# Thread: how to size of conductor 3 phase motor or frequency driver in UL508A

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## how to size of conductor 3 phase motor or frequency driver in UL508A

Hi,
How shall the sizing be done for a 3 phase frequency driver according to UL508A?
If the frequency driver input rating is 21 amps, and the motor marking is 19 amps.
The table 28.1 is saying AWG12 for 20 amps and AWG10 for 30 amps if copper and 75C.
Due to 3 phase input rating of 21 amps for the driver, shall the amps be devided by square(3) i mean 1.73?
23/1.73 = 12.138, so the conductor size per table 28.1 is AWG14 which is for 15A.
Is that correct? Or should it not be divided by 1.73 and why?

For the motor the 19 amps shall be multiplied with 1.25 (125% of the motor FLC).
19*1.25 = 23.75 Amps
Is that correct thinking? Or shall it be divided by 1.73 and then multiply 1.25 and then look into table 28.1?
I have seen training pdfs where they do not divide the numbers with 1.73.
So I am little bit confused how the amps shall be reflected by the table, if it is a 1 phase or 3 phase load.

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Originally Posted by larsahl
Hi,
How shall the sizing be done for a 3 phase frequency driver according to UL508A?
If the frequency driver input rating is 21 amps, and the motor marking is 19 amps.
The table 28.1 is saying AWG12 for 20 amps and AWG10 for 30 amps if copper and 75C.
Due to 3 phase input rating of 21 amps for the driver, shall the amps be devided by square(3) i mean 1.73?
23/1.73 = 12.138, so the conductor size per table 28.1 is AWG14 which is for 15A.
Is that correct? Or should it not be divided by 1.73 and why?

For the motor the 19 amps shall be multiplied with 1.25 (125% of the motor FLC).
19*1.25 = 23.75 Amps
Is that correct thinking? Or shall it be divided by 1.73 and then multiply 1.25 and then look into table 28.1?
I have seen training pdfs where they do not divide the numbers with 1.73.
So I am little bit confused how the amps shall be reflected by the table, if it is a 1 phase or 3 phase load.

by frequency driver do you mean variable speed drive? UL508a refers to these as power conversion equipment.

in any case the input current rating will be in amps and you would not divide it by anything.

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## power conversion equipment.

Yes, true power conversion equipment it is.
But why not see it as amps per phase (I1, I2, I3)?
Is it because they call it conductor = cable ?
wire = a wire in a conductor with one or several wires in that conductor?