On Oct 25, 5:09*am, (Doug Miller) wrote:
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.

If I can add an additional question - just for my personal
knowledge...

At a junction box, why is it bad to step down the gauge of a wire to
match the draw of the load on that wire?
In my case, if I had 12 gauge wire to a junction box, why would 14
gauge from the box to the individual heaters be a violation? If each
is only capable of drawing a few amps, and the "pipe" to the box is
big enough?

Tnx