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Topic Title: Volt drop?
Topic Summary: getting a mV/A/m figure from r, x, z values
Created On: 14 January 2012 05:25 PM
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 14 January 2012 05:25 PM
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ben42

Posts: 3
Joined: 14 January 2012

Hi,

Im trying to work out the volt drop for a multicore armoured 70°C thermoplastic insulated cable (copper conductors) it's a 95mm² cable and from table 4D4B in the bs7671 wiring regulations it has three values r, x, z. How do I get a mV/A/m value from these figures?

Thanks
 14 January 2012 06:36 PM
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Legh

Posts: 3537
Joined: 17 December 2004

r = resistance
x = reactance
z = impedance

Generally, with higher current rated cables where a large amount of current is expected to flow then a back emf will be naturally generated causing an opposition to the current flow. This is called reactance.

A combination of Reactance and Resistance gives us the Impedance.

Impedance is then the total opposition to current flow in a circuits (z)

So which term should be used?

Legh

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 14 January 2012 07:47 PM
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ben42

Posts: 3
Joined: 14 January 2012

OK thankyou,

so impedance (z) should be used in the formula:

V.D = mV/A/m x L x Ib/1000
 14 January 2012 08:13 PM
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micjamesq

Posts: 777
Joined: 23 January 2009

Yip - however doing so may make you end up with either small circuit lengths or overly sized cables needed depending on circumstances.

Have a wee read of Appendix 4 (Informative) of the Big Green Bible under heading 6 TABLES OF VOLTAGE DROP for a little more information on the subject.

Regards

-------------------------
E & OE
 18 January 2012 09:57 AM
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HarryJMacdonald

Posts: 259
Joined: 15 May 2002

For a simple calculation use the impedance z.

If you know the phase angle of the load, then you can calculate the real and imaginary components separately using r and x and work out the voltage drop from that, but it is unlikely to make much difference unless you have some very odd loads, e.g. motor starting currents.

If you don't know what I'm talking about, don't worry, just use z.
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