In article , gwandsh wrote:
This weekend I had planned to add a 1000 watt electric heater to an
existing circuit with three similar heaters on it. The other heaters
are 750 watt, wired 240, and draw 2.7 amps each.

750W / 240V = 3.1A, not 2.7A.

The circuit is 30 amps,

Are you sure? A 15A double-pole breaker is a 15A 240V circuit, not 30A.

so I expected to pony the new heater (4.5 amps @ 240V) off the
wire to one of the existing heaters, and still not challenge the
breaker.

1000W / 240V = 4.2A, not 4.5A.

I was surprised to find the wire to the circuit I planned to pony from
was a 14/2. I traced it back to the junction box, and found each of
the heaters was serviced by a 14 gauge wire. Then I was horrified to
find that the wire from the panel to the main junction box for all
circuits was also a 14 gauge. Even at low amperage, I would expect at
least a 12 gauge, maybe 10?

14-gauge wire is perfectly fine for the existing load: 15A * 240V * 80% =
2880 watts, and you're using only 2250.

Adding another 1000 watts puts you over the limit, though.


So, I have shut off the circuit pending some advice on how to wire it
safely. I expect my options are :

1 - Panel-Junction box 10 gauge. Junction box to each heater 12
gauge.
2 - Panel-junction box 12 gauge. Junction box to each heater 12
gauge.
3 - Panel- junction box 12 gauge. Junction box to each heater 14
gauge.

None of the above. 1 is completely unnecessary; 2 works fine, but is much more
trouble than it's worth; 3 is a Code violation with either a 20A breaker
(because the 14ga wire is undersized for the breaker) or with a 15A breaker
(because the breaker is undersized for the load).

Instead, run a new 240V circuit, using 14ga wire and a new 15A double-pole
breaker, to supply the new heater, and make sure that the existing circuit
uses a 15A double-pole breaker as well. *Much* less effort than rewiring the
existing circuit.


The final load on the circuit at 240V will be about (if all was on at
full blast) 13.5 amps. We have never turned on any more than two of
the heaters, but I am pretty sure the proper calculation would require
the worst case scenario.

Yes, it would. This is the worst case: all four heaters drawing maximum
current for more than three hours, which meets the Code's definition of a
"continuous load" and therefore limits the circuit to 80% of its rated
capactiy. Three 750W heaters plus one 1000W heater = 3250W, or just over
13.5A, as you said. Eighty percent of 15A is 12A, so you need a 20A circuit
instead, and that means 12ga wire instead of 14. Don't waste your money on
10ga wire, though. There's no benefit.