Calculating Voltage Drop

[Cookie]

How is voltage drop of a circuit normally calculated in the UK? And why are wires 16mm2 and under listed as a single unit, but 25mm2 and over in R, X and Z? How do I work with Z?

 

[telectrix]

in the UK we use tables showing mV/A/m for different cable sizes. e.g. if we have a cable 50m long, 20A, and the table shows 7mV/A/m, then VD = (7 x 50 x 20)/1000 i.e. 7000mV or 7V.

 

[Cookie]

Super!

What about larger sizes such as 95mm2?

 

[telectrix]

pass on that. I never use cables that big

 

[Spoon]

r = Resistive load
x = Inductive reactance
z = Total impedance

 

[Cookie]

Isn't R the AC resistance (as taken from skin effect) of the cable itself rather than the load?

 

[Lucien Nunes]

For small cables at 50Hz, the ohmic resistance dominates, so it is not important to consider the DC resistance and AC impedance separately. A single figure is used for all applications.

For larger cables with lower resistance, the inductive reactance starts to become significant on AC, so the voltage drop arising from each effect, and their vector sum, is given. For DC, the r figure alone would be used. For AC, the impedance. By tabulating both the reactive and resistive contributions, the total drop for a load of known non-unity pf can be calculated (where the drop is displaced in phase from the line voltage) for greater accuracy.

 

[Cookie]

But even for small cables, doesn't load power factor effect voltage drop considerably? Does DC resistance or AC resistance give the worst figures vs AC reactance (Z)?
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Here is the NEC's Table 9. Reactance, even load power factor when known is taken into account for smaller conductors down to 2.08mm2.

 

[Lucien Nunes]

You make a good point. I am not sure why the NEC and BS take different approaches here, and looking at the NEC table the ratio is greater than I thought on small circuits (i.e much smaller than the crossover point around #2). I would like to plug some numbers in and see how much impact it has but time doesn't permit. @marconi will be along presently, he often seems to have more patience for number crunching than I do!

 

[Cookie]

Much appreciated, thanks in advance to Marconi!

 

[mydigitalhome]

Hi, for those sizes tend to use software like Cymap or Amtech, I use the first option. For smaller sized cable I use http://jarsoftelectrical.co.uk/ very good and cost effective.

 

[Cookie]

Obvious Question: is the software free?

Also, can you calculate short circuit current using this table and ohms law with the assumption of an infinite source?

 

[mydigitalhome]

Jahsoft is the others are expensive but worth it.

 

[Cookie]

I'll look into it.