Choosing the correct Breakers Rating
Choosing the correct Breakers Rating
(OP)
Guys I have a question regarding in choosing the correct breaker rating of a motor. Given a 20hp pump motor with 230V, 3Phase, 60Hz. what should the the ampere rating of the correct breakers to be used? Is this correct 20Hp, 3ph, 230v in table is equivalent to 54amps. therefore we multiply full load current which is 54amps by 125% that gives us 67.5amps. our breaker should be 100at, 3p, 230v. Should we multiply it by 125% or 175%? also how do we compute for its KAIC(kilo ampere interrupting capacity)?






RE: Choosing the correct Breakers Rating
The 125% is for conductors.
There is another table for breaker or fuse sizing. These values are based on the motor current from the table, (54 Amps).
You may find an example in Chapter 9.
The interupting current is based on the transformer full load amps divide by the impedance, or the interupting rating of any current limiting deices ahead of the breaker.
yours
RE: Choosing the correct Breakers Rating
RE: Choosing the correct Breakers Rating
Overcurrent protection for motors.
Overcurrent protection for motor branch circuits.
Overcurrent, motors.
In the NEC, Chapter 9 is titled "Tables and Examples". It is in the back, after the rules but ahead of the appendicies.
yours
RE: Choosing the correct Breakers Rating
The breaker sizing I use for a typical application is the motor FLA times 1.5 rounded up to the next available size.
If you have a high inertia load or a special motor then you have to dig deeper into the starting requirements to determine what the breaker size needs to be.
RE: Choosing the correct Breakers Rating
h
David Baird
Sr Controls Designer
EET degree.
Journeyman Electrician.
RE: Choosing the correct Breakers Rating
LionelHutz
dbaird,
Thanks guys for the info. i'l look it up.
RE: Choosing the correct Breakers Rating
Mike