What is the Formula for Sizing Circuit Breakers?

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Sizing circuit breakers for motors involves considering inrush currents, with recommendations suggesting a breaker rated at 2.5 times the full load current (FLC) for adequate protection. For a 40 HP motor with an FLC of 37.5 amps, a 100 amp circuit breaker is proposed to handle starting currents effectively. The discussion also highlights the importance of the 125% rule for conductor sizing, ensuring that conductors can handle the motor's load. Additionally, participants emphasize the need to understand the inductive nature of motors and the potential for nuisance trips with improperly sized breakers. Proper calculations and adherence to manufacturer specifications are crucial for optimal circuit breaker sizing.
  • #31
Although I agree that inductive loads have huge inrush, he states that this is running off of a drive that will severely limit inrush to the motor. Most panels I service with VFD use 1.25 X FLA and Part wind and across the line use 1.5 X FLA generally. I work in a lot of control panels for water/wastewater applications from fraction HP up to 250 HP motors. Just my 2 cents
 
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  • #32
Hmmmm-- if you are using a 40HP VFD which no one seems to have mentioned / 30KW -- that will limit the starting current and provide the protection to the motor. The sizing of the feeder breaker should be > 40A and suited for the conductors. So I would use min of 50A and cable accordingly.

Depending on the electrical code in your locality - you may have an issue with the feeder being too small - for example you may not be allowed to have a 50A or 60 A breaker down stream from another 60 A Breaker.
 
  • #33
psparky said:
The above question should actually read...

Say you have a three phase 10 KVA motor with a full load current of 100 amps...with a power factor of .7. If you correct to power factor to .95 with a capacitor in parallel...

What is the new current and KVA of the motor?

With PF = 0.7 -> Power = 7KW
& P= sqrt(3)*I*V , with I = 100 A, solve for V-> V = 7000/(sqrt(3)*I) -> V = 40V!
The given in not correct, if it is a 3 phase motor -> V = 380V then I will not 100A for 10KVA motor!

+ We go very far away from the question! it is regarding CB sizing.
 

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