Motor Calculations...

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Qckrun

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I am a 4th yr apprentice(soon to be 5th yr) and am currently installing some motor start Disconnects for some Fan coil Units on the job I am on. Well the foreman took off for a couple weeks and I was left with no information on the fuses or the heaters to be installed. So the lead journeyman said calculate it out, I am to busy to do it. Well I go to calculate it out according to what I believe is correct from my schooling and he asks where I got the figures. I have multiple FCU's but I'm just going to do two calculations, 1 3phase and 1single phase and tell me if my calculations are correct.

3 phase, 480v
Hp 2, Amps- 2.1, S.F. 1.15 (all name plate)

Dual element TIme Delay fuse: FLC= 3.4a

3.4a X 175% = 5.95% = 6a fuse

Overload(heaters)=
2.1a X 125% = 2.625a = In this case a Square D B4.95 Heater

1 Phase 115v
1/4 HP, A-3.9, S.F. 1.15(assuming doesn't list on nameplate)

Dual Element
5.8a X 175% = 10.15a = 10a or 11a fuse (were special ordering fuses, currently have a few 4a, 5a, and 7a).

O.L.
3.9a X 125% = 4.875 = B4.95Heater from Square D.

Did I calculate this right?
 
Fuses look correct.
I would suggest you check your SqD catalog in regard to OL Heaters. There are things to be taken into consideration as far as ambient of the motor & starters and S.F., but, I believe, the "standard" would be based on NP amperage without your 1.25%.
 
augie47 said:
Fuses look correct.
I would suggest you check your SqD catalog in regard to OL Heaters. There are things to be taken into consideration as far as ambient of the motor & starters and S.F., but, I believe, the "standard" would be based on NP amperage without your 1.25%.
Click to expand...

Ok I will check it, I was going based on the SqD chart inside of the Motor Starter Disco.

I always thought that the overload's were based on the Nameplate amperage, the S.F. factor, and then referring to 430.32 to determine if the S.F. is greater tan 1.15 or less than 1.15. But is it different with a motor starter disco?
 
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