Hello, My mother-in-laws house built in the late 50's has Aluminum wiring throughout. I need to run a new leg ( one-20 amp outlet) from an existing circuit, 20amp AL 12-2/g. Can I do this with copper 12-2/g or need to run new circuit from panel. Also what about GFI outlets on aluminum?

Do NOT combine aluminum and copper on the same circuit.

It would be better to have the aluminum wire removed from the house all together, and replaced with copper throughout, this may not be the answer you wanted, but it's probably the best.

What ever you do DO NOT combine the two metals on the same circuit.

To use 20 amp on aluminum you would need to run 10/2 not 12/2. 12/2 AWG is 20 amp copper.

If you are going to tun a home-run shot from the main panel to the new outlet, I would indeed select 12/2 copper along with a 20 amp breaker.

If you are going to cap onto an existing end run then the problem gets a little trickier.

The problem with mixing aluminum and copper is that they are dissimilar metals. Aluminum will loosen the connections because of the way it expands and contracts. The chemical properties of the two metals also cause "Oxidation". There are two favored ways around this problem, three if you count removing all of the aluminum.

1) COPALUM connectors. - A licensed electrician uses a press to reduce the alum. And copper to almost a gaseous state and then clamps the two metals together.

2) Wire Nuts with an "Antioxidant filling" coat the dissimilar metals and prevent them for oxidizing and loosening.

Personally, I prefer to run a new copper line from the panel, but that's just a opinion.

Also, whenever, you use a device, it must be listed for CO/AL (Copper / Aluminum)