# Motor overloads



## mightyjoe (Sep 20, 2010)

Trying to apply NEC 430.32.A.1 to size motor overloads for an airhandling unit (ahu). Motor 460v, 3phase, 2hp, FLA 3.4amps, SFA 1.5%. Applying 3.4 x 1.25 = 4.25 . As I read 430.32.C this is increased even higher.
After an air balance the motor is over amping and the electrician refuses to increase heater size. Am I right in how this is applied and when beyond motor FLA is not the motor in an overload condition?


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## nitro71 (Sep 17, 2009)

What's over amping? Is it blowing the overloads or tripping the breaker feeding it?


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## Jlarson (Jun 28, 2009)

mightyjoe said:


> After an air balance the motor is over amping and the electrician refuses to increase heater size. Am I right in how this is applied and when beyond motor FLA is not the motor in an overload condition?


Sounds like you are trying to move too much air.


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## jwjrw (Jan 14, 2010)

Did it ever occur to you he wouldn't upsize the heater for a reason?:whistling2:


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## nitro71 (Sep 17, 2009)

If the motor is overloading increasing heaters won't fix it. Motor will just burn up then.


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## RIVETER (Sep 26, 2009)

Check the makeup air. If you are starving the fan on the motor it will draw more.


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## Jlarson (Jun 28, 2009)

RIVETER said:


> Check the makeup air. If you are starving the fan on the motor it will draw more.


Moving less air should cause a drop in amps.


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## RIVETER (Sep 26, 2009)

Jlarson said:


> Moving less air should cause a drop in amps.


Moving less air...but trying to...and can't.


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## Magnettica (Jan 23, 2007)

Wait a second... if this is an air handling unit won't there already be overload protection built in to the motor?


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## mattsilkwood (Sep 21, 2008)

RIVETER said:


> Moving less air...but trying to...and can't.


 It's not a pump, if no air is moving it will cause a drop in amps.

Is this motor the right rpm for the application? On a centrifugal fan if it is spinning too fast the draw will go up alot. 

Google centrifugal fan curve.


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## wdemos (Nov 27, 2008)

*It's not a pump*



mattsilkwood said:


> It's not a pump, if no air is moving it will cause a drop in amps.
> 
> Is this motor the right rpm for the application? On a centrifugal fan if it is spinning too fast the draw will go up alot.
> 
> Google centrifugal fan curve.


A centrifugal pump will also draw less current.
A positive displacement pump will will draw more.
I would double check fan motor rpm as well.


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## mightyjoe (Sep 20, 2010)

nitro71 said:


> What's over amping? Is it blowing the overloads or tripping the breaker feeding it?


over amping 4.1 amps


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## Jlarson (Jun 28, 2009)

mightyjoe said:


> over amping 4.1 amps


So this is an AHU right, any dampers and/or registers you can close "down stream" of the fan?


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