Hi,
I was just wondering if anyone could explain that to me.
The 14/3 wire has two hot wires and one neutral. Assuming full load on both wires of 15amps the single neutral wire that uses the same gauge of wire as each of the hot wires is handling twice as much current, 30amps.
Am I misunderstanding something? Is the gauge of the wire for 15amps selected based on the need for the 14/3. As in if there was no 14/3 wire could then make the 15amp cable out of say 16 gauge with the same safety limits factored into the 14/3.
Thank you,
learner
Replies
The power is running on each hot leg in a phase, which you can imagine to be a wave. The wave on each leg (red and black) is 100% opposite of each other (or 180 degrees) so that when one wave it peaking, the other is at its lowest point.
When both legs meet back on that single neutral leg (white) they cancel each other out. The power (amperage) doesn't double, it cancels.
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
If your hair looks funny, it's because God likes to scratch his nuts. You nut, you.
"When both legs meet back on that single neutral leg (white) they cancel each other out. The power (amperage) doesn't double, it cancels."does the NEC say how it should be wired in the breaker panel?
bobl Volo, non valeo
Baloney detecter WFR
"But when you're a kibbutzer and have no responsibility to decide the facts and apply the law, you can reach any conclusion you want because it doesn't matter." SHG
Que?Rebuilding my home in Cypress, CA
Also a CRX fanatic!
If your hair looks funny, it's because God likes to scratch his nuts. You nut, you.
As I understand it, if you run14/3 wire off of 2 breakers on the same side of the breaker panel, you tain't 180 degress out of phase. does the code require such a set up to be from different sides of the panel?
bobl Volo, non valeo
Baloney detecter WFR
"But when you're a kibbutzer and have no responsibility to decide the facts and apply the law, you can reach any conclusion you want because it doesn't matter." SHG
Never seen a 240 breaker from both sides of a panel. Mine is set up so every other breaker is on a different leg.
Pushmatic is accross from each other not up and down.
No, the NEC doesn't show how to wire up a shared-neutral circuit (also called multi-wire circuit or Edison circuit). It's also called a 240/120V circuit, and the most common use in residential is the electric clothes dryer circuit. Actually, a house utility feeder is a 240/120V circuit, too.
The NEC is a set of minimum criteria for electrical installation, not a manual of techniques. An electrical apprenticeship is 4 years of on the job traininng with classrom instruction at night.
Not that it'd take that much study and training to understand residential electrical. There are a bunch of good "how to" books, I like the Ortho and the Black and Decker ones. For something more focused on theory and key Code requirements, John Traisters "Wiring a House" is real good.
The National Fire protection Assn. publishes the NEC, and also puts out a little handbook that includes the parts of the Code that apply to residential wiring. It's called the "Pocket Guide to Electrical Installation under NEC (year) Volume I: Residential". The NEC is revised every three years and the latest version is 2008. Check with your local building dept., as it's the local authority that adopts the NEC, and sometimes amendments. Some authorities having jurisdiction (AHJs) work off of old versions of the Code, others adopt the most recent fairly quickly.
O.K., now that you've suffered through the sermon, you can eat...
The way you wire up a multiwire circuit is to put the two hots (ungrounded wires) on the two different legs or poles of the service. For most modern breaker panels, that's two adjacent fullsize breakers--a breaker and the one above or below it, or a breaker and the one to it's right or left. But it depends on the configuration of the panel. The critical thing is to hook the two hots of the multiwire ckt to 240 volts.
