430.6 and T430.250

[tortuga]

When I am dealing with a motor voltage not in T430.250 I go by the nameplate.
But technically this is a violation of 430.6.
How is that table 430.250 determined? I can see it goes way back in the code and its not a linear relationship between HP and amps.
Is it for example safe to assume a 400V motor (416V system) is 15% greater than a 460V motor (480V system).

 

[don_resqcapt19]

That is not really a voltage system that is addressed by the NEC. My understanding is that the table values are "worst case" to make sure that the conductors are large enough if a less efficient motor would be used as a replacement.

 

[tortuga]

Thanks for the reply, the thing is if I use the 460 volt column I come in way below the nameplate (obviously). After doing some research on full load currents of 416 and 400 volt 60Hz motors, I think its safe to increase the corresponding 460 volt motor full-load current by 15 and 20 percent respectively.

 

[don_resqcapt19]

Thanks for the reply, the thing is if I use the 460 volt column I come in way below the nameplate (obviously). After doing some research on full load currents of 416 and 400 volt 60Hz motors, I think its safe to increase the corresponding 460 volt motor full-load current by 15 and 20 percent respectively.

In the real world, I would just use the nameplate. It is unlikely that a future replacement motor of the same horsepower will have a higher nameplate current.

 

[tortuga]

I do, but also in the real world are plans checkers that are trained on the NEC and seem trust the NEC more than me and question the use of the nameplate.
I am submitting a PI for 416 volt systems as by the letter of the code we're supposed to use the table, this figures back into feeder calcs etc.
My PI would add a note at the bottom of the table just like 208V motors used to have:

For full load currents of 416 and 400 volt motors, increase the corresponding 460 volt motor full-load current by 15 and 44 percent respectively.

That 20% number in my last post should be 44% for 400V motors.
I have only checked it against Baldor, but there are not many other suppliers of 400V or 416V 3 phase 60Hz motors I know of.

 

[don_resqcapt19]

I do, but also in the real world are plans checkers that are trained on the NEC and seem trust the NEC more than me and question the use of the nameplate.
I am submitting a PI for 416 volt systems as by the letter of the code we're supposed to use the table, this figures back into feeder calcs etc.
My PI would add a note at the bottom of the table just like 208V motors used to have:

That 20% number in my last post should be 44% for 400V motors.
I have only checked it against Baldor, but there are not many other suppliers of 400V or 416V 3 phase 60Hz motors I know of.

Maybe a better PI would be to change 430.6(A)(2)(3) to permit the use of the nameplate values for voltage systems not included in the tables. Could even require the use of a multiplier so that it would provide a worst case current like the table does now. That section currently permits nameplate values only for large motors exceeding the horsepower in Part XIV of Article 430.