What is the Formula for Sizing Circuit Breakers?

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Discussion Overview

The discussion revolves around the sizing of circuit breakers for a motor application, specifically addressing the protection against inrush currents in a 415VAC, 50Hz system. Participants explore various formulas and rules of thumb for determining appropriate circuit breaker ratings based on motor specifications and load conditions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about a formula for sizing circuit breakers to protect against inrush currents, providing specific motor parameters.
  • Another participant suggests a circuit breaker rating of 40 Amps based on the motor's full load current, noting that inrush may increase the maximum amperage to 45 Amps.
  • Some participants propose that conductors should be sized at least 125% of the motor's full load current (FLC) for protection.
  • There is a suggestion that circuit breakers should be sized at 2.5 times the full load current to accommodate inrush currents, with a recommendation for a 100 Amp breaker in this context.
  • Concerns are raised about the adequacy of a 60 Amp main distribution panel for starting the motor, with suggestions that a higher rating may be necessary.
  • One participant emphasizes the importance of considering the inductive nature of motors and the complexities involved in sizing breakers compared to resistive loads.
  • Several participants engage in a mathematical exploration of power factor correction and its effects on motor current and KVA, with varying interpretations of the implications for circuit breaker sizing.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate sizing of circuit breakers, with no consensus reached on a single formula or method. Some advocate for the 125% rule, while others argue for a higher multiplier based on inrush current considerations.

Contextual Notes

Participants reference various assumptions about motor characteristics, inrush currents, and the relationship between circuit breaker ratings and load conditions. There are unresolved mathematical steps and differing interpretations of how power factor correction affects current and KVA calculations.

Who May Find This Useful

This discussion may be useful for electrical engineers, technicians, and students interested in motor control, circuit protection, and electrical system design.

  • #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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