[RE-wrenches] calculating DC voltage line loss

[Hugh]

>
>
>Can anyone pass on a good website address that provides the means to 
>calculate DC line loss over distance?
>

I normally just use the resistivity of copper and make an estimate of 
the operating temperature.  Most of the standard tables that you find 
in books assume that the temperature of the copper wire is at the 
limit for PVC insulation (around 70 degrees C) but in reality you 
will be working much lower (for energy efficiency) and thus have 
lower resistance.

As usual it's a lot easier for us with metric units of measure (wires 
are sold according to their cross section in square mm) but you can 
also do this with AWG.  Remember that increasing wire gauge by 3 
halves the area, so #6 has half the sectional area, and hence twice 
the resistance of #3.

Here are some resistances I worked out for an article in HP magazine 
134 (back page basics) using "English" units.  But here I am giving 
you the figures at 3 different temperatures to emphasise the effects 
of temperature:

FEET PER OHM OF TWIN CABLE
===	===	===	===
#AWG	0	35	70 degrees C
===	===	===	===
0	 5,541	 4,639	 3,989
1	 4,394	 3,679	 3,164
2	 3,484	 2,917	 2,509
3	 2,763	 2,313	 1,989
4	 2,191	 1,835	 1,578
5	 1,738	 1,455	 1,251
6	 1,378	 1,154	 992
7	 1,093	 915	 787
8	 867	 726	 624
9	 687	 575	 495
10	 545	 456	 392
12	 343	 287	 247
===	===	===	===

The figures are FEET ONE WAY to make one ohm for a 2-core cable.  For 
example if you use #8 at an operating temperature of 35C then you get 
726 feet of wire run (actually using 1452 feet of actual wire) to 
make one ohm.

It's easy to use.  Say your current is I and the wire run is L. 
Volt-drop will therefore be

	V = I x L / 726 (in this case).

Say 10 amps and 100 feet then you will lose 1000/726 = 1.4 volts.

No sense in calculating a string of decimals, since the temperature 
is only estimated anyway.

Using this stuff you can build your own spreadsheet or even do the 
calcs quickly in your head to a useful level of accuracy once you 
have a bit of experience.  As I say its a lot easier with metric 
units where the wires sizes are multiples of 1 sqmm, and about 28 
metres of that stuff is one ohm.
-- 
Hugh Piggott

Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk