Motor short circuit protection

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Rickhe

Member
Location
NC
Hello Mike
Need a little help in understanding.
I have three 3 phase 208 volt 3.5 amp thermally protected motors. All on same 20 amp branch circuit controlled by one three pole contactor. Do I need to provide individual over current protection for each motor? If I tap with conductor rated At 20 amp. To feed my disconnect to each motor.
Thanks
Rick
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Rickhe said:
Hello Mike
Need a little help in understanding.
I have three 3 phase 208 volt 3.5 amp thermally protected motors. All on same 20 amp branch circuit controlled by one three pole contactor. Do I need to provide individual over current protection for each motor? If I tap with conductor rated At 20 amp. To feed my disconnect to each motor.
Thanks
Rick
Click to expand...

It could be an a 15A CB but not a 20A CB.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Over CURRENT protection is different from Short Circuit protection. Technically, on a thermally protected motor, the over CURRENT protection is in the motor. So the breaker is only really supplying the Short Circuit protection.

Doesn't change anything though, it still needs to be a 15A CB. Even though your conductors might be sized for 20A, the motor windings are going to need SC protection too, and the code allows for the fact that 15A is the smallest standard breaker, thereby allowing you to use that. 20A violates that exception.
 
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