Need help with wiring questions

Posted by Doug Miller on October 26, 2009, 7:23 pm

>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."

And you're right about that -- I'd forgotten that the 80% rule was already
built in to the overcurrent ratings for 14- and 12-gauge 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..

Agreed.

Posted by on October 26, 2009, 7:47 pm

On Mon, 26 Oct 2009 23:23:56 GMT, spambait@milmac.com (Doug Miller)
wrote:

wrote:

>>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."

>And you're right about that -- I'd forgotten that the 80% rule was already
>built in to the overcurrent ratings for 14- and 12-gauge 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..

>Agreed.

No problem partner. As I say, those kind of questions will usually
start an argument at an IAEI meeting.
The real thing to remember is you have to use 240.4(D) to size wire
when you have receptacles and lampholders, where the installer has no
control over what the user plugs in. That is pretty much what 210.23
is all about

Posted by terry on October 31, 2009, 6:40 pm

On Oct 26, 8:23=A0pm, spamb...@milmac.com (Doug Miller) wrote:
m wrote:

> >On Sun, 25 Oct 2009 18:32:00 GMT, spamb...@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 por=

tion_

> >>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."

> And you're right about that -- I'd forgotten that the 80% rule was alread=

> built in to the overcurrent ratings for 14- and 12-gauge 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..

> Agreed.- Hide quoted text -
> - Show quoted text -

Exactly:
Suggestion; Change the double pole breaker to 15 amp, 15 x 240 =3D3600
watts and that size breaker protects the 14AWG wiring. Retain existing
14AWG wiring, use 14AWG for the new 100 watt heater.
The existing three 750 watt heaters draw 3 x 750 =3D 2250; add 1000 for
the new heater 2250 + 1000 =3D 3250.
That 3250/240 =3D 13.54 amps. That's within the 15 amp rating of the
whole circuit.

Posted by zxcvbob on October 25, 2009, 2:03 am

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. 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.
>
> 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?

The existing circuit is fine with 14 ga wire (but it should have a
15A breaker or fuses, not 30A). You can load it up to 12A, because
resistive heating circuits are limited to 80% of their ampacity --
so adding a 1000W heater (14 guage wire is good enough) takes you
pretty close to the limit but you're still under.

Make sure that the breakers or fuses are the right size for the
wires -- 15A. This is important. The rest, not so much.

Bob

Posted by Doug Miller on October 25, 2009, 8:22 am

wrote:

>The existing circuit is fine with 14 ga wire (but it should have a
>15A breaker or fuses, not 30A).

Right...

>You can load it up to 12A, because
>resistive heating circuits are limited to 80% of their ampacity --

Right...

>so adding a 1000W heater (14 guage wire is good enough) takes you
>pretty close to the limit but you're still under.

.. and wrong, and wrong. Do the math. It takes him well *over* the limit, and
14ga wire is *not* "good enough".

3 * 750W existing + 1000W new = 3250W total = 13.5A @ 240V.

He needs 12ga if he's going to put all of them on the same circuit. It's
probably a lot easier to run a new 15A circuit on 14ga wire for the new heater
than it will be to rewire the existing circuit with 12ga.

Page 3 of 6 < 1 2 3 > last >>

Similar Threads	Posted
Wiring questions:	May 27, 2006, 10:07 am
More wiring questions	October 6, 2006, 7:48 pm
Wiring a shed questions	August 1, 2005, 5:33 pm
crawlspace wiring questions	July 9, 2006, 2:08 pm
Outside Light Wiring Questions	September 3, 2006, 4:32 pm
telephone wiring questions	December 24, 2006, 11:23 pm
Electrical wiring questions	January 23, 2007, 11:21 pm
electrical wiring questions	March 4, 2007, 10:58 am
Electrical wiring questions	September 15, 2007, 12:45 pm
Electrical Wiring Basic Questions	September 3, 2007, 11:37 pm

Need help with wiring questions - Page 3