What is the Formula for Sizing Circuit Breakers?

Click For Summary
SUMMARY

The formula for sizing circuit breakers for motors, particularly when considering inrush currents, is critical for ensuring proper protection. For a 40 HP VFD motor operating at 415VAC and a full load current (FLC) of 37.5 amps, the circuit breaker should be rated at 50 amps, calculated as 125% of the FLC. However, inrush currents can necessitate a breaker size of up to 100 amps, especially if the main distribution panel is rated at 60 amps. Proper wire sizing, such as using #2 wire for higher amperage, is also essential for effective circuit protection.

PREREQUISITES
  • Understanding of motor specifications, including Full Load Current (FLC) and inrush current.
  • Knowledge of circuit breaker ratings and sizing rules, particularly the 125% rule.
  • Familiarity with electrical wiring standards, including wire gauge sizing.
  • Basic principles of power factor and its impact on motor performance.
NEXT STEPS
  • Research "circuit breaker sizing for motors" to understand various sizing methodologies.
  • Learn about "inrush current calculations" and how they affect circuit protection.
  • Explore "wire gauge selection for electrical circuits" to ensure compliance with safety standards.
  • Investigate "power factor correction techniques" to optimize motor efficiency and reduce inrush currents.
USEFUL FOR

Electrical engineers, maintenance technicians, and anyone involved in motor control and protection systems will benefit from this discussion, particularly those working with variable frequency drives (VFDs) and motor circuit 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
 
Engineering news on Phys.org
  • #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.
 

Similar threads

Synchronous Motor Capacitor Sizing
  • · Replies 11 ·
Replies
11
Views
5K
Float charging batteries: what really happens?
  • · Replies 7 ·
Replies
7
Views
2K
What materials do I need to wire a VFD?
  • · Replies 4 ·
Replies
4
Views
2K
What is the power curve of the American Bosch Motor/Generator?
  • · Replies 1 ·
Replies
1
Views
11K
Help- using NiMh batteries for high load applications (150A peak)
  • · Replies 3 ·
Replies
3
Views
3K