# Disconnects and breakers for panels



## Kiddanimal (Dec 16, 2009)

I know there is a thread for something like this already posted but I am still having a bit of trouble. I am pricing out a panel for a mill that has 3 50hp motors and they will each have a softstart on them. There will be a control power transformer also of 250va. Do I have to have a breaker or a disconnect? The customer has not gotten back to me as to the voltage. It will be 240 or 480. I will be sequentialy starting these motors so they are not all stating at the same time. Any help will be much appreciated. 
Thank you.


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## JayH (Nov 13, 2009)

The customer probably should have a good fire insurance policy.


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## electures (Oct 23, 2008)

Kiddanimal said:


> I know there is a thread for something like this already posted but I am still having a bit of trouble. I am pricing out a panel for a mill that has 3 50hp motors and they will each have a softstart on them. There will be a control power transformer also of 250va. Do I have to have a breaker or a disconnect? The customer has not gotten back to me as to the voltage. It will be 240 or 480. I will be sequentialy starting these motors so they are not all stating at the same time. Any help will be much appreciated.
> Thank you.


Need more info;

1) Voltage
2) Type of GF/SC/OC device for each motor branch circuit.
3) Type of GF/SC/OC device for the feeder.
4) Other loads


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## McClary’s Electrical (Feb 21, 2009)

Tell us what YOU think you need,,,,and we'll go from there.


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## Kiddanimal (Dec 16, 2009)

The voltage: 480
Customer feed is a 300A breaker in the main service panel.
Fuses for each soft start 
The soft start will monitor OL in the bypass mode.
There are timer realys to delay the starting of motor 2 and 3 so that they don't start at the same time.
There are no other loads on this panel and a 3 wire start circut will start this panel. 
This is all there is to this panel.


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## McClary’s Electrical (Feb 21, 2009)

Kiddanimal said:


> The voltage: 480
> Customer feed is a 300A breaker in the main service panel.
> Fuses for each soft start
> The soft start will monitor OL in the bypass mode.
> ...


 
If you have all this in one big cabinet (MCC), I would size a feeder and breaker to 125% of the largest motor, and 100% of the remaining motors. This breaker will protect your feeder. Then, once inside of the MCC I would branch to all loads seperately and either set fuse blocks or breakers , for each motor,whichever you choose or specify. Some equipment will demand fuse protection. You could have a combination of both in this MCC. You can possibly apply 430.26 to adjustthe size your feeder. Keep in mind fuses will not meet as disconnecting means.


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## electures (Oct 23, 2008)

Kiddanimal said:


> The voltage: 480
> Customer feed is a 300A breaker in the main service panel.
> Fuses for each soft start
> The soft start will monitor OL in the bypass mode.
> ...


The following is based on;
1) No other loads in the panel.
2) Each motor branch circuit originates from this panel and is protected by a TD fuse.
3) The feeder breaker to the panel is an ITCB.

*430.6(A) 1* states;
*(1) Table Values. **Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate*. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.

Table 430.250 indicates 65 amps for a 480V 50HP three phase motor.

*430.22(A) General* states;

*(A) General. *Conductors that supply a single motor used in a continuous duty application shall have an ampacity of
not less than *125 percent of the motor’s full-load current rating as determined by 430.6(A)(1).*

65A x 125% = 81.25A

Table 310.16 indicates #3AWG copper is rated for 85A.

Next we look at sizing the ground fault, short circuit, overcurrent protective device for each motor.

430.52 states;

*430.52 Rating or Setting for Individual Motor Circuit.*
*(A) General. *The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.

*(C) Rating or Setting.*
*(1) In Accordance with Table 430.52. *A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)

Table 430.52 states that for a Dual Element (time delay) fuse protecting an AC polyphase motor it is to be sized at 175% of the FLA as stated in Table 430.250.

65A x 175% = 113.75A

Based on the above calculation a 125A TD fuse is correct.

If the motor fails to start and run then 430.52(C) (1) Exception No. 2(b) can be applied which states;

_(b) The rating of a time-delay (dual-element) fuse shall be permitted to be increased but shall in no case exceed 225 percent of the full-load current._

_65A x 225% = 146.25A_

_Since we can’t exceed 225% of the motor FLA the code will only permit a 125A TD fuse on a #3AWG copper conductor._

_Next we address sizing the feeder._

_43-.24 states;_

*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 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads.

65 + 65 + 81.25 = 211.25

Table 310.16 indicates #4/0 AWG copper is rated for 230A.

_Now we size the feeder protective device._

_430.62 states;_

*430.62 Rating or Setting — Motor Load.*
*(A) Specific Load. *A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting *not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52*, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group. Where the same rating or setting of the branch-circuit short-circuit and ground-fault protective device is used on two or more of the branch circuits supplied by the feeder, one of the protective devices shall be considered the largest for the above calculations.

65 + 65 + 125 = 255A

The feeder protective device shall be set at 250A

If the panel supplies other loads in addition to the motor loads then 430.63 applies and reads;

*430.63 Rating or Setting — Power and Lighting Loads.*
Where a feeder supplies a motor load and, in addition, a lighting or a lighting and appliance load, the feeder protective device shall have a rating sufficient to carry the lighting or lighting and appliance load, plus the following:

(3) For two or more motors, the rating permitted by 430.62

Merry *Christ*mas! Enjoy the holiday season!


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## Kiddanimal (Dec 16, 2009)

Wow thank you for the response to my question. I understand the method to figure out how to size for a motor load. Thank you for including the pasages from the NFPA also. I am an electronics/automation guy and have not had to size conductors and branch protection for motors before. 
This panel will be checked before it is built at the UL508 panel shop but now I atleast won't send something that is unsafe and incorrect.
That mean changes and that is not good for deadlines.


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