Current capacity of electrical wire having no. / gauge of the conductors as: 7/20,7/18, 7/16 , 7/14 etc?

Answer 1

It depends on application, the longer the run, the lower the ampacity, as in "what voltage drop can you tolerate?" For example,

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

take the ohms/1000ft number of the 20 gauge and divide by 7 (the number of strands).

10.15/7 = 1.45 ohms/1000ft

Now look for the gauge of wire with the closest ohms/1000ft value. This gives an equivalent of almost 12 gauge. The chart indicates that 12 gauge can be used for up to 41A for chassis wiring (something like under 10 feet), or up to 9.3A for "power transmission", which probably means up to several hundred feet or maybe 1000 feet. In residential use, 12 gauge is used for up to 20A.

Use the same method for the other sizes, depending on your application. If your app is household wiring, use 1/2 the chassis rating as your ampacity.

Answer 2

Are you asking about a single stranded conductor,
or a multi-conductor cable of separate wires?
You fail to specify insulation temperature limit or
voltage and voltage drop requirements.
What ductwork, if any?
What ambient temp.?
We are engineers, not psychics.

Answer 3

Check this site for copper wire tables:
http://en.wikipedia.org/wiki/American_wire_gauge

For example, #14 AWG is good for 15 amps for a 60 degree raceway temp rise. 7 strands of that is good for 105 amperes.

Depending on your application and distance, you might not want to use #14 for 15 amps over a long distance ( > 100 ft) if voltage drop is a concern. Also, if large electrical motors or refrigeration equipment is to be used, size the wire to handle the starting surge without excessive voltage drop.