OT: Breaker Sizing For Control Panel

I need to ask for help with Sizing a MCC bucket / breaker for a control panel. This panel has the following motors

5 HP 5.7 FLA
30 HP 36.0 FLA
3 HP 2.3 FLA
1 HP .25 FLA

Adding these up a 60 amp service should handle it but one of the guys here at work said to multiply the largest motors fla times 2 then add the rest and round up to the next size service. Doing it this way would be a 100 amp breaker?

Also we already have a spare 125 amp breaker bucket in the MCC would this be okay to use? WE have not ordered the control panel disconnect which the boss wants to be a breaker also. Would the control panels breaker need to be 100 amp or 125 amp in this case?

I have heard 1.25 X largest and adding the rest.

Breaker Sizing

Our licensed (not me) electrician said to add all the motor amperages, then multiply times 1.25. There are other factors involved that are shown in the 2011 NEC code book. Please look under article 430.
Hope this helps.

I would probably use the 125 Amp bucket since its spare, and then use a smaller breaker in the disconnect panel. That way, the only thing the 125 amp breaker is protecting is the wire between the bucket and the panel since theoretically the smaller breaker will trip before the larger one does. As for the smaller breaker, if those are your only loads, i'd probably go with around a 70 Amp breaker.

a 60 amp breaker would be fine cos , it is impossible to reach the optimum current for all the motors in the same time , even if it happens , you still covered by 100% of the load consumption , we used to calculate 1.25 of the full load for a single motor breaker , but this is different cos you need one breaker for all , which would be fine to go through 60 amp breaker

I use 1.5x the largest, then add the rest, then the next available size. So I'd be recommending the 100A

For what I read here, I know you are not going to agree with me, but I always size the main breaker to protect the power cable I will be using. Of course the cable has to be size according to the FLA. But since the panel needs some space for the future, usually the cable is oversized. To size the cable you have to take into account the cable distance, the temperature, the amps, etc.

I am in nonuke's camp. What I used to (for main breaker in the control cabinet) was add all motor amps x 1.25 plus all other significant loads (heaters ect...) plus educated guess for all miscellaneous. Take that total then use next size breaker. If the total was close I would jump a breaker size for future stuff. All motors and heaters transformers had their own breaker or fuse protection.

Tim Ganz:
I would run the correct wire size for the 125 amp breaker to the control cabinet. Connect this wire to your 100 amp breaker in the control cabinet.

Tim,

Use the 125 Amp breaker in the MCC, run Size #1 AWG wires with ground to the new control panel. Inside the panel, you could use a 60, 70, or 100 Amp breaker or fused disconnect. The size of the control panel overcurrent device depends somewhat on the voltage supply level (575, 480, 460, 400, volts?) AND also on what equipment the 30 HP motor is running. If it is a 30 HP 480 volt air compresser, then go with 2X the rated motor FLA for this one, which probably means a 100 Amp control panel circuit breaker powering 4 motor starters (one NEMA Size 3, two Size 0, and one Size 00).

On the other hand, if the 30 HP motor is running a fan, then you can get by with a 60 or 70 Amp main control panel breaker.

Would the control panels breaker need to be 100 amp or 125 amp in this case?

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Obviously you are confused about what is important here. For the existing 125 Amp MCC breaker, because it is larger than needed, you must size the wire connected to this breaker to handle the full 125 possible Amps in a short-circuit situation. On the other hand, the device in the panel needs to be sized for the largest operating (running) load that it will see. You want to keep this device as small as possible, yet large enough to handle the largest total motor starting current that is possible. The control panel breaker should never see 125 Amps, but it might see 100 (depending on the type of motor loads). It might never even see 70 Amps, so that is why it is important to know (not only the motor size, volts, and FLA), also what type of device the motor is running. Another factor is whether all 4 motors can start at once, or do they start seqentially? If sequentially, and there is no heavy starting load such as a compressor or loaded conveyor, then you possibly could use a 60 Amp breaker. This type of problem depends a lot on knowledge, but also experience.

don't worry so much about the size of the main breaker as the size of the individual breaker(fuse) feeding each motor. The main disconnect in the pannel could even be non-fused (200amp) I would use a distrubiton block to feed the other breakers. as lancie said you must use feeder wire rated for your mcc breaker. Cost has a lot to do with your selection the difference between a 100 and 200 amp fuse/breaker and a non fused should be considered.among many other things such as are you Qualfied (by law) to build this panel and install the feeder see: http://www.license.state.tx.us/electricians/eleclaw.htm

Well, according to Sec. 1305.003 Exemptions; Article 7, I could build the panel and ship it to him. If it is a retrofit, repair, or maintenance, then Tim could do it without a license. Of course it is always best to know what you doing!

Sec. 1305.003. Exemptions;
(7) work involved in the manufacture of electrical equipment that includes the on-site and off-site manufacturing, commissioning, testing, calibrating, coordinating, troubleshooting, or evaluating of electrical equipment, the repairing or retrofitting of electrical equipment with components of the same ampacity, and the maintenance and servicing of electrical equipment within the equipment’s enclosure that is performed by an authorized employee or authorized representative of an electrical equipment manufacturer and limited to the type of products manufactured by the manufacturer;

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(7) work involved in the manufacture of electrical equipment that includes the on-site and off-site manufacturing, commissioning, testing, calibrating, coordinating, troubleshooting, or evaluating of electrical equipment, the repairing or retrofitting of electrical equipment with components of the same ampacity, and the maintenance and servicing of electrical equipment within the equipment’s enclosure that is performed by an authorized employee or authorized representative of an electrical equipment manufacturer and limited to the type of products manufactured by the manufacturer;

yes he could work on a panel you built for him. NO he can not run the feeder or build the panel without a license! (legally)

NO he can not run the feeder or build the panel without a license! (legally)

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That is not what the law that you quoted says. It says that there are many exceptions where a person without a license COULD be qualified to do certain work. For example, if he is an authorized employee of a Texas plant involed in the production or manufacture of oil. You need to read the list of exemptions before you quote this law. It does not say what you have been told.

Sec. 1305.003. Exemptions;
(14) electrical work performed at a business that operates:

(A) a chemical plant, petrochemical plant, refinery, natural gas plant, natural gas treating plant, pipeline, or oil and gas exploration and production operation by a person who works solely for and is employed by that business; or
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Yes Lancie there are exemptions!!! thats wy i posted this:

should be considered.among many other things such as are you Qualfied (by law) to build this panel and install the feeder see: http://www.license.state.tx.us/electricians/eleclaw.htm

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