New wall oven. Old one had 4 solid heavy gauge copper wires in conduit (i think they are black, red, white, gray?). New on has 3 tiny braided copper wires: black, red, green. First two hot 110s, green is ground.

The extra one is the neutral. What do I do with it?

Manufacturer said that new thin wires because no self clean. Oven maxes at 3800 watts.

But how do I connect 3 oven wires to 4 house wires? Can I simply cap off the neutral?

Other issue is 2 tandem 40 amp fuses in breaker box. I think I can just pop them out and replace with tandem 20 amp fuses right?

Should I disconnect neutral at the box too?

 

Here's a pretty good explanation of wiring 220.
http://www.nojolt.com/Understanding_...circuits.shtml

I'm assuming you mean the fuses are in parallel, one for each 110v circuit.
Yes, you can, and should, replace them with fuses rated for what you're running on the circuit. There's little chance of your new stove overloading the existing wiring, but using fuses more closely matched to the appliance will help protect the appliance wiring in the event of a short.

(not an electrician, but picked up a bit from my dad who was)

 

So the other day I took the big old solid copper wires from house conduit and wired them into back of oven. Black and red went to hot, and the white house neutral went to where the oven's green wire had been. The house ground wire just got secured to outside of metal oven case.

That's pretty much the way old oven had been set up.

Everything seems to work.

Woo hoo!

Oh, and swapping the 40amp double pole? breakers for 20 amp double pole breakers took all of 3 minutes at the breaker box.

Yeah - I went from 40 amp to 20 amp because all new oven needs is really 15-20. Without the 'self clean' feature, ovens don't suck nearly as much power.

 

you should just cap off the extra neutral wire from the house if your oven doesn't need it, and wire the ovenhow the directions show. in actuallity the neutral and grounds are all connected in the main panel, but code requires them to stay seperate til they get back to the main breaker.