MCC PDC Sizing

[Snyder5150]

The main for one of my MCC's that the contractor has installed is under sized as far as i see it. I ran the numbers and i come up with do not add up. using 430.24 and no other exception are needed. 1471.8 FLA looks like it comes out to 1839.7. I have a total of 1471.8n hp. The main that is instaled is a 1200 amp Masterpact NT. any thoughts?

 

[petersonra]

The main for one of my MCC's that the contractor has installed is under sized as far as i see it. I ran the numbers and i come up with do not add up. using 430.24 and no other exception are needed. 1471.8 FLA looks like it comes out to 1839.7. I have a total of 1471.8n hp. The main that is instaled is a 1200 amp Masterpact NT. any thoughts?

The main is there to protect the bus of the MCC. presumably it is 1200A bus.

430.24 Several Motors or a Motor(s) and Other Load(s).
Conductors supplying several motors, or a motor(s) and other
load(s), shall have an ampacity not less than the sum of each of
the following:
(1) 125 percent of the full-load current rating of the highest
rated motor, as determined by 430.6(A)
(2) Sum of the full-load current ratings of all the other
motors in the group, as determined by 430.6(A)
(3) 100 percent of the noncontinuous non-motor load
(4) 125 percent of the continuous non-motor load.

I think you have multiplied the combined FLC by 125% to get the ampacity required.

It is only 125% of the biggest motor FLC plus 100% of the FLC of the other motors.

But this is just for the CONDUCTOR ampacity. Has nothing to do with sizing the main of a MCC.

 

[Snyder5150]

I see the correction. Thank you. My load is 1558.05 The MCC is a 1200 a bus. The feeders are parallel 500 at 1824 amp but the PDC is fused with bussman KLU 1000. What is it I'm missing?

 

[petersonra]

I see the correction. Thank you. My load is 1558.05 The MCC is a 1200 a bus. The feeders are parallel 500 at 1824 amp but the PDC is fused with bussman KLU 1000. What is it I'm missing?

Your initial observation was the MCC main was undersized. If it has 1200A bus and a 1200A main CB it is not undersized.

I don't know what you mean by PDC. power distribution ????

I don't know why someone would fuse such an installation with a 1000A OCPD.

Maybe they did a power study and found 1000A was adequate.

Why do you care other than just curiosity. Idle speculation about why it may or may not have been done this way won't solve the problem if there is a problem.

 

[topgone]

I see the correction. Thank you. My load is 1558.05 The MCC is a 1200 a bus. The feeders are parallel 500 at 1824 amp but the PDC is fused with bussman KLU 1000. What is it I'm missing?

Remember that motors of different functions means different demand factors. Also, not all motors run at the same time so you have to apply a diversity factor. Maybe the designer found out that the actual loads do not reach the full loads.

 

[Snyder5150]

Your initial observation was the MCC main was undersized. If it has 1200A bus and a 1200A main CB it is not undersized.

I don't know what you mean by PDC. power distribution ????

I don't know why someone would fuse such an installation with a 1000A OCPD.

Maybe they did a power study and found 1000A was adequate.

Why do you care other than just curiosity. Idle speculation about why it may or may not have been done this way won't solve the problem if there is a problem.

The root concur for the mater is that all of the motors are countesses duty and running all shift. The four largest motors are 300 hp at 345 fla. I will see 1300 to 1400 amps on the CT's during heavy loading.

The Main CB has had some over current faults. I am not wanting the fuses at the PDC to fault.

Even with a power study i have a hard time understanding why the OCP is lower then the calculated load?

 

[Ingenieur]

The root concur for the mater is that all of the motors are countesses duty and running all shift. The four largest motors are 300 hp at 345 fla. I will see 1300 to 1400 amps on the CT's during heavy loading.

The Main CB has had some over current faults. I am not wanting the fuses at the PDC to fault.

Even with a power study i have a hard time understanding why the OCP is lower then the calculated load?

if 4 x 300 hp at 345 fla/each all runnng simultaneously and continuously is accurate there is a problem
even if the motors are not fully loaded the 1200 mcc mcb and 1000 a feeder fuse look too small
even the mcc bus bar might be too small depending on arrangement/layout
what size starters? what type? full voltage, reduced voltage, vfd, etc
what size cb on each motor starter?

have you amp'ed the motors?

 

[Snyder5150]

if 4 x 300 hp at 345 fla/each all runnng simultaneously and continuously is accurate there is a problem
even if the motors are not fully loaded the 1200 mcc mcb and 1000 a feeder fuse look too small

have you amp'ed the motors?

Yes and a High Pot test on all four. A clean bill of health . With out going in to all of the math i have found that 1558 for the MCC is correct. I am trying to find out why one would sing off on this install. The single line is close with 1268 HP and the name plate on the equipment is 1280. I have no consensus about the 12 HP as much as much as printing the single line with the sizing as is. BOO on them or am i missing something?

