Power System Sizing

[Stanley Straughter]

When a system has two motors connected to a bus, with one of the motors being a stand-by, and will only run when the other is not running. Is the non-running motor defined as non-continuous, and have to follow the NEC sizing of 100% non-contiuous plus 125% continous?

 

[topgone]

When a system has two motors connected to a bus, with one of the motors being a stand-by, and will only run when the other is not running. Is the non-running motor defined as non-continuous, and have to follow the NEC sizing of 100% non-contiuous plus 125% continous?

Continuous - maximum current is expected for 3 hours or more
Non-continuous - maximum current expected less than 3 hours

 

[bluesmoke...oops]

I would call the two loads non-coincident and if they are the same size, use the same circuit if possible. Job requirements don't always let you do that though.

 

[LarryFine]

Stan, welcome to the forum!

When a system has two motors connected to a bus, with one of the motors being a stand-by, and will only run when the other is not running.

If the motors cannot run simultaneously, you should not need to count them as two loads. Each should still be supplied as if it's the only one.

 

[kingpb]

I agree with Larry. There is one question you need to ask yourself, that is, will the second motor be started and operating before the other is shutdown, or can the first be shutdown, then the second one started.

In the first case you would need to design for it's operation. In the second case, I would not count it in the load calc.

 

[Stanley Straughter]

Power System Sizing

Thanks for the welcome, and to all that replied.

 

[weressl]

I agree with Larry. There is one question you need to ask yourself, that is, will the second motor be started and operating before the other is shutdown, or can the first be shutdown, then the second one started.

In the first case you would need to design for it's operation. In the second case, I would not count it in the load calc.

The AHJ may stipulate that a simple interlock of the two contactors, IF it is fed my two independent means of disconnect may not be an acceptable assumption as a simple control rewiring in the future can change that, and we are all familiar with Mr. Murphy and his laws. However if there are two contactors feed by a selector switch then the system can only 'see' one load at a time.

The above argument is based on the same logic as the argument of NFPA/OSHA of not accepting control circuit as an acceptable means of disconnect.