# conductor rating

Discussion in 'Electrical Engineering' started by Warren Thai, Dec 6, 2006.

1. ### Warren ThaiGuest

given the required current rating of a conductor, how do you go about
determining the required cross section of the conductor. Any documents would
be great.

2. ### TimPerryGuest

you can search "wire gauge size" and find any number of tables or
calculators. for example http://en.wikipedia.org/wiki/American_wire_gauge

different tables would apply if the conductor is in a metal conduit or
subject to heat. aluminum wire uses a different table then copper.

http://www.powerstream.com/Wire_Size.htm

3. ### Guest

| given the required current rating of a conductor, how do you go about
| determining the required cross section of the conductor. Any documents would
| be great.

Cross section alone does not determine the current capacity rating of a
given wire size. While you will see resistance being inversely proportional
to the cross section area, consideration needs to be given to thermodynamics
(the course that often makes or breaks engineers in school). Voltage drop
across resistance means power dissipated as heat. But the heat must be
removed. Unfortunately, wire cross section increases faster (squared) than
the outer surface area (unless it is a hollow wire). At power frequencies
for most cases, there is no skin effect, so the heat will come from the
inside parts of the wire as well as the outside. Stranded wire will boost
the outer surface area a bit. Then you have to consider the insulation.
It will slow the heat dissipation rate (more so for thicker insulation
required on higehr voltage wires), as well as being a limitation on the
permissible heat rise. The ambient temperature is also an issue. And
this gets more complicated with other heat dissipating conductors close by,
especially if inside a cable sheath or conduit.

If you are an engineer, this should all be recalled/researched from that
thermodynamics perspective. If you are an electrician, your training in
sizing and load calculations should give the information you need. But if
you are a hobbyist, or do-it-yourself-er, your best bet is to be rather
conservative and overrate for safety, after finding out the basics of what
sizes can carry what (and don't stuff many wires together).

It's simpler in most cases to just look up the capacity ratings in the
tables in the National Electrical Code (for USA) or your Austrailian
equivalent. There will also be some rules, particularly for smaller more
common conductor sizes, that just spell out a maximum current that may be
used. I can cite some of these from memory for US standard wire gauges,
but since you are down under, you are likely using different wire size
standards which would likely have different size points to consider.

4. ### John GGuest

The australian Equivelent--
AS/NZS 3000:2000
Electrical installations (known as the Australian/New Zealand
Wiring Rules)