Good luck,
Cliff
Cliff,I like to use 3-wire circuits when running A/B outlet circuit runs, like kitchens or workshops. I thought I recalled reading somewhere that in those instances you should use a double-pole breaker or breaker ties so that you have to turn off both circuits or they both cut out if one trips. The reason being that someone might try and work on one side of it while the other was still hot, and there could still be current flowing through the neutral that was serving both sides, exposing the person handling it to shock. Isn't that what this section of the 2006 IRC says:E3601.5 Multiwire branch circuits...Where two or more devices on the same yoke or strap are supplied by a multiwire branch circuit, a means shall be provided at the point where the circuit originates to simultaneously disconnect all ungrounded conductors of the multiwire circuit...Steve
Steve,
Yep, it's an NEC requirement to put a multiwire ckt on a two-pole breaker or use a proper tie bar or clip to tie the handles of the two breakers together, when the two legs land on terminals on the same strap--i.e., the same device. An example is when you split a duplex receptacle (considered two devices if it's split by breaking off the tab between the brass screws), and feed each receptacle from a different leg of the multiwire ckt. You're right, it's a safety thing, and makes sense.
This configuration (split duplex receptacles, each half fed by a different pole) is useful for situations where you expect to have a couple of heavy loads in one location. This configuration is not commonly used in residential wiring in the U.S., but I believe it's required in Canada for kitchen countertop receptacles. Of course, because the small appliance receptacles have to be GFCI-ptrotected, if you use a multi-wire circuit for this application, you have to use a two-pole GFCI breaker. Those are expensive, and it's more convenient to re-set a GFI receptacle than to troop to the basement panel to reset the GFI breaker.
Another option is to run a multiwire to the first box and split the circuit there. You then split the hot and neutral side of each downstream duplex receptacle, and run two 12/2 cables from box to box, or use the new Southwire 12/4 with ground cable--two hots and two neutrals in one jacket. (the neutrals are identified separately). Then use two GFI receptacles, one first in line for each circuit. This makes for a lot of wires in the box, but that can be dealt with easily enough by using 4" square boxes and a plaster ring. It's also a little confusing to the homeowner or resident, as you might imagine. Top plugs go dead, there are two GFI receptacles in the kitchen, which GFI is it? Some of the newer GFIs are pretty good on this count, with an indicator light that goes on when they trip.
In the 2008 NEC, all multiwire circuits have to be protected by a two-pole breaker or by breakers with tied handles. This is a safety thing, too, as some unqualified person may cut power to one of the multiwire ckts, then go into a box and test one leg and not the other.
I think this is trading function/utility for overprotection...if I'm supplying two duplex receptacles in one box with a multiwire, I may not want both circuits to be shut down if only one has an overload or fault. It's up to the person doing the work to understand this stuff and verify power is off. What next, a requirement on the NEC that there can be only one circuit in any given box, or that they have to be common trip? Sheesh! Next thing you know, all house wiring will have to be low-voltage to protect the uninformed, stupid, or careless.
Cliff
Edited 5/13/2008 1:58 pm ET by CAP
Hi Cliff,It was my impression that the code is there to prevent bad installations and while it isn't an installation manual it does have clauses to require proper installation. I would think that the 14/3 connection would require a clause. If the 14/3 is connected incorrectly then you have the potential for the neutral to catch on fire - maybe it is over engineered enough that this is unlikely but there are lots of things in the code that are over engineered for safety. There are some things that are covered under "workmanship" but I wouldn't expect this to fall in that category and while the code shouldn't really say how to hook it up, it should require that you hook it up to the 240volts, does it not have a clause to require that?If you are the Popejoy of FHB fame, thank you for all the articles over the years. I've always particularly enjoyed them.learner
Thanks for the reply.Is it then a requirement to hook up the hot wires to specific areas in the break in order to ensure you are using the split phases? I am not sure exactly how the phase layout works in all breakers, it is likely every other so if you are using 14/3 chances are you would hook it up to the two closest spots and happen to have +120 and -120 but in the case where your panel was already fullish and you had two spots open that happened to be +120 and +120 you could hook the 14/3 up in correctly?Thank you,leaner
To wire it up, you really need to have a look inside your panel. All the ones I've seen have every slot in the box on an alternating hot, this way a double wide breaker covers a hot from both sides of the split phase.
