MOTOR FEEDER / EXAM PREP BOOK

[Scottjo]

Unit 8 challenge questions 7&9. Ask to size the feeder over current protection. I am thinking that the answers are wrong. I read the code that you take 125% of the largest motor plus the sum of the other motors. The answer states 250% of the largest motor plus only one of the full load currents of the remaining motors. Anyone if that is wrong / I'm wrong / both?
Thanks!

 

[ActionDave]

Unit 8 challenge questions 7&9. Ask to size the feeder over current protection. I am thinking that the answers are wrong. I read the code that you take 125% of the largest motor plus the sum of the other motors. The answer states 250% of the largest motor plus only one of the full load currents of the remaining motors. Anyone if that is wrong / I'm wrong / both?
Thanks!

Welcome to the forum.

I think you are mixing up conductor sizing with overcurrent protection.

 

[Martin B]

Unit 8 challenge questions 7&9. Ask to size the feeder over current protection. I am thinking that the answers are wrong. I read the code that you take 125% of the largest motor plus the sum of the other motors. The answer states 250% of the largest motor plus only one of the full load currents of the remaining motors. Anyone if that is wrong / I'm wrong / both?
Thanks!

Doesn't the 250% rule applies to Branch Circuit OCPD. Feeder OCPD are calculated differently.

 

[Scottjo]

Thanks for the replys, I was mixing them up. I still don't get why the answers don't include all the motors in the calculation.
The question
There are THREE motors one 5hp, 230v single phase motor, and 2 1 1/2hp motors

 

[Scottjo]

My last reply got cut off, I'm not to good at the forum thing. The questions are based on 3 / 4 motors but they only use the largest and only one of the remaining motors for the over-current protection device calculation. That dose not seem correct to me?

 

[ActionDave]

My last reply got cut off, I'm not to good at the forum thing. The questions are based on 3 / 4 motors but they only use the largest and only one of the remaining motors for the over-current protection device calculation. That dose not seem correct to me?

Don't worry about the forum thing, you get better without any formal ttraining.

The question you are pondering may be wrong, it happens. More often you are glancing over a detail.

 

[Scottjo]

Don't worry about the forum thing, you get better without any formal ttraining.

The question you are pondering may be wrong, it happens. More often you are glancing over a detail.

The answers were correct. I failed to notice that smaller motors needed to be balanced under the larger motors. Thanks for the help!

 

[precise8128]

Thanks for the replys, I was mixing them up. I still don't get why the answers don't include all the motors in the calculation.
The question
There are THREE motors one 5hp, 230v single phase motor, and 2 1 1/2hp motors[/QUOTE]

I got 6Awg wire feeding them 3 motors and 150A breaker Correct?