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Topic Title: How to get per phase impedance from volt-drop tables - please help!
Topic Summary: 3phase impedances
Created On: 30 January 2013 05:09 PM
Status: Post and Reply
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 30 January 2013 05:09 PM
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SimonStickley

Posts: 4
Joined: 23 November 2010

I'm currently stuck on what is probably quite simple and need some help.

I need to know the impedance seen at a generator referred through a transformer. The cable is at LV, line voltage 400V. I've gone to the wiring regs, looked up the volt drop tables and got the impedance for the correct length of cable. For the example (See table 4E4A) 215m of 95mm2 4c SWA so Z=(0.09245 + j0.02795) ohms. This is where I get stuck because this gives a voltdrop of line voltage if multiplied by current, so is this a line impedance? I need a per phase impedance.

What I have thought is if this is a line impedance, and all phases are balanced (true in this case) then a delta to wye impedance transformation should work; which should be Zph = Zl/3. Is this correct?
 30 January 2013 10:08 PM
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Jobbo

Posts: 915
Joined: 08 July 2010

For 3 phase circuits the resistance of one conductor is given by the tabulated mv/a/m divided by root 3 (1.732) x 1000

This will be based at its operated temperature, which can be adjusted accordingly.
 31 January 2013 09:12 AM
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SimonStickley

Posts: 4
Joined: 23 November 2010

Thanks Jobbo, that is very helpful.

It makes sense really; line voltage = phase voltage / root 3
 31 January 2013 09:18 AM
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John Peckham

Posts: 7579
Joined: 23 April 2005

Simon

I think that should read phase voltage = line voltage/ root 3.

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John Peckham

http://www.astutetechnicalservices.co.uk/
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