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Posted by gwandsh on October 25, 2009, 12:56 pm
On Oct 25, 5:09=A0am, spamb...@milmac.com (Doug Miller) wrote:
> In article <f90ce47e-3d33-4189-9685-86193efa4...@z4g2000prh.googlegroups.=
> >This weekend I had planned to add a 1000 watt electric heater to an
> >existing circuit with three similar heaters on it. =A0The other heaters
> >are 750 watt, wired 240, and draw 2.7 amps each. =A0
> 750W / 240V =3D 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 =3D 4.2A, not 4.5A.
> >I was surprised to find the wire to the circuit I planned to pony from
> >was a 14/2. =A0I traced it back to the junction box, and found each of
> >the heaters was serviced by a 14 gauge wire. =A0Then I was horrified to
> >find that the wire from the panel to the main junction box for all
> >circuits was also a 14 gauge. =A0Even 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% =
=3D
> 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. =A0I expect my options are :
> >1 - Panel->Junction box 10 gauge. =A0Junction box to each heater 12
> >gauge.
> >2 - Panel->junction box 12 gauge. =A0Junction box to each heater 12
> >gauge.
> >3 - Panel-> junction box 12 gauge. =A0Junction 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 break=
er
> (because the breaker is undersized for the load).
> Instead, run a new 240V circuit, using 14ga wire and a new 15A double-pol=
e
> breaker, to supply the new heater, and make sure that the existing circui=
t
> 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. =A0We 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 =3D 3250W, or just ove=
r
> 13.5A, as you said. Eighty percent of 15A is 12A, so you need a 20A circu=
it
> instead, and that means 12ga wire instead of 14. Don't waste your money o=
n
> 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
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>are indeed a 15 amp double poled circuit, rather than a 30 amp as I
>reported. It wasn't until this "Eureka" moment that it all made
>sense, and I appreciate all the feedback here.