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Posted by zxcvbob on October 25, 2009, 3:20 pm
gwandsh wrote:
> On Oct 25, 5:09 am, spamb...@milmac.com (Doug Miller) wrote:
>> In article
>>> 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
There is nothing wrong with that, but (with a few exceptions*) the
breaker has to protect the smallest sized wire. So with a long run
of 12 gauge wire (to reduce voltage drop) that then taps off with a
14 gauge wire, you need a 15A breaker.
*there are some exceptions, like the "10 foot tap rule" which might
be used for a kitchen with a split cooktop and oven on the same 40A
circuit.
Bob
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