 3 75 HP motors to a system that has the following

Transformer 12470 - 480Y/277V

Braker 1600AF/16000A

4 600AF/600A breakers that go to an MCC column each

MCC1 load is 405 HP
MCC2 load is 413 HP
MCC3 load is 188.5 HP
MCC4 load is 374 HP

for a total of 1380 HP...

we are looking to add these 3 75 hp motors to MCC3

These motors are 3 phase, 460V, 96FLA

Can anyone help me how to do the load calculations for these to check if we will be ok?

If I understand this correctly, you will add 3ea - 75HP motors to MCC3. MCC3 has 188.5 total HP connected. If I use the FLA (table 430.250, 75HP has 96FLA) that will be 3x96FLA=288FLA. 188.5HP is approx. 289FLA. Adding 288FLA+289FLA...the total for MCC3 is 577FLA. MCC3 should have a 1600A bus...with 1600AF/1600AT incoming brk. Your MCC3 has the capacity ...1600A, so you have "no worries".

3-#1 Feeder to each 75HP motor should have an ampacity greater than (96FLA x 1.25= 120A) if no voltage drop (maybe apprx 50' away?).

 

[Jraef]

If I understand this correctly, you will add 3ea - 75HP motors to MCC3. MCC3 has 188.5 total HP connected. If I use the FLA (table 430.250, 75HP has 96FLA) that will be 3x96FLA=288FLA. 188.5HP is approx. 289FLA. Adding 288FLA+289FLA...the total for MCC3 is 577FLA. MCC3 should have a 1600A bus...with 1600AF/1600AT incoming brk. Your MCC3 has the capacity ...1600A, so you have "no worries".

Feeder to each 75HP motor should have an ampacity greater than (96FLA x 1.25= 120A) if no voltage drop (maybe apprx 50' away?).

You made one small error, the SWITCHGEAR feeding the MCCs is 1600A, but each MCC is fed by a 600A breaker, so likely the MCC bus is only 600A.

Still OK though.

 

[kaichosan]

You made one small error, the SWITCHGEAR feeding the MCCs is 1600A, but each MCC is fed by a 600A breaker, so likely the MCC bus is only 600A.

Still OK though.

LOL... thanks for the eagle eye. For me, it's easier and faster to look at a single line than read the text description.

 

[tmag28]

You made one small error, the SWITCHGEAR feeding the MCCs is 1600A, but each MCC is fed by a 600A breaker, so likely the MCC bus is only 600A.

Still OK though.

Thanks for the help. Yeah the switchgear feeding the MCCs is 1600A. So I will be ok as long as we don't run at full load right? (not that we will ever run a full load)

 

[dicklaxt]

You don't want to deal with connected loads but with operating loads,,,from what it looks like at a glance it's probably okay. If you look at your onelines and minus out all the spares then put a 60-70 % diversity on all non process related loads it will give you a pretty close picture,If by chance the 3 you are adding has a spare in the mix it ought to be added to one of the other MCC's even if you have to do a little juggling of users.In fact if you are adding primary ,spare and backup it has been done where all 3 need to be on separate MCC's.The norm tho would not be that ,it would be primary either/or with a common spare.

dick