Need help with wiring questions

Posted by Doug Miller on October 25, 2009, 8:15 am

>Methinks OP is over in the UK, perhaps?

His IP address indicates Portland, Oregon.

>Seeing as how he is running 240 on 14-2? They do some strange (by our
>standards) things with wiring over there.

There's nothing one bit unusual about running a 15A 240V circuit on 14-2. Why
do you think there is?

Posted by Doug Miller on October 25, 2009, 8:13 am

>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. The circuit is 30
>>amps, 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.

[...]

>You are talking about 10.4a or so. (2500w @ 240v = 10.42a)

As I pointed out in another post, his numbers are wrong, but so are yours.
3 * 750W + 1000W = 3250W = 13.5A @ 240V.

>You do need to size at the smallest conductor on the circuit.
>I would just put a 2 pole 15a breaker on that circuit and get on with
>your life being perfectly legal.

This would be good advice if your numbers were right. They're not, and he will
create a hazard if he adds that 1000W heater to the existing circuit.

Posted by on October 25, 2009, 12:21 pm

On Sun, 25 Oct 2009 12:13:18 GMT, spambait@milmac.com (Doug Miller)
wrote:

wrote:

>>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. The circuit is 30
>>>amps, 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.

>[...]

>>You are talking about 10.4a or so. (2500w @ 240v = 10.42a)

>As I pointed out in another post, his numbers are wrong, but so are yours.
>3 * 750W + 1000W = 3250W = 13.5A @ 240V.

>>You do need to size at the smallest conductor on the circuit.
>>I would just put a 2 pole 15a breaker on that circuit and get on with
>>your life being perfectly legal.

>This would be good advice if your numbers were right. They're not, and he will
>create a hazard if he adds that 1000W heater to the existing circuit.

Hazard?, What hazard? I agree I read two 750w not 3 but the 15a
breaker will still protect the 14 ga wire.
You size the largest load at 125% and the rest at 100% so he just
squeaks in.
1000 x 1.25 = 1250 / 240 = 5.3a
750 x 3 = 2250 / 240 = 9.43a
Calculated load 14.7a

Posted by Doug Miller on October 25, 2009, 2:32 pm

>On Sun, 25 Oct 2009 12:13:18 GMT, spambait@milmac.com (Doug Miller)
>wrote:
> wrote:

>>>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. The circuit is 30
>>>>amps, 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.

>>[...]

>>>You are talking about 10.4a or so. (2500w @ 240v = 10.42a)

>>As I pointed out in another post, his numbers are wrong, but so are yours.
>>3 * 750W + 1000W = 3250W = 13.5A @ 240V.

>>>You do need to size at the smallest conductor on the circuit.
>>>I would just put a 2 pole 15a breaker on that circuit and get on with
>>>your life being perfectly legal.

>>This would be good advice if your numbers were right. They're not, and he will

>>create a hazard if he adds that 1000W heater to the existing circuit.

>Hazard?, What hazard? I agree I read two 750w not 3 but the 15a
>breaker will still protect the 14 ga wire.
>You size the largest load at 125% and the rest at 100%

Code cite on that, please?

I'm specifically questioning the use of the 125% factor for _only a portion_
of a continuous load, as opposed to the _entire_ continuous load.

> so he just squeaks in.
>1000 x 1.25 = 1250 / 240 = 5.3a
>750 x 3 = 2250 / 240 = 9.43a
>Calculated load 14.7a

Posted by on October 26, 2009, 12:41 am

On Sun, 25 Oct 2009 18:32:00 GMT, spambait@milmac.com (Doug Miller)
wrote:

>>Hazard?, What hazard? I agree I read two 750w not 3 but the 15a
>>breaker will still protect the 14 ga wire.
>>You size the largest load at 125% and the rest at 100%

>Code cite on that, please?
>I'm specifically questioning the use of the 125% factor for _only a portion_
>of a continuous load, as opposed to the _entire_ continuous load.

You are right, I should not have said 125% of the largest load, it is
125% of all of them (16,92a) BUT 240.4(D) applies to the breaker
allowed, not the circuit ampacity, since the 80% rule is already built
into that number.
The ampacity of 14ga wire is 20a, the maximum O/C device is 15a unless
otherwise allowed.(not here)

Look at 210.19 FPN No. 1: "See 310.15 for ampacity ratings of
conductors."

BTW these are the kinds of questions that will divide a room full of
inspectors.
The bottom line is a 15a breaker will adequately protect 14ga wire, no
matter what you do with it and those heaters should not trip a 15..

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