Choosing the correct Breakers Rating

[mnnc]

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)?

 

[waross]

Hello mnnc
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

 

[mnnc]

Hello warossare you referring chapter 9 in the NEC? We don't use nec here. we use pec, but almost all of its contents are gathered fro nec. we might have a mix up on the chapters. what article should i look for. Thanks

 

[waross]

Check the index for:
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

 

[LionelHutz]

To get the official rules regarding breaker sizing you really do need to look it up in the governing electrical codes.

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.

 

[dbaird]

Check Bussmann Reference table based upon NEC:

http://s37.yousendit.com/d.aspx?id=240XXB1ESI40025NBDM4PX5H05

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[mnnc]

waross
LionelHutz
dbaird,

Thanks guys for the info. i'l look it up.

 

[mpparent]

To determine the AIC rating, you really should do a short circuit analysis. I don't disagree with waross's method for a quick and dirty number. However, it doesn't take into account any motor contribution you may have.

Mike