Motor Circuit Protection

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dvcraven052256

Member
Location
St. Louis, missouri
Occupation
Electrical Engineer
I have a question relative OCPD of motors.

As an example 40HP motor rated a 480/3. NEC Table 430.52 for Inverse Time Breaker say 250% of FLA 52 x 250% = 130A thus a 150CB.
My Square-D motor calculator says 90 Thermal Magnetic trip rating. Obviously their must be some characteristics of the Square-D type CB that allows for the
90ampere rating but I am not sure what that is.
 
dvcraven052256 said:
I have a question relative OCPD of motors.

As an example 40HP motor rated a 480/3. NEC Table 430.52 for Inverse Time Breaker say 250% of FLA 52 x 250% = 130A thus a 150CB.
My Square-D motor calculator says 90 Thermal Magnetic trip rating. Obviously their must be some characteristics of the Square-D type CB that allows for the
90ampere rating but I am not sure what that is.
Click to expand...
Some motors and loads are hard to start and under those conditions the 250% is allowed. Typically the motor will start with the 90 suggested. Most likely even smaller.
 
The 250% rule is for MAXIMUM, the NEC does not deal in “recommended”, that’s up to you. But the smaller you get, the more risk of nuisance tripping of the magnetic trips you will face. Newer “energy efficient” motors can have significantly higher magnetic inrush current because of the steps taken to reduce losses in the motors. Breakers with adjustable mag trips are often going to max out at 10x the thermal rating, so a 90A breaker will top out at 900A on the mag trips. Breakers with FIXED mag trips are often KESS than 10x. But it’s not at all unusual for a new EE motor to be at 17x, so a motor with 52A FLC may cause an inrush of 884A, which is right on the ragged edge of that 10X sizing. But it’s not unheard of for these motors to occasionally go to 20x FLC magnetic inrush under certain circumstances, which may cause nuisance tripping. So I like to not go with minimal breaker sizing. I would probably use 100A or 125A.
 
ptonsparky said:
Some motors and loads are hard to start and under those conditions the 250% is allowed. Typically the motor will start with the 90 suggested. Most likely even smaller.
Click to expand...
10 HP 480V three phase motor is allowed up to 35 amp inverse time breaker per NEC as the general rule. Square D motor calculator like OP mentioned recommends 25 amp. I have many times seen them work fine even on a 15 amp breaker.

Starting inrush can be limited by conductor size and length and same motor/driven load that holds in one application may not hold in some other application where more current is available just because of less impedance between the load and the source.
 
Eddie702 said:
I usually used 175% of the FLA unless it was something that is really hard starting.
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General rule I learned long ago is 2xFLA and next standard size up. Looking at some breaker manufacturers recommendations that usually kind of lines up with their charts on standard size motors.
 
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