No electrician who deserves to be called that uses anything other than green or bare wire for the equipment ground (on the sizes of wire we are talking about).
The smaller wire is a hint that it might be an illegal ground installation, you'll need to look back at the panel to verify that it is indeed the grounding conductor. If so, do yourself and all others who come by later a favor and wrap it with green tape on both ends.

Hi ronnatalie, thanks for the reply. I don't know if I was clear or not...the second black wire isn't "smaller" per se...it's the same size as the red and white ones. However, the other black wire is significantly heavier than the other three, which is why I believe it's the "actual" hot black wire.

Having said that, I'm just a DIYer so I've never done anything with a panel...I know panel work can be significantly more dangerous than in-home wiring. Is it dangerous to even just open it up to look at the wiring? Do I need to shut off the main house breaker or anything? Or just don't touch anything and I'll be fine?

Thanks,
~D

Unless you know what you're looking for, perhaps it's time to call the electrician. You can carefully remove the panel cover being careful not to touch anything. Even turning off the main breaker won't kill all the power inside that panel.

Ok, here's some more information and two more questions....I talked to the guy who helped me to disconnect it, and what HE said was that he wrapped electrical tape around the wires he disconnected, and that there were black, red, and white wires connected, but the green wire was grounded to the junction box. Sure enough, there are three wires wrapped with electrical tape as he says, and the extra black wire does NOT have electrical tape on it. So assuming he's remembering correctly:

1. is it safe to ground an oven to a junction box rather than an explicit ground wire?

2. is it safe to have a black (presumably hot) wire inside a junction box that is connected to *nothing*, but capped off?

1. is it safe to ground an oven to a junction box rather than an explicit ground wire?
With small exception (metal conduiting) no, the box must be grounded and the connection to the equipment must be made seperate from the box.

2. is it safe to have a black (presumably hot) wire inside a junction box that is connected to *nothing*, but capped off?
That would be fine *EXCEPT* it's not safe to leave the stove ground unconnected.

Are these individual wires in a conduit? If so, then you ground to the box and the conduit serves as the ground.

Thanks for the info. As far as I can tell (since I can't see inside the wall) it appears that the junction box is attached to metal conduit...which sounds like that means it's ok to ground the oven to the metal box?

I strongly urge you to get an electrician involved at this point. If you make the wrong assumption about the ground wire and a house fire starts there, you'll be up the creek on getting your insurance to pay for it.

My last older house had lots of metal boxes, but the metal strain reliefs were just that-- strain reliefs. You have to be fairly educated in the subject to tell the difference by just looking.