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# 3 ph cable calculations

Discuss 3 ph cable calculations in the Commercial Electrical Advice area at ElectriciansForums.net

K

#### kevinreilly

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

Hi all,

Can anyone help me as to how to calculate the maximum demand on a 3 ph supply for cable sizing?

Can i just quickly estimate the demand by the breakers and multiply by a g factor i got from the IET design guide for sparks. eg 250 amps counting the breakers x 0.7 factor for mixed process mills

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G

#### Guest123

Hi Kev.

For initial design brief this would be ok.

I would though be careful when using this estimation method as you need to be sure on the type of loading you will have at your dis board i.e inductive/resistive, aswell as the amount.

For your paper mill I would use the 0.7 factor also, to air on the side of safety.

Obviously when all info is available then a more detailed max demand calc should be done.

Cheers.

K

#### kevinreilly

cheers lenny,

Another quick one is it correct that i divide by sqrt of 3 to find the demand per phase ie 250amp x 0.7 =175 then 175amp divided by sqrt of 3 =101amp?

Thanks in again

G

#### Guest123

Hey Kev.

Unless you've got a purely balanced 3phase load then that isn't going to be very accurate.

Cheers

K

#### kevinreilly

Hi again

The thing is Lenny ive gone into this job today and counted the breakers and come up with 250amp say and multiplied it by the 0.7 factor and got say 175 amp only on one of the boards and another spark has come in after ive gone and said there is only 75 amp per phase so a 35mm supply cable will do for both boards. ive already ordered the 70mm armoured i think is right its just proving to the client why i think 70mm. so do i just go off the 175amp i got and add it to the 2nd board and then sqrt to get an overall load? or dont i even have to use the sqrt?

confused

G

#### Guest123

Hi Kev.

I would use the total load of 175A for board 1.
For board 2 can you do the same, total all circuits and multiply by 0.7 again.

Add these two for the total load of both boards and you should be covered for whatever the situation then.

Cheers.

K

#### kevinreilly

Your good man thanks alot mate

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