In article , 46erjoe wrote:
I need to install baseboard electric heating units in two rooms I'm
refurbishing. One will need a 48" 2000 watt unit; the other a 36" 1500
watt unit.

The instructions in each box say that I will need to use a 220v

220, or 240? It makes a difference. If the heater is rated 2000 watts at 220V,
it will produce almost 2400W at 240V -- which is almost certainly the
voltage that you actually have in your house. And never mind the installation
instructions. Look at the rating plate on the heater.

circuit with "amperage according to local code".

You'd have to talk to a local electrical inspector to find out what your local
code is.

Both rooms are next to each other and I'm wondering if instead of
fishing two wires, I can go with one 10/2 30amp circuit and have both
units branching off the main line. What might be the minimum wiring
and amp circuit? What would be safest? What's the most amperage I can
get out of a 10/2 line?

Under the National Electrical Code, 10/2 is limited to 30A overcurrent
protection (breaker or fuse), and continuous loads (such as electric
resistance heating) are limited to 80% of the overcurrent rating -- which
would be 24A for a 30A breaker. 24A at 240V is 5760 watts; your heaters total
3500, so one 10/2 30A circuit will be just fine.

Related question: I've got tons of 12/3 wiring with ground laying
around unused. Can I turn this into, say, 10/2 by simply clamping
the black and red wires together at the panel and at the end point

NO. First off, that's a Code violation: connecting conductors in parallel is
not permitted. Second, even if that was allowed, that would take care of only
*one* of the two conductors in the circuit anyway. What about the other one?
It would still be 12ga. And don't even think about doubling up the white and
bare wires -- you could wind up making the case of the heater live.

However, if the heaters are rated 2000 and 1500 W at 240V, you don't need a
30A circuit anyway, and you can use 12ga wi 20A * 240V * 80% = 3840 W,
which is adequate for the heaters you have.

OTOH, if they're rated 2000 and 1500 W at 220V, and you run them on 240V, then
you will need a 30A circuit, because the heaters will produce almost 20% more
power: 2380 and 1785 watts respectively, for a total of 4165 watts -- too much
for a continuous load on a 20A circuit.

This is why I asked above if they're rated at 220V or 240V. It does matter.

and then painting the red wire with black marker pen to indicate power?

That would be unnecessary -- red is assumed to be power anyway -- but as noted
above, it's a Code violation, and it's not safe.

I hate to waste wire with the cost of copper these days.

It's come down quite a bit since June. It's still more than double what it was
two years ago, but I saw 250' of 12/2 NM at Home Depot last week for $67...
and just a few months ago, it was over $100.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.