On 2007-11-12, bent wrote:

I traced the cabling back into the fuse box. This receptacle box
(the one I want to change the broken plastic on) has two lines
coming into/ going out of it: a 14/3 (black/white/red, + gnd) going
back to the fuse box, and a 14/2 (black/white, + gnd) going to a
single under the counter type ~20W fluorecent light with a little
on/off button.

[. . .]

So: Bare ground to bare ground to bottom ground screw of the
receptacle. The two are to the rear of the box, and another single
wire to the receptacle screw. White wire to white wire on the
silver coloured/unbroken side of the new receptacle. So they become
connected, as are the grounds.

Red wire from fuse box to where?
Black wire (from the fuse box) to where?
Black wire (to supply power to the single, 20W F. light) to where?

First, if you have fully traced the circuit and know all the loads on
it, you can wire the circuit any way that does what you want and is
code compliant.

Now we can say with fair certainty how the original circuit was wired:
red and black wires from the fuse box to opposite hot sides of the
receptacle (with the connector tab broken), black wire to the light
connected to one side of the receptacle (either side, doesn't matter).

Unfortunately this is not a code compliant way to wire the circuit
now. Two problems:

If you have a multi-wire branch circuit (two opposite hot phases each
serving 120V loads with a common neutral), then the connectivitiy of
the neutral can not depend on the device being installed. This just
means you have to take the two neutrals and pigtail them to a short
piece of wire that is then connected to the receptacle.

If you have a multi-wire branch circuit, then when there are any
devices that are connected to both phases (like your split
receptacle), the circuit needs to be opened by a single overcurrent
device.

For this second requirement, if the circuit is protected by circuit
breakers, this means that a handle tie is required so that both
circuit breakers open at once. I don't know how this requirement is
applied to fuses: my instinct would be that you can't use two separate
fuses, so you simply can't wire a split receptacle at all. Perhaps
someone more experienced than I am can comment.

As to what to do to comply with the second requirement, more
information about the receptacle is required. Where is it located?
Do you need the extra capacity that the split receptacle requires, or
will the total current draw at the receptacle be less than the 15 amps
the 14 gauge wire can provide?

Cheers, Wayne