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
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