If I have a 15 amp circuit, and 14 gauge wire, why can't I install a 20-amp outlet? If a 20-amp device is plugged into the outlet, won't the circuit trip, preventing overload? Also, 20-amp outlets and 15-amp outlets appear to have the same connection for devices (two prongs and a ground hole), yet on this blog I think I am reading contrary information (I thought I read that a 15-amp outlet is constructed in such a way as to avoid having a 20-amp device plug into it - am I missing something? Thanks for your help (whomever is kind enough to reply)

The 20 amp breaker may not trip because the load is under 20 amps, but still high enough to overload the 15 amp wire.

15 amp plug has two blades parallel up and down, and a 20 amp plug has one sideways from the other.

Thanks - I probably wasn't as clear as I could be in my question. What I was wondering was, if I have a 15 amp breaker, and 15 amp wire, is it a problem to connect them to a 20 amp plug? I have always assumed the 15 amp breaker would trip before overheating the 15 amp wire.

A 20 amp receptacle cannot be used on a 15 amp circuit. A 20 amp load will exceed the 80% rating of a 15 amp breaker and wire.

Can you use all 20 amp receptacles on a 20 (12/2) amp line, even if it is not the only 20 amp receptacle on the circuit? In other words, in a typical 12/2 receptacle circuit, can all the outlets be 20 amp?

I ask because I happened to have 3 boxes of 20 amp commercial receptacles that I have been using on my receptacle only 20 amp lines.

Yes.