Simultaneous Motors Starting, 9 @ 75HP-3PH AC motors

[AHE Tom]

I have an application where 9 75hp 3ph AC motors will start simultaneously all being fed out of the same control panel.

I must size the control panel feeder, feeder breaker accordingly, and the service disconnect (circuit breaker) that will be integral to the control panel.

The NEC indicates to take the first motor at 125% and add the remaining motor FLA's.
I am thinking this is incorrect as taking the first motor at 125% will take into account the starting current of the largest motor, and this is correct, but the remaining 8 75HP motors will also start at the same time.

The only item I found in the code is in 430.24 (handbook note) that indicates that the
"feeder protective device of higher rating or setting is based on the simultaneous starting of two or more motors, the size of the feeder conductors is required to be increased accordingly"
I take this as the feeder shall be sized upon the starting loads of the motors and not just taking the largest at 125% and adding the FLA's of the rest.

Can someone point me to another code are that would address my concerns

Thanks

 

[kwired]

I have an application where 9 75hp 3ph AC motors will start simultaneously all being fed out of the same control panel.

I must size the control panel feeder, feeder breaker accordingly, and the service disconnect (circuit breaker) that will be integral to the control panel.

The NEC indicates to take the first motor at 125% and add the remaining motor FLA's.
I am thinking this is incorrect as taking the first motor at 125% will take into account the starting current of the largest motor, and this is correct, but the remaining 8 75HP motors will also start at the same time.

The only item I found in the code is in 430.24 (handbook note) that indicates that the
"feeder protective device of higher rating or setting is based on the simultaneous starting of two or more motors, the size of the feeder conductors is required to be increased accordingly"
I take this as the feeder shall be sized upon the starting loads of the motors and not just taking the largest at 125% and adding the FLA's of the rest.

Can someone point me to another code are that would address my concerns

Thanks

That 125% of the largest motor is not so much for starting as it is for minimum ampacity adjustment for a continuous load. If these were intermittent duty motors you don't have that 125% rule to follow.

If you wish to increase conductor sizes because of voltage drop during starting nothing in the NEC prohibits that, and it doesn't tell us how to do it either, but no matter what they want a minimum of 125% of the largest motor plus all others for minimum conductor ampacity.

 

[david luchini]

The only item I found in the code is in 430.24 (handbook note) that indicates that the
"feeder protective device of higher rating or setting is based on the simultaneous starting of two or more motors, the size of the feeder conductors is required to be increased accordingly"

This seems to be referring to 430.62 and 430.24.

430.24 would allow you to size the feeder for your 9 motors (assuming 480V) at 96*9.25 = 888A (say 3 sets of 350mcm.)

430.62 would allow you to protect the feeder with the 888A ampacity at 250 + 96*8 = 1018 => 1000A c/b.

The quoted text suggests that if the 1000A c/b won't hold for the simultaneous starting of all 9 motors, you can increase the feeder c/b (say to 1200A) BUT you must increase the feeder conductor size to be protected at its ampacity by the 1200A c/b. So you would have to increase your feeder size to say, 3 sets of 600mcm, even though the motor load is the same. Hope this makes sense.

 

[augie47]

There are folks on here far more qualified to comment on things like "demand factor billing, etc" but with that possibility plus another handful of service/POCO equipment issues (unless this is a huge facility), I can see lots of reason that a few time delay relays would be well worth the investment. Is there any reason the start-up can't be staggered a bit ?

 

[AHE Tom]

Looking through 430.110 Ampere Rating and Interrupting Capacity, C, (1) For Combination Loads, and C, (2) Ampere Rating

I am thinking 430.110, C, (2) is where I should be looking

It indicates to take the sum of the FLA's at 115%

Is this correct


Thanks again

 

[augie47]

As I read it, you would also have to satisfy 430.110(C)(1) taking the sum of the FLA and LRA currents and treating that as a single motor.

 

[Jraef]

As I read it, you would also have to satisfy 430.110(C)(1) taking the sum of the FLA and LRA currents and treating that as a single motor.

that's how I would interpret it too. You have, in essence, a 675HP motor.

