I'm wiring up a small project, and I'm not sure what gauge of wire
to use.  I couldn't find an answer in _The Art Of Electronics_.

I assume that 30 gauge wire is sufficient for carrying digital
signals, since that's what Wire-Wrap used.

Is 30ga sufficient for Vcc and ground wires, which might be carrying
20mA?  How about motor supply wires which need to handle (in my
project) a couple of amps at 6 volts?

Answers to these particular questions appreciated; a formula or
rule of thumb to cover all situations would be even better.

Brian