"True, thats because the #12 circuit trips the circuit when it reaches 20 amps."

I have seem 20A breakers hold 20 -30 amps for a long long time.

Also during direct or partial shorts this not exactly true either..... a short, acts slightly different than an over-load, and has more to do with the AIC rating of the breaker/fuse and size of transformer, and distance to it via the conductors than simply "20A". The main reason you'll get that fireball/arch-flash effect on a service short, as opposed to a feeder or branch circuit is that circuit breaker or fuse has an AIC rating determined by many varying factors, and it should clear below those ratings safely. Exceed the ratings, as many people dont even think about them during most installs, and you could have a breaker or fuse that fails to clear a fault in time, explodes in the proccess, or worse not at all.

Most residential ditribution systems the idea is to limit to be 10,000 AIC or below, and likewise some comercial, but some 22,000, 30,000, and upwards for some types of industrial work.

Theres kind of an explaination here: http://www.currenttechnology.com/pdf/Current_Terminology.pdf
http://www.mikeholt.com/mojonewsarchive/EES-HTML/HTML/ElectricalCircuitBreakers~20030621.htm

And, I'm sure some of the other guys will have sometime else to add.

Mark Heller
"Well - I oughta....." -Jackie Gleason