 

[david luchini]

Looking through 430.110 Ampere Rating and Interrupting Capacity, C, (1) For Combination Loads, and C, (2) Ampere Rating

I am thinking 430.110, C, (2) is where I should be looking

It indicates to take the sum of the FLA's at 115%

Is this correct


Thanks again

430.110 refers to disconnect sizing, not to overcurrent protection as you referenced earlier. Generally, each motor controller would require its own disconnecting mean, rather than treating the group as one motor (unless you comply with 430.102(A) ex 2.)

You mentioned control panel. Maybe you should be looking at Article 409?

 

[AHE Tom]

Yes, no time delays, these motors all serve a single production line in where the motors must all start at the same time so the product is not damaged due to one or other motors mot starting in a synchronous fashion

 

[petersonra]

Yes, no time delays, these motors all serve a single production line in where the motors must all start at the same time so the product is not damaged due to one or other motors mot starting in a synchronous fashion

I have a hard time believing you can make all the motors start simultaneously and still keep everything synched up. They won't all shut down in the same relative position after the power failed and they won't all start up and run with the same accel rate.

Having said that, it is a lot of motor load to start across the line in one chunk. A lot of utilities would look at such a thing very suspiciously.

 

[augie47]

are the across-the-line start ?

 

[AHE Tom]

Yes, must size the circuit breaker feeding the control panel, feeder, and the disconnect (circuit breaker) that is going to be located in the control panel
409.30 Disconnecting Means references Part IX 430
409.20 Conductor Minimum Size and Ampacity indicates to take the larges motor at 125% and add the remaining FLA's, this will work for the largest motor starting, all 9 75HP motors will start at the same time, so this is not of help either.

I can not find anywhere in the code that specifically addresses my situation

 

[AHE Tom]

All motors are on VFDs, almost no ramp times, starting under a high torque situation

 

[AHE Tom]

all motors a 460VAC 3ph, each motor will have it's own respective circuit breaker within the control panel
there are also other loads that will be served from this control panel, I was just trying to keep my question simple

I am going to go with 430.110 C (1) and (2) for now, as even the 409.30 refers me to this section, and size the feeder accordingly

This is not the only line like this in this plant many more to upgrade, they use a lot of energy

 

[GoldDigger]

All motors are on VFDs, almost no ramp times, starting under a high torque situation.
.
.
.
I am going to go with 430.110 C (1) and (2) for now

OK, so the OCPD needs to be large enough that the listed input current for all of the VFDs combined will not trigger the instantaneous trip of the common breaker.
In addition, the combination of the full input current of the 9 VFDs over the starting period must be well within the current-time curve of the breaker to avoid nuisance trips.
Both of those are design issues rather than NEC issues.
Then you have to figure out what minimum size breaker you need according to the NEC. In your case the NEC says that you may need engineering input to decide that. See above.
But 430.11(C) looks like a good hook to hang from.
And finally, whatever size breaker you end up with, you must then size the conductors so that they are protected by the common breaker and by the individual motor overloads in the case of taps.

Sounds simple. :happysad:

 

[AHE Tom]

Yes in regards to the VFD input current (105A), if I take the sum all my loads (motor FLA's and misc loads) at 115% it just appears to be lower than I am comfortable with

I spoke to the VDF mfg tech. and they are recommending to take the VFD input current at 150% (as their VFD can produce 150% for up to 30 seconds) due to the constant torque starting of the motor/VFD combination, this will definitely work, and could be the answer.

I am just looking for the correct and accurate way to size the feeder and the circuit breaker as per the NEC that is to feed the control panel.


Thanks again for all the input on this topic

 

[kwired]

All motors are on VFDs, almost no ramp times, starting under a high torque situation

Very little ramp time will still be a much lower surge at starting then the surge you will see if they are started across the line, for the most part you will not accelerate any faster than across the line acceleration would be, but you are still going to start out at a reduced voltage and frequency which will result in starting at lower current as well.

 

[Jraef]

... A lot of utilities would look at such a thing very suspiciously.

I'd call that an understatement...