Thanks. Tom M
If u require a 20amp system you dont have to use 12 wire but you do have to use 12 wire if you want to comply to code. If you are taking the trouble to run thourgh the wall and into the attic etc. instead of conduit exterior to the finished walls you should use a 12/3 wire just incase you require another curcuit at a later date.
Physics require 12-2 wire for for 20amp circuit. The code is not written to be annoying.
Do not even think of using 14 guage wire with a 20 amp breaker unless you are looking to collect on your fire insurance. You are defeating the purpose of the breaker. A breaker is supposed to trip before the wire overheats. Size 14 wire cannot handle 20 amps. A saw and shop vac can easily overload the 14 wire.
You must use 12 guage wire for a 20 amp breaker. 12-2 is ok. 12-3 is used for 3-way switches and getting power to a switch at the end of a circuit.
Since this is for a workshop - get industrial grade outlets. If there is water nearby, get a 20amp GFCI. They cost more but what price is a life?
#14 - 15 amps max
#12 - 20 amps max
#10 - 30 amps max
#8 - 40 amps max
The circuit breaker or fuse is sized to protect the wire. But, don't always assume that because someone has a #12 wire protected with a 30amp overcurrent device that it is wrong. A wire's ampacity and a max voltage is determined by the insulation protecting that particular wire. Some size 14 wires can handle 30 amps... House wire or sheathed electrical cable is similar to THHN or MTW wire insulation. Hope this helps, but definately run a #12 wire if you want 20 amp service!