BobH wrote:

Thanks for the answers, but I'm not quite clear on some details. I
probably should have posted the oven and cooktop questions separately.
Let me try this again.

First, three clarifications:

A. In the cooktop junction box, one of the wires has red markings on
it, so there I can distinguish between red and black. In the oven
junction box, the two blacks look identical.

B. The cooktop clearly takes 240 volts. It is marked 240/120V.

C. I don't have a multimeter.


Questions:

OVEN

1. Is there any difference between the two black leads in the oven box?
Does it matter which one goes to the black oven wire and which goes to
the red oven wire?

No difference/doesn't matter.

COOKTOP

2. Does everyone agree I should just put a wire nut on the white-stripe
wire and not connect it to anything from the cooktop?

Assuming it is the likely neutral I agree.
If they are on the same circuit (same fuse/breaker) IIRC the max size is
50A.

GROUNDS

3. In both junction boxes, the stranded aluminum ground is attached to
a lug screwed to the metal box. The oven ground wire is relatively thin
copper, and the cooktop ground is relatively thick copper. What is the
best way connect them to the box ground wire? Should I use Al/Cu wire
nuts on the part of the aluminum that extends past the lug?

Reasonable. If the wiring to the oven/cooktop is in flexible metal
conduit (not flexible cord) that is part of the ground.

MULTIMETER

4. If I go buy a multimeter, is it going to be obvious which wire is
neutral? How will I know?

120V to each hot, 0V to ground

CONNECTORS

5. I have seen expensive Al/Cu nuts from the Ideal company (about $5
for only two nuts). I think they have antioxidant inside. They look a
little small for my wire, though.

I also have some larger wire nuts and a tube of antioxidant. As I
understand it, the preferred practice when using wire nuts is to tape
them after twisting the wires together and embedding antioxidant in the
nut.

Another recommendation I have seen is to buy a special
aluminum-to-copper "bug" that has separate screws or clamps for the two
wires, and to tape it up after connecting the wires.

There is information on making aluminum wire connections at
http://www.inspect-ny.com/aluminum/alreduce.htm
which is based extensive research, primarily for the Consumer Product
Safety Commission. It does not like the Ideal wire nuts, prefering 3M
wire nuts and antioxide paste.

Someone discovered a more recent product
http://www.alumiconn.com/details.html
but the max wire size is only #10

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