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Silent_Bob

05:58AM | 09/15/00
Member Since: 09/14/00
5 lifetime posts
Bvelectrical
I have a question regarding use of 12/3 cable to wire the two 120V/20A branches in a kitchen wiring scheme.

I believe the NEC allows 12/3 cable to be used to wire two branch circuits in a kitchen, where half of the outlets use the red wire for hot and the other half use the black. Usually ever other outlet gets a red and vice versa for black. Every outlet uses the same 20AWG white wire for neutral. At the circuit panel the red is wired to one 20A breaker and the black wire to a separate 20A breaker. The white is connected to the neutral bar like any other 120V branch.

Given this wiring, the 12AWG white wire could be carring as much as ~40A if a both circuits are just under maximum loading. The circuit breakers would not trip if each branch was using, say, 19A. 19A would be going through the red wire, 19A through the black wire. These would be safe. However, the white wire would be carrying 38A, which is way above its capacity of 20A. Why does the NEC allow this? I know this is an unlikely occurance(such high loading), but it is possible, especially in an kitchen. It seems like an inconsistancy in the usually very conservative code.

GFCI's would not help in this situation either. As I understand, a GFCI trips if current is detected going throuh the ground pin instead of the neutral. This is usually caused by a short to ground in an appliance, say water gets in there and shorts power to the chassis of the device. In my example, the appliances are operating normally, there is just a lot of high power devices.

To any of you who may think that because the neutral wire is at 0V, there is no current flowing throuh it, that is not true. That would only be true if there was no load in the circuit. The current(Amps) in a circuit depends on the voltage supplied and the size of the load. And, in home wiring, the current flows through the hot wire, through the load(appliance or light, etc), then through the neutral wire back to the panel. Current ALWAYS flows in a loop in this manner.

If there is anyone out there who knows why the code would allow this, please respond. I think this is not a safe wiring scheme and I would always use two separate cables for anything like this.

ElectrcBil

11:39AM | 09/15/00
Member Since: 07/21/00
76 lifetime posts
The answer is very simple. At no time does the current equal the sum of the two circuits. The current alternates on the different circuits. When it is traveling on the black towards the neutral the red circuit is traveling away from the neutral. This is true when the circuits are installed on opposing legs of the service which is how it is supposed to be done. If you put both circuits on the same legs then yes you would have a situation as described. In commercial three phase wiring it is not uncommon to have three circuits share the same neutral. As for your gfi situation refer to the instructions on or in the box, they usually indicate how to wire the device to this type of circuit. Let me know if you don't understand and i will try to explain it better.

Silent_Bob

06:11AM | 09/19/00
Member Since: 09/14/00
5 lifetime posts
This makes sense if the code requires that a double pole breaker be used or at least the red and black get attached to different phases in the panel.

Is there a copy of the NEC available on-line? If so, could the location be added to the FAQ. This would be a pretty useful reference to have.

Thanks for explianing this.

bink

10:30AM | 09/22/00
Member Since: 01/18/99
47 lifetime posts
ElectricBil is correct. In a mult-wire circuit, each curcuit must be on a different phase. You use two single pole breakers, one on one phase and the other on the other phase. My garage/workshop is set up this way.
No, there is no NEC manual online, that I know of. Either go to the library and check it out and buy one for 40+ dollars.

ElectrcBil

09:55PM | 09/24/00
Member Since: 07/21/00
76 lifetime posts
Yes Silent-bob there is a website for the NEC here it is.http://www.nfpacodesonline.com

[This message has been edited by ElectrcBil (edited September 25, 2000).]

Deep

10:26PM | 09/28/00
Member Since: 09/28/00
3 lifetime posts
Some good responses. SilentBob, look up Thomas Edisons 3-wire circuits for more info, if you want more in depth info. Usually the most common problem that can occur with a 3-wire circuit is getting an open nuetral (its like opening a can of worms)If you look closely they are probablly a few 3-wire circuit currently in your house. Where I am and most likely where you are the Code requires that your counter plugs not be adjacent when on the same feed and that you use a double pole breaker or two single poles with an approved tie bar. So as to prevent someone only turning off half the receptacle off and thinking its dead.

gmpirate

10:01PM | 08/10/01
Member Since: 10/03/00
15 lifetime posts
ElecticBill,

I have just installed this same setup with a 2-pole 20amp breaker. I was planning to put 2 GFI's at the beginning (one for red, one for black). How do I wire the whites?

Anonymous

07:59AM | 08/19/04
The currents ALWAYS add. The whole idea of a three wire feed is that when wired correctly ON A TWO POLE BREAKER the red current is + when the black is - and they ADD up to zero. If the original poster puts them on two of the same phase, his white turns brown and smells funny, and gets the full 40amps. [The Volts have to measure 240 Black to red 120 phase to neut.]

Wiring correctly cuts the line loss on the neutral [I squared x R] to zero which makes the properly wired and balanced 12/3 circuit 50% more energy efficient, and less EMF radiated in your walls. That's why it is allowed.

The GFI's are another issue, this 3 wire technique is on the way out for GFI'd kitchen appliance branch circuits. Canada allows only custom split circuit two pole 15 amp GFI's,or single circuit 20 A GFI's so I give up trying to argue for 3 wires on kitchen counters. A fridge and Dishwasher/disposal would be a better place to use a two pole 3 wire.
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