6 steps to sizing motor protection & wires

 

****Audio of sample problem at bottom click link

 

NEC 2014 reference

For 3phase motors with 1phase motors

 





Step 5 ------------------------------------------AUDIO 7 MIN 23 SECONDS
Feeder Conductor Size

• - Take largest motor in FLC X 125%.
• - With result add all other motor(s) on same feeder (select either a,b,or c phase whichever may connect to feeder with said largest FLC motor)
• - With result see chart p. 161 to determine conductor size

Step 6
Feeder Over Current Protection

• -Take the largest Branch circuit overcurrent device (from step 4 B)
• -ADD all the FLCs of All the other Motor(s) connected on the same feeder.                                [Follow feeder to either a,b,or c phase whichever it is connected to and ADD the motor(s) FLCs]
• With that result refer to page 96 to determine Feeder overcurrent protection           ((((REMEMEBER feeder protection CAN NOT go up to next standard size.. so ... Decrease once (if not spot-on) to the nearest standard size DOWN.)

Steps 5 & 6 are different for motors that are 3phase and 1phase that share the same feeder.

In Step 5

Select the Largest FLC and multiply it by 125% (1.25).
With the result add the next largest FLC connected to it. With that result go to page 161 to determine wire size.

Select the Largest FLC and Add the FLCs that are connected.
***
Note:
Since 3 phase motors have A, B, and C phases that balance between the single phase motors.
(ex. 1, 3phase with 3, single phase motors.) Each single phase connects to the 3, phase and only one at a time does the current travel on each A,B& C phase. (confusing yet?)

Step 6
From step 4 select the Largest Branch circuit Over-current protection device.
Add this to the FLC connected on the same branch.
(in this case this is a single phase connected to one of the 3phases of the Three phase motor either a,b, or c.
With that result refer to page 96. for the Feeder over-current protection
(((Remember for the feeder if the closest value is higher go DOWN 1, NEVER UP---you could create damage))))


_________________________________
In addition to the six steps the
FORMULA FOR EFFICIENCY 

*VOLT-AMPS ARE WATTS
* 1 hp= 746 W
* IF THE OUTPUT IS IN HP JUST MULTIPLY IT BY 746 TO CONVERT THE VALUE SO THAT IT CAN BE DIVIDED INTO VOLT-AMPS

EFFICENCY= ___OUTPUT____
                                INPUT

EFFICIENCY=_____HORSEPOWER_____
                                     VOLT-AMPS

What was on the board__**click picture for full screen**__

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