Hi there,
The allowable current in a wire depends a lot on what it is
to be used for, ie the application. Wires that are short can
handle more current than wires that are long, and sometimes
voltage drop is the more important thing anyway.
For the application of 120vac house wiring, here are a few
examples:
#12 is good for 20 amps
#14 is good for 15 amps
A little math tells us that these ratings are calculated roughly
by allowing 300 circular mils per amp of current.
Other applications however, such as transformer coil design,
may require 600 circular mils per amp of current, which would
mean #12 wire would only be good up to 10 amps.
Even with the above numbers it is possible to require a heavier
wire if the wire is very long and the voltage drop is critical.
If so, the diameter of the wire has to be increased. For these
applications usually the resistance of the wire is calculated and
the voltage drop calculated from that and the normall running
current. The wire size is increased until an acceptable voltage
drop is obtained.
For a rule of thumb, calculate the wire size using 300 circular mils
per amp (assuming wire is in free air, or 600 circular mils if not)
and use that wire in the application. Run the application for
several hours and at elevated ambient and see if the wire gets
hot. If it gets too hot increase the wire size or run a second
length of wire parallel to it (one for each wire in a pair).
If you are worried about voltage drop, calculate the resistance
of the wire and use Ohms Law to calculate the voltage drop
at the expected current:
V=I*R
where
V is the voltage drop
I is the current
R is the resistance the wire
Note that if there are two wires (a pair) then R will be the sum of the resistances
of both wires.
A convenient formula for voltage drop is as follows:
V=22*Lenght_In_Feet*Current_In_Amps/Area_In_CMils
or in words:
The voltage drop is equal to twenty two times the length of the wire in feet
times the current in amps divided by the area in circular mils.
Note that in the above formula if there are two wires from point A to point B
and the distance from point A to point B is 10 feet, then the total length of
the wire is 20 feet (2 times 10).