Three Phase Motor Wire Size

[doug glasscock]

I have a 480VAC/3ph/400Amp panelboard with a 150Amp/3p Breaker providing power for a 50HP/3ph motor starter located about 75FT away.
I searched and read several of the threads concerning single motor circuits.
430-22 says the wire shall not be less than 125% of the motors FLA.

Table 430-150; 50HP motor FLA=65
65FLA x 1.25 = 81.25 therefore the wire should be sized for 100amp (min.)Table 430.52 says the panelboard breaker size is okay since 150 is less than (65 x 2.5 = 162.5).
Table 310-16/75deg says #3 is good for 100Amps.
Table 250-122 says a #6 EGC is required for a 150Amp Breaker

Therefore the wire from the Panelboard's 150Amp breaker to the starter and then from the starter to the motor is 3-#3 & 1-#6 GND

Just want to confirm, does this sound correct?

 

[DGrant041]

Doug-

Looks pretty good to me except

Table 430-150; 50HP motor FLA=65
65FLA x 1.25 = 81.25 therefore the wire should be sized for 100amp (min.)

Therefore the wire from the Panelboard's 150Amp breaker to the starter and then from the starter to the motor is 3-#3 & 1-#6 GND

II. Motor Circuit Conductors
430.22 Single Motor
(A) [B]General[/B] Conductors that supply a single motor used in a continuous
duty application shall have an ampacity of not less than 125 percent
of the motor's full-load current rating as determined by 430.6(A)(1).

My Ugly's book told me #4 so I had to figure out why. #4 is good for 85 A (75C). If you're using 90C conductors, they should be good for up to 113 F ambient.

Breaker size: couldn't tell ya--we usually used fuses

Motor overloads: 430.32 (A) (1) says you can go upto 115 - 125% FLA on motor. I'm curious: Who does this? I've only seen 100% rated. Lose the bearings but save the motor windings.

What do you folks think? (Doug-I hope you don't think I'm hijaking your thread. . .:twisted: )

 

[hmspe]

65FLA x 1.25 = 81.25 therefore the wire should be sized for 100amp (min.)

Why the jump from 81.25A to 100A? The minimum ampacity is 81.25, not 100. If your terminations are 60 degC that would mean #3 CU minimum. For 75 degC terminations that would mean #4 CU minimum. In both cases the Table 310.16 ampacities are 85A.

Martin

 

[doug glasscock]

Your right, no need to jump up to 100.

 

[doug glasscock]

Just want to confirm one Last thing:
The disconnect located at the motor?
430-110 says 115% of motor FLA minimum.
Therefore, 65 x 1.15 = 74.75Amps
Next size disconnect is 100Amp/3P.