If your box is pretty full, you may need to move some breakers up or down to give yourself a double wide slot... keep an eye out for lines that are red going into the box, those might also be setups like you are doing!Rebuilding my home in Cypress, CA
Also a CRX fanatic!
If your hair looks funny, it's because God likes to scratch his nuts. You nut, you.
Of course the other solution is to not use 14/3 but instead use two 14/2 cables.
What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell
14/3 wire is commonly used in homes to run between switches where one lead is always hot and the other is switched hot. The neutral in the wire may or may not be the only return lead in the eventual circuit.
FYI, AFAIK the degree separation is 120 between phases, not 180. There is no more 2 phase power commonly in use today unless someone has installed specialized transformers to do so.
The separation in 120/240V setups is indeed 180 degrees. You can (by analogy to 3-phase) call it "two phase", but folks here seem to get their shorts in a knot if you do.14/3 wire is used anywhere 3 wires are needed in a single circuit -- can be a 240V circuit or ceiling fan/light or a 3-way switch leg or any number of other things.
What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell
I didn't say 120/240 setups. No, I was referring to 3 Phase power. My mistake... Sorry for confusing the issue. Whenever I hear phase, I automatically jump in that bucket. You are correct that the two leads in a home breaker box are 180 degrees apart since they came from the same wave form initially.
People get upset when hearing home circuits referred to as 2 phase because the more correct term is split phase. 2 phase power would be 90 degrees out of phase. Split phase is still single phase power, just split across two conductors to minimize thick cabling and cost in transmission. The overall amperage is now just split between the two hots with a 1/2 reduction in voltage. The phase is always relative to the center. Oh, I need a wye diagram... Nevermind, the rabbit hole is deep.
Simply put, domestic power typically comes from a single conductor of a 3 phase high voltage transmission line stepped down to 240v which is then run into a center tapped transformer to produce two 120 VAC leads of opposing, but single phase.
As for the two leads cancelling each other out (bucking vs boosting), I suggest that is a confusing and dangerous way to describe the situation as someone might believe they were somehow safe in handling two hot leads in a dryer connector, etc.
In a 240v circuit under load, electrons will keep flowing on either or both conductors to the neutral unless one has a perfectly balanced load (between the hots) in which case, the neutral would carry no current. Balanced loads are hard to come by in typical household appliances and motors though.
Someone contacting either or both hots in such a circuit will soon find that the only cancellation they will experience will be their life. I would also not be inclined to grab the neutral either as I suspect one would find a few electrons there as well.
I know that there is split phase in your residential power, I believe this is how your 240v appliances are able to get 240 (-120 and +120) for a relative difference of 240volts. Is this the same as the split phase you are talking about?As far as I know when you use 14/3 for two circuits instead of running two 14/2 wires there is nothing that says you need to connected them to specific phases in the breaker panel. In the case of an appliance like your oven you have to ensure that the two hot wires are connected to different phases in panel or the oven just won't work as the difference b/t +120 and +120 is not going to be the 240 required to run the oven. Are you saying there is a requirement when using 14/3 to connect the two hots up to different phases in the breaker panel?Thank you,learner
Yes, when 14/3 is used for two circuits (with separate breakers), the two circuits MUST be on opposite legs of the 240V line. There's some argument over whether they must be on a single 240V breaker -- a good idea even if not code required.
What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell
Others have already replied... the answer is yes.
In a standard breaker panel this is no big deal as a 240VAC breaker will span both legs. Every other breaker (going down one side) in your box is on an alternating leg.
Regardless of cable choice, when wiring a 240VAC circuit you MUST use a single double pole breaker at the box otherwise half the receptacle could still be hot.
It's also not good practice to mix circuits in receptacle boxes. (ran into that once in a remodel situation - surprise, 2 for 1 sale!) Basically when you throw that breaker at the panel, it had better kill that receptacle box completely.