2 Motors and 1 Motor Starter.

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MattJ

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I have a main disconnect which supplies a 3 phase motor starter. From that starter supplies 2 service disconnects one for each motor. Only 1 motor runs at a time and is continuous duty. The second is wired up as a back up for quick change over if the first motor should burn up. I just wanted another opinion if there should be a second main disconnect and starter installed for the back up motor to isolate them or if this set up is up to code.


Thanks
 
I don?t see anything non-compliant in what you described, though you haven?t told the entire story. I can?t, for example, pass judgment on the wire sizes, the disconnect ratings, or the impact of the motor being continuous duty. What you are risking, with the situation you describe, is a single failure that can take out both motors. If the motor starter were to fail, having two ?service disconnects,? as you call them, would not change the fact that both motors would now be out of service. So it is a matter of choice for the owner, as to whether to spend some extra money to improve the overall reliability of the system. That will depend on the potential cost of the down time, should the system be out of service for an extended period.
 
MattJ said:
I have a main disconnect which supplies a 3 phase motor starter. From that starter supplies 2 service disconnects one for each motor. Only 1 motor runs at a time and is continuous duty. The second is wired up as a back up for quick change over if the first motor should burn up. I just wanted another opinion if there should be a second main disconnect and starter installed for the back up motor to isolate them or if this set up is up to code.


Thanks
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I do not believe this is compliant for various reasons. Take a look at these provisions and see if you meet all of them.

430.24 I do not think you meet exception # 3.
430.62 (A)
430.63
 
The big one...

430.87 Number of Motors Served by Each Controller.
Each motor shall be provided with an individual controller.

Exception No. 1: For motors rated 600 volts or less, a
single controller rated at not less than the equivalent horsepower,
as determined in accordance with 430.110(C)(1), of
all the motors in the group shall be permitted to serve the
group under any of the following conditions:
(a) Where a number of motors drive several parts of a
single machine or piece of apparatus, such as metal and
woodworking machines, cranes, hoists, and similar apparatus
(b) Where a group of motors is under the protection of
one overcurrent device as permitted in 430.53(A)
(c) Where a group of motors is located in a single
room within sight from the controller location

Exception No. 2: A branch-circuit disconnecting means
serving as the controller as allowed in 430.81(A) shall be
permitted to serve more than one motor.
Click to expand...
 
I'm thinking the only way around 430.87 is to have a transfer switch such that it is both electrically and mechanically impossibly for both motors to be energized simultaneously. Having a disconnect for each motor does not provide the assurance necessary for both to not be energized simultaneously.

Perhaps another option is to rewire a reversing starter, so the reversing relays act as the transfer switch.
 
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or wire the motors plug and cord so you can just unplug the bad one and plug the new one in.

personally, I am not convinced a spare motor does much for you. the motor drives something. is there a spare for it as well?
 
petersonra said:
or wire the motors plug and cord so you can just unplug the bad one and plug the new one in.

personally, I am not convinced a spare motor does much for you. the motor drives something. is there a spare for it as well?
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Yes you are following similar thought process as I am, there has to be a driven load, then comes the question of how are the motors coupled to the load? If your "spare" motor is spinning whenever the load is in operation you are still providing wear on bearings on something that is essentially not doing anything until it is called upon to do its thing, and you are wasting some energy just to spin the rotor. But maybe more information on the driven load will explain better why you would want a spare motor that is connected and basically ready to go. Otherwise many times the spare motor is just something that is on hand and ready to replace the original when the time comes it is needed.

Now some things where alternating relays are used - usually has not only separate motors, but also separate driven load devices, like say a dual pump pumping station.
 
I have seen arrangements like this on conveyors where the motor shaft of the spare motor is not coupled to the conveyor unless it is needed.

never seen anyone try to use the same motor starter for the primary and backup motor though.
 
MattJ said:
I have a main disconnect which supplies a 3 phase motor starter. From that starter supplies 2 service disconnects one for each motor. Only 1 motor runs at a time and is continuous duty. The second is wired up as a back up for quick change over if the first motor should burn up. I just wanted another opinion if there should be a second main disconnect and starter installed for the back up motor to isolate them or if this set up is up to code.
Click to expand...

Trying to read between the slang.

If your service disconnect is really the motor disconnect (also called a LOTO disconnect), it may be possible for it to also be the motor controller if this is a HP rated switch (430.83), especially if it is fusible.
 
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petersonra said:
I have seen arrangements like this on conveyors where the motor shaft of the spare motor is not coupled to the conveyor unless it is needed.

never seen anyone try to use the same motor starter for the primary and backup motor though.
Click to expand...

If you want to be real nit-picky I guess you could say it can't be done and back it up with code sections, but I really don't have much of a problem with it from an application point of view. If you for some reason have both disconnect switches on and run both motors, if the load is light they may run with no troubles, but if load is high enough you will draw too much current and trip the motor overload, kind of sort of one of the things motor overload is for - to trip when there is abnormal load conditions.
 
petersonra said:
I do not believe this is compliant for various reasons. Take a look at these provisions and see if you meet all of them.

430.24 I do not think you meet exception # 3.
430.62 (A)
430.63
Click to expand...

430.24 Ex #3 is easy to meet if a manual transfer switch is included in the circuit. Key interlocks added to the motor disconnect (service) switches should also be acceptable (the NEC does not define "interlocked").
430.53(B) would require each switch be able to provide individual overload protection (e.g. be fused at 125% of FLA) per 430.32(A)(1).
430.62(A) is about maximum size of devices, not minimum. It effectively says the feeder protective device cannot be larger than the largest "active" load protective device.
430.63 is not applicable unless non-motor is also being fed.
430.87 can be complied with if the motor disconnect (service) switches are also controllers per 430.82 and 430.83(A).
 
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