# Motor feeder calculation

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#### kware

##### Member
I am an apprentice hoping to gain an understanding of motor calculations, the question I am working on is this.

"What is the minimum size inverse time breaker needed to protect a feeder with "TW" insulation supplying two 1hp single-phase 115V motors?"

This is what I have come up with so far:

1. FLC = 16A (Table 430.148)

16A x 1.15% = 18.4A rated heater 430.32 (B,1)

3. Branch circuit wire size:
16A x 1.25 = 20A # 14 TW Table 316.10

4. Branch circuit OCP:
16A x 2.50 = 40A ocp 430.52, Table 430.52

5. Feeder wire size:
16A x 1.25 = 20A + 16A = 36A # 8 TW 430.24, Table 310.16

This is where I lose my train of thought. Is this method correct?

6. Feeder OCP (using inverse time breakers):
16A x 2.50 = 40A + 16A = 56A but this is a motor feeder so I need to down size to a 50A ocp??? 430.62, 240.4 (G)

Is the math in step six the correct method? Any help would be great.

Thank,
Kevin

#### tom baker

##### First Chief Moderator
Staff member
Re: Motor feeder calculation

What size motor? As this is a feeder, is there more than one motor?

#### iwire

##### Moderator
Staff member
Re: Motor feeder calculation

First I want to ask where you got your figure of 16 A for the overload heater, that figure should not be from the tables.

Part of 430.62
shall be provided with a protective device having a rating or setting not greater than the largest rating or setting

#### kware

##### Member
Re: Motor feeder calculation

For Mr. Baker, there are two 1hp 115V single-phase motors.

For Mr. Badger, I got the 16A figure from the motor tables only because no name plate information was given in the question. Yes sir I am aware of 430.6(A)(2)

Thanks guys, the help I recieve here is worth allot....

#### bob

##### Senior Member
Re: Motor feeder calculation

Your math looks Correct. You said
"6. Feeder OCP (using inverse time breakers):
16A x 2.50 = 40A + 16A = 56A but this is a motor feeder so I need to down size to a 50A ocp??? 430.62, 240.4 (G) "
The reason you down size is because 430.62 says that the feeder OC may not be larger than the largest OC setting plus the sum of the FLA of the other motors. In this case it is as you say
40+16 = 56. You can't go to a 60 amp because you would exceed the 56 amps per 430.62. Therefore you
choose the 50 amp OC.

[ April 19, 2004, 04:54 PM: Message edited by: bob ]

#### iwire

##### Moderator
Staff member
Re: Motor feeder calculation

Bob we are not supposed to give the answer away to students.

Kevin, your welcome but if you continue in calling me Mr. I will give you bad info.

Bob B

#### bob

##### Senior Member
Re: Motor feeder calculation

Mr. Bob
I didn't think I was giving away the answer. He looked like he already had the answers. I hoped I was able to clear up the question on #6.

#### kware

##### Member
Re: Motor feeder calculation

Yes bob your answer was a big help. No matter how many times I would read 430.62 it never sank in but now I believe I'm fine with it.

Thanks again,
Kevin

#### kware

##### Member
Re: Motor feeder calculation

For iwire:

Sorry about being so formal, I just believe in being respectful to people who know more than me and are willing to pass the knowledge on. This site is full of great information and great people. Thanks for your input and your help.

Kevin

#### iwire

##### Moderator
Staff member
Re: Motor feeder calculation

Originally posted by kware:
For iwire:

Sorry about being so formal, I just believe in being respectful to people who know more than me and are willing to pass the knowledge on.
Kevin that's how I was brought up too.

I guess I just think I am to young to be called Mr.

Originally posted by kware:
This site is full of great information and great people.
You are on the money there, the assembled knowledge here is a great resource and while we will not do your work for you we are glad to help someone that was obviously making an effort to do it on their own.

Bob

#### ryan_618

##### Senior Member
Re: Motor feeder calculation

Originally posted by iwire:

I guess I just think I am to young to be called Mr.
Bob
Thats certainly not the case!

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