Motor circuit protection

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david luchini said:
Don, why is the 20A OCPD too big for the circuit "no matter what?" The 20A c/b is a motor feeder OCPD, if the fuses are the motor branch circuit OCPD. The motor feeder is #12 awg. A 20A c/b will protect the #12 feeder.
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That was my thought as well, But think he is saying that the starter itself need protection.
 
Jakewhis said:
That was my thought as well, But think he is saying that the starter itself need protection.
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Interesting thought, but I don't think that's the case. Even if it was, the 10A fuses would certainly provide protection, so the 20A c/b in the panel, protecting the #12 feeder should still be OK.
 
iwire said:
There were two motors.

1/2 HP 208 3 phase

1/3 HP 208 3 phase
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Jakewhis said:
In this building I have a 3phase 480v exhaust fan. The FLA is 3amps. It has overload protection. The breaker that feeds it is on the first floor and it is a 20 amp breaker. The disconnect, starter and fan is in the attic. The disconnect is fused 20amps and the conductors are 12 THHN. I would say this is ok, but the starter on the inside where is says what size overloads to use also says maximum fuse size 10 amps. Does this mean that the fuses need to be 10 amp fuses?
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Am I missing something? in the OP there was one 480V motor with an FLA of 3 Amps.
 
david luchini said:
Am I missing something? in the OP there was one 480V motor with an FLA of 3 Amps.
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No you are not, I was just being entirely self centered and assumed you were talking about my installation and not the OPs.

I will shut up now.
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iwire said:
No you are not, I was just being entirely self centered and assumed you were talking about my installation and not the OPs.
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Cool! Two posts for the price of one. And they'll throw in a set of Ginsu Knives. How much would you pay now?!?! :grin:
 
mxslick said:
To be clear I'm not arguing that the fuses aren't needed, but:

IF the O/L's had NOT been bypassed, I wonder if they would have survived as the extreme quick heating caused by the short circuits would have caused them to act and open the contactor. I know they are not intended to be short circuit protection, but in this case I'm guessing that if they were properly wired they would have possibly survived.

If the available fault current at the starters is high though, all bets are off.

I agree that IEC stuff is not as robust as NEMA...somewhere I had a guide wich compared life expectancy and max number of rated operating cycles and the IEC stuff was all short of what NEMA gear can take.
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If the overloads were not bypassed they would have shut the overloaded motor down before it degraded to the point of short circuit or ground fault and then they would not have had to carry the fault current. Still not a good excuse for not proper fuse but is the reality of the situation
 
kwired said:
If the overloads were not bypassed they would have shut the overloaded motor down before it degraded to the point of short circuit or ground fault and then they would not have had to carry the fault current.
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That is my thought as well. :)

But I still feel that the OLs were just as likely to self destruct if the field wiring between the motors and OLs faulted. There is a definite reason UL and AB requires the 'small' fuses ahead of the starter. In this case there was a 30 amp breaker ahead of the starter.

Here is the page from AB showing max fuse sizing. :)

Bulletin 193-K Bimetallic Overload Relays
 
david luchini said:
Don, why is the 20A OCPD too big for the circuit "no matter what?" The 20A c/b is a motor feeder OCPD, if the fuses are the motor branch circuit OCPD. The motor feeder is #12 awg. A 20A c/b will protect the #12 feeder.
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I was looking at the circuit as a branch circuit and not a feeder.
 
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