conductor sizing

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drhoads

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I have a 3PH, 460V, 25Hp motor. The nameplate FLA is 30.1. The starter is a size 2 and is fused for 60A. The overloads are rated for 32.5A and the conductors already in service are XHHW #8 AWG. To me this is wrong in every way. 1.)The FLC is (125% x 34)=42.5A 2.)From the nameplate data the overloads should be sized higher (115% x 30.1)=34.6A 3.)The wire size is only good for 40A. #6 AWG should be used. 4.) the starter should be fused for 45A. Does anyone agree??
 
Assuming this is a single motor feed, continuous duty, squirrel cage, Design B,C or D, with a S.F. of 1.0;

125% x 34A = 42.5A, therefore #8AWG at 75 deg C is plenty sufficient (50A)

O/L @ 115% x 30.1A = 34.6 A (Max). The O/L may be set a little tight at 32.4, but without looking at the protection curves it is not possible to know for sure. If it;s not tripping then it must be alright.

NEMA size 2 is acceptable

Fuses are 175% x 30.1 A = 52.7 A but can be increased up to 225% (67.7A) therefore 60A fuses may be correct. Again, need to see the protection curves.

All in all, no red flags.
 
drhoads said:
I have a 3PH, 460V, 25Hp motor. The nameplate FLA is 30.1. The starter is a size 2 and is fused for 60A. The overloads are rated for 32.5A and the conductors already in service are XHHW #8 AWG. To me this is wrong in every way. 1.)The FLC is (125% x 34)=42.5A 2.)From the nameplate data the overloads should be sized higher (115% x 30.1)=34.6A 3.)The wire size is only good for 40A. #6 AWG should be used. 4.) the starter should be fused for 45A. Does anyone agree??
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From Table 450.250 the FLA of a 25 hp motor at 460 volts is 34 amps.
From table 310.16 #8 xhhw capacity = 50 amps at 75C.
Wire size ampacity required = 125% x 34 amps = 42.5 amps
OL = 115% x 30.1 amps = 34.6 amps. Present rating per your post is 32.5 amps.
From table 430.52 Max breaker size = 250% x 34 = 85 amps
Max TD fuse = 175% x 34 amps = 60 amps
Everything you have listed is set at or below the max rating allowed by the code. You are allowed to set the ampacity of devices at a lower rating than listed in the NEC tables. I see nothing wrong per the code.
 
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Thanks for all the info. But, as far as the fuses are concerned, shouldn't the wire rating be higher than the fuses? In other words, shouldn't the fuses be sized to blow before the rated ampacity of the wiring? To protect it?
 
drhoads said:
Thanks for all the info. But, as far as the fuses are concerned, shouldn't the wire rating be higher than the fuses? In other words, shouldn't the fuses be sized to blow before the rated ampacity of the wiring? To protect it?
Click to expand...

No, the fuses are only providing short circuit and ground fault protection. The OL's are protecting the conductors.
 
I don't see a problem either.
NEMA 2 starter rated for 25 hp at 460v.
Dual element fuse 175% of 34 amps (table 430.250) 59.5 round up to 60amp.
Running overload protection maximum is 115% nameplate FLA (unless SF 1.15+) having it lower is fine, it is still slightly higher than the FLA.
The wire ampacity must be 125% 34amps, 42.5amps. #8 XHHW is rated 50 amps. at 75 degrees C in couduit. NEC allows motors wiring to use 75 degree column. Check example D8 in annex D of the NEC.
 
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