Voltage Drop - which load?

[msteiner]

For voltage drop calcs of a motor branch circuit, should I use the NEC FLA table or the motor nameplate FLA as the "load" portion of the equation?

 

[broadgage]

For voltage drop calcs of a motor branch circuit, should I use the NEC FLA table or the motor nameplate FLA as the "load" portion of the equation?

IMHO you should use whichever figure is greater.
If the installed motor has a higher FLA than that in table, then clearly this figure should be used to avoid excessive voltage drop whilst in use.

If the installed motor has a lower FLA than listed in the NEC , then in my view the NEC figure should be used to avoid any problems if the motor fails and is replaced with a standard one.

 

[iwire]

You may also want to run the numbers based on the locked rotor amps and see what kind of VD you will have at start up.

 

[ptonsparky]

You may also want to run the numbers based on the locked rotor amps and see what kind of VD you will have at start up.

But don't loose a lot of sleep over the answer. Just file the information back in the recesses of your mind for those really hard starting motor loads that seem to take forever to get up to speed.

 

[mayanees]

You may also want to run the numbers based on the locked rotor amps and see what kind of VD you will have at start up.

I look at fla from the tables, but also look at the locked rotor amps (from the tables using the lra based on the NEMA motor design code) to see if the voltage drop is in excess of 15% - which is the 695 Fire Pump criteria for startup, then upsize cable to accommodate.

John M



'sup Matt!