 

[Smart $]

...
Even with a power study i have a hard time understanding why the OCP is lower then the calculated load?

Has the loading been increased since the MCC was first installed?

It is common practice in an industrial setting for some electrical engineers to design with extra capacity for the addition of future load... but then apply the age-old concept of diversity, ending up with a capacity that really has no extra for future load, if not less than enough to handle the true calculated load. It all looks good on paper, but it really is not NEC compliant.

 

[Ingenieur]

Yes and a High Pot test on all four. A clean bill of health . With out going in to all of the math i have found that 1558 for the MCC is correct. I am trying to find out why one would sing off on this install. The single line is close with 1268 HP and the name plate on the equipment is 1280. I have no consensus about the 12 HP as much as much as printing the single line with the sizing as is. BOO on them or am i missing something?

what were the values when amp'ed?

the mcc must be running very hot

 

[Snyder5150]

Has the loading been increased since the MCC was first installed?It is common practice in an industrial setting for some electrical engineers to design with extra capacity for the addition of future load... but then apply the age-old concept of diversity, ending up with a capacity that really has no extra for future load, if not less than enough to handle the true calculated load. It all looks good on paper, but it really is not NEC compliant.

No new loads added

 

[Snyder5150]

Amped between 195 and 300 depending on difference in the material.

 

[Ingenieur]

Amped between 195 and 300 depending on difference in the material.

only load is the 4 300 hp motors
and all run at the same time
and each amp at 200 to 300 amp
not as bad then

 

[Snyder5150]

only load is the 4 300 hp motors
and all run at the same time
and each amp at 200 to 300 amp
not as bad then

That is per leg per motor. the other motors on the same MCC are contentious as well. If we have a heavy load the CT's will read upper 1350 to 1480 for the MCC

 

[topgone]

That is per leg per motor. the other motors on the same MCC are contentious as well. If we have a heavy load the CT's will read upper 1350 to 1480 for the MCC

How long a time does 1480 amps persist? If it is surging for afew seconds, there's no problem for the 1200A main, IMO.

 

[Snyder5150]

How long a time does 1480 amps persist? If it is surging for afew seconds, there's no problem for the 1200A main, IMO.

About 12 sec every 40 sec or so. I will post all of the numbers.

 

[Snyder5150]

The PDC is fused at 1000 amps
The feeder conductors are parallel 500 kcmil ratted for 1840 amps
The MCC has a max capability of 1200 amps on the horizontal buss so the 1200 amp breaker is sized for the MCC only
The total HP of the MCC is 1280 HP. All are counted as continuous duty
The calculated load of the entire motor load is 1558 amps

I'm thinking that i will be requesting that the contractor needs to re engineer the MCC with a 1600 amp service. I was trying to play the devils advocate and find something that says one can under size the PDC to an overdue MCC. sucks. But the coast and safety to personal and equipment is something i do not want to endure.

I still want some advice

 

[topgone]

The PDC is fused at 1000 amps
The feeder conductors are parallel 500 kcmil ratted for 1840 amps
The MCC has a max capability of 1200 amps on the horizontal buss so the 1200 amp breaker is sized for the MCC only
The total HP of the MCC is 1280 HP. All are counted as continuous duty
The calculated load of the entire motor load is 1558 amps

I'm thinking that i will be requesting that the contractor needs to re engineer the MCC with a 1600 amp service. I was trying to play the devils advocate and find something that says one can under size the PDC to an overdue MCC. sucks. But the coast and safety to personal and equipment is something i do not want to endure.

I still want some advice

My quick calcs tell me that 1600 A is good for 4 X 300 amperes motors! If you go for the full load amperes, you need a bigger main breaker, IMO. But that breaker size fits the 5X500MCM cable you have.

 

[Cow]

We put our Fluke logger on MCC's regularly with a 15 minute averaging sample rate. Give it some time to record. Simply amp clamping one motor at a time for a few seconds isn't real world.

 

[Smart $]

My quick calcs tell me that 1600 A is good for 4 X 300 amperes motors! If you go for the full load amperes, you need a bigger main breaker, IMO. But that breaker size fits the 5X500MCM cable you have.

We put our Fluke logger on MCC's regularly with a 15 minute averaging sample rate. Give it some time to record. Simply amp clamping one motor at a time for a few seconds isn't real world.

I do not understand what all the hoopla is about. Code only permits two ways to determine demand for an MCC (i.e. its feeder). One is a standard load calculation under 220 part III. The other is 220.87 using demand log as noted by Cow. Under standard calculation, we cannot use partial loading or nameplate data for motors. Code requires using Table data for motors. Just (4) 300HP 460V 3Ø coincidental-load motors puts the minimum feeder ampacity at 361A × 425% = 1534A. With additional motor loads on top of that, what more needs to be said about the MCC being 1200A???