sizing motor cables

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j_braddock

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we had a rather long meeting today in which several engineers were talking about sizing motor cables. during the discussion i had to take a call for a minute and missed enough of the conversation to get confused.

lets say a 75 hp fan motor, 480 volts 3 phase, distance from contactor to motor is 150 ft. continuous duty.
i know to derate the wire based on 90 deg C column for temperature and number of conductors.

here is where i get mixed up.
75 hp = 96 amps at 480 volts.
#2 wire @ 90 deg c = 150 amps X .87 temp correction factor = 113.1 amps. (only 1 set of wires). #2 wire @ 75 deg c = 115 amps, so i use 113.1 amps as the wire size.
since the motor pulls 96 amps the #2 wire is ok according to several people.
do i need to have the wire rated at 125% of the 96 amp load (120 amps) based on NEC 2002-art 430.32(A) or have i missed something?

thanks in advance.
 
Re: sizing motor cables

Unless I know the motor is short term or intermitent duty, I have always used 125% per 430.22.

Roger
 
Re: sizing motor cables

The conductors that feed motor load that is continuous requires the 1.25 factor. Using your example, 75 hp at 96 amps would be 96 x 1.25 =
120 amps. If you are required to derate for temp
the derated capacity must equal 120 amps. Using the 90 degree col #1 cu capacity is 150 amps x 0.87 = 130 amps. This meets the requirement of 120 amp capacity.

[ October 17, 2003, 06:07 PM: Message edited by: bob ]
 
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