On Thu, 25 Aug 2005 17:03:11 GMT, "John Blessing"
wrote:

"Mr Fizzion" wrote in message
.. .
On Thu, 25 Aug 2005 10:13:08 +0100, Cordless Crazy
wrote:

3. What size of cable would be required on this 55m run with, what I
assume will be, a huge voltage drop?

Voltage drop is given by V=IR where I is the current in amps and R is
the resistance of the wires in ohms. The resistance of the wires is
found from the resistivity of copper - 1.68*10^-8 ohm metres: R =
(length in metres * resitivity) / cross-sectional area of the wire in
square metres.

Mr F.

Isn't that formula valid only for DC not AC current?

If you are considering a section of cable then it's valid for AC too
unless the cable is tightly coiled into an inductor.

You do however get the "skin effect" with AC where all the current is
concentrated in the outside of the cable and none of it flows through
the central core.

The skin depth in copper at 50Hz is around 10mm, which I believe in
practice means that there isn't any point in having copper cable whose
radius of cross section is larger than 10mm. (but many years since I
studied that kind of thing!)

Mr F.