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Posted by Doug Miller on October 25, 2009, 2:27 pm
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>It occured to me last night that the "paired" circuit breakers I saw
>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.
That's a common misunderstanding.
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>I think I will return the 1000 watt heater (forget to get one with
>thermostat anyways, yeesh), and get a 750 watt version. That one I
>will pony off one of the existing heater sub-circuits. We have never
>turned on more than one heater at a time at this place, and never even
>close to max capacity (weekend summer application) so the circuit load
>will never be an issue.
Perhaps *you* won't ever turn on all the heaters at once, but what about the
next homeowner? Resistance heating is definitely subject to the 80% rule,
which means that the circuit is limited to 15A * 240V * 80% = 2880 watts. Four
750W heaters is 3000 watts. Not much over the limit, but still over. With all
four in use, the breaker won't trip, but it is possible to overheat the
circuit conductors.
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>Again, thanks
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Posted by gwandsh on October 26, 2009, 12:34 am
On Oct 25, 11:27=A0am, spamb...@milmac.com (Doug Miller) wrote:
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> In article <9e61369c-23c4-4166-999d-adb26d21e...@s21g2000prm.googlegroups=
> >It occured to me last night that the "paired" circuit breakers I saw
> >are indeed a 15 amp double poled circuit, rather than a 30 amp as I
> >reported. =A0It wasn't until this "Eureka" moment that it all made
> >sense, and I appreciate all the feedback here.
> That's a common misunderstanding.
> >I think I will return the 1000 watt heater (forget to get one with
> >thermostat anyways, yeesh), and get a 750 watt version. =A0That one I
> >will pony off one of the existing heater sub-circuits. =A0We have never
> >turned on more than one heater at a time at this place, and never even
> >close to max capacity (weekend summer application) so the circuit load
> >will never be an issue.
> Perhaps *you* won't ever turn on all the heaters at once, but what about =
the
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> next homeowner? Resistance heating is definitely subject to the 80% rule,
> which means that the circuit is limited to 15A * 240V * 80% =3D 2880 watt=
s. Four
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> 750W heaters is 3000 watts. Not much over the limit, but still over. With=
all
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> four in use, the breaker won't trip, but it is possible to overheat the
> circuit conductors.
> >Again, thanks- Hide quoted text -
> - Show quoted text -
I hear your concerns. While I plan to own this place for many years,
I am a big proponent of doing things right and not handing my problems
off to someone else. I certainly could not handle it if any harm came
to someone downstream from my actions.
I will add a new circuit and string a new 14 gauge wire for the new
heater.
Again my thanks to the group knowledge base.
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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:
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> 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
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> 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
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> 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=
> (because the breaker is undersized for the load).
> Instead, run a new 240V circuit, using 14ga wire and a new 15A double-pol=
> breaker, to supply the new heater, and make sure that the existing circui=
> uses a 15A double-pole breaker as well. *Much* less effort than rewiring =
the
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> 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=
> 13.5A, as you said. Eighty percent of 15A is 12A, so you need a 20A circu=
> instead, and that means 12ga wire instead of 14. Don't waste your money o=
> 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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Posted by Doug Miller on October 25, 2009, 2:28 pm
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>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?
Because the Code says so. There are only a few exceptions, and AFAIK this
isn't one of them.
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Posted by aemeijers on October 25, 2009, 1:44 pm
Doug Miller wrote:
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> In article
>
>> 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?
>
> Because the Code says so. There are only a few exceptions, and AFAIK this
> isn't one of them.
I asked an actual electrician about this once, and he said it was
because the NEXT silly SOB might add yet something else to that leg of
the circuit. (Hey! I can grab power from that box right there! etc...)
aem sends...
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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.