Breaker Sizing

[akhan_08]

Dear all,

I have a question regarding breaker sizing as per NEC codes which I use mostly. Will we consider locked rotor Amps of motor as noncontinuous current for breaker sizing in addition to 125% time continuous load or not?

I use this calculation when selecting breaker sizing
= 100% locked rotor current of motor + 125% x Full load rated motor current

for motors, compressor motors loading. Is that corrent? some told me that you will only consider full load rated current & multiply it with 1.35, which is OK.

How about unwanted trippings of breaker due to inrush currents if we select it with only full load rated motor current times 1.25?

please help me!

 

[raider1]

Welcome to the forum.

First off when sizing the branch circuit, short circuit and ground fault protection you start by getting the full load current value for the motor that you are protecting from the Tables in Part XIV of Article 430.

Then you take the tabular value from Article 430 and you are permitted to use Table 430.52 and multiply the tabular value by the percentage given in the Table for the type of motor and type of overcurrent protective device.

So for example if I have a 460 volt 3 phase 5 HP Squirrel cage motor the full load amps would be, 7.6 amps form Table 430.250. If I am using a standard inverse time circuit breaker for short-circuit and ground fault protection then I would look at Table 430.52 and see that the breaker could be 250% of the motor tabular value. So I come up with 19 amps. Exception No 1 permits us to use the next standard size breaker where the calculated number does not correspond to a standard size, so I could use a 20 amp breaker.

Hope this helps.

Chris

 

[augie47]

Welcome.
According to the NEC, if you are using the breaker for Short-circuit/Ground-Fault protection, you normally follow Table 430.52 which is based on the FLA from the NEC Tables 430.247-250. Normally that value is 250% of FLA for Inverse Time and 800% for I.T. breakers but it varies by motor type.

(Chris, as often happens I was typing as you answered...I'm slow)

 

[raider1]

No problem Gus.

Chris

 

[akhan_08]

Breaker for Short circuit & ground fault

Breaker for Short circuit & ground fault

Thanks for helping me. it means we are no gonna select breaker by simply multiply FLA of motor by 1.25 but either use 2.5 or 8 multiplying factor for inverse time & Instantaneous breakers respectively?

What hazards are involed if we select breakers 135% of FLA instead of 250% for short circuit & ground fault protection? Kindly elaborate by example.

Awaiting your response

 

[raider1]

Thanks for helping me. it means we are no gonna select breaker by simply multiply FLA of motor by 1.25 but either use 2.5 or 8 multiplying factor for inverse time & Instantaneous breakers respectively?

What hazards are involed if we select breakers 135% of FLA instead of 250% for short circuit & ground fault protection? Kindly elaborate by example.

Awaiting your response

You can size the breaker by applying 125% or 135% if you choose but this may not be wise as the breaker may trip under inrush current during motor start up.

When a motor starts the inrush current can be as much as 6 times the running load current for a short time. this is the reason we are permitted to use an inverse time circuit that is size upto 250% of the full load current to provide short circuit and ground fault protection.

Hope this helps.

Chris