Can I add Motor to existing feeder and sleep at night?

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factoryrat

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We have an existing control panel feeding: six 3-phase, 480v motors. Three of the motors are 20hp, one mtr is 7.5hp and two mtrs are 1hp. The plant wants to add one additional 1hp, 3ph, 480v mtr.

When I use the NEC to calculate the existing motor load I come up with a feeder that has to supply approximately 103 amps. I also have to supply additional loads: Daykin = 3a, PLC5 = 2a, Remote I/O rack = 4a. Now my calculated load for this existing feeder is up to 112amps.

With everything running (motors, processor and Daykin)I read the full load current, on the supply feeder, to the panel to be only 34 amps per phase. The motors run conveyor chain and roller beds. The motors are drawing less than half their FLC (approximately 9amps for each 20hp Mtr).

My supply feeder to panel is #2 AWG (good for 115a). The panel disconnect is currently fused with 80a fuses. The supply feeder comes from a fused-100a buss plus/disco.

My question is can I add another 3ph, 480v - 1hp mtr? If I did this I come up with a calculated load of 114 amps. The calculations tell me to go a little larger on the buss plug size but the actual situation seems to indicate an additional couple ampere load would not overload the existing wiring.

The motors do not all start at the same time but can all run at the same time.

We have never had any problems with existing panel – We are looking to save cost by using existing wiring,buss plug,etc.. I would appreciate the thoughts any of you may have on this. I think I’ll be o.k. adding the additional motor. Am I violating anything in the NEC?
Thanks in advance.
 
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Sounds like you could change your 20 hp motor to a 10hp.

Would 430.26 do you any good? " the character of their loads and duties"

I'd be inclined to do it.
 
I would not even worry about it. Another 1 HP motor is not going to hurt anything, even if it might be a tad off per NEC load calculations.
 
realolman said:
Sounds like you could change your 20 hp motor to a 10hp.
Would 430.26 do you any good? " the character of their loads and duties"
I'd be inclined to do it.
Click to expand...

Yes, I think these 20HP motors were originally used to handle larger/longer conveyors.
Would there be any savings by replacing the 20HP motors with 10HP? Probably not if you had to go out and buy three new 10HP motors and then replace the existing 20HP motors.
430.26 is interesting. I will have to read and think about it for some time.


petersonra said:
I would not even worry about it. Another 1 HP motor is not going to hurt anything, even if it might be a tad off per NEC load calculations.
Click to expand...

Reading both replies (I respect your opinions) I feel safe adding the additional 1HP motor. I want to show my superiors I know what I am talking about. Also I do not want to screw this up.

I feel more confident now. Thanks for feedback
 
factoryrat said:
Yes, I think these 20HP motors were originally used to handle larger/longer conveyors.
Would there be any savings by replacing the 20HP motors with 10HP? Probably not if you had to go out and buy three new 10HP motors and then replace the existing 20HP motors.
...
Click to expand...

You will have higher losses on an unloaded 20HP motor compared to a more fully loaded 10HP motor. So much so that if your utility has an energy retrofit assistance program (as many do), they may even contribute towards the cost of paying for 10HP energy efficient motors!
 
factoryrat said:
Would there be any savings by replacing the 20HP motors with 10HP? Probably not if you had to go out and buy three new 10HP motors and then replace the existing 20HP motors.
Click to expand...
The short answer is changing the motors to 10HP would save on energy costs, particularly if you use high efficiency motors as Jraef has suggested. It would also improve power factor. Whether the savings would justify the costs in changing motors and protection devices is something that would need to be calculated for the particular circumstances. It gets done so there should be some merit in considering it.

A thought.....
If you do go down the route of considering a motor change, you might want to monitor what power the motors are actually taking. Power, not just current. You mentioned that you had measured 9A in a 20HP motor. That's less than half rated current and likely to be considerably less than half rated power. It's not unusual for a three phase motor to take 30% FLC or more at no load, so current, by itself, is not a particularly good indicator of motor load.
It's possible that you could get by with motors smaller than 10HP - provided they had enough torque to start the conveyers, of course.
 
Thank You

Thank You

Thanks again for great replies. I will share these posts and ideas with my boss. Me and my fellow electricians would love to do the work of downsizing the motors.

Right now our Corporate Bosses & Union Leader is in Washington DC begging for billions of $$$.

Things are bad here in Detroit?s Auto Industry! - Layoffs and plant closings do not surprise me anymore. I do not have any answers but I hope this mess gets better soon or I'll (along with many friends) be looking for a new job.
 
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