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The neutral carries the DIFFERENCE of those currents, not the SUM. If the phase currents are equal, the neutral has I=0. Wade

Reply to
wade_h
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Right, so long as the two branches are on different phases. And in office buildings where there are three separate phases of 120, that used to be true, too. But PCs and monitors have switching power supplies that pull a high current spike at the peak of the AC line cycle, and the spikes from the three phases don't align in time, so they don't cancel, and fires have resulted from the huge neutral currents.

New directives regarding line harmonics (requiring unity-power-factor power supplies) fix this issue.

John

Reply to
John Larkin

This is true so long as someone competent does the work. I have seen situations where someone added so much stuff on to a circuit that they tied another breaker on to the kluged end of their mess probably because the smaller gauge and resistive connections caused so much line drop downstream. It took two breakers to turn off this mess! Was I swearing. I saw this twice last week. I also saw one where someone had replaced a 220 AC outlet with a standard outlet. The lady wondered why her Christmas lights were the brightest on the block and why they kept blowing out so much!

Richard

Reply to
spudnuty

Well, I'm an electrical engineer, and we don't refer to "legs". By "different phases" I meant that the voltages are, well, not in phase.

Zero degrees and 180 degrees are different phases to me. Zero and 120 are also different phases, by my standards. Zero and zero are the same phase. A good test is to short them together and see what happens.

Unless you're an engineer, in which case they are often correctly called 'Phases.'

But definitions are personal property, so call them what you will.

John

Reply to
John Larkin

I'd like some feed back on this one. When I built my house, I had a very honest Father and Son contractor who were jack of all trades. They did the wiring also. Sloppy but functional. A few years later, while making some changes, I noticed they used 14-3 wiring to run 2 seperate circuits. These 2 circuits shared the neutral. I questioned them on it and they said that is perfectly ok. I shared my concerns with them so they asked an electrical inspector and he confirmed their beliefs. It's ok to do that. I understand the load is such that it doesn't tax the neutral but what happens years down the road when the homeowner throws an extra load here and an extra load there on these circuits until the neutral is carrying more amperage than it is designed for? The breakers don't trip because the hots are carrying just under 15 amps each. 14.5 + 14.5 = 29. The 15 amp neutral can't carry 29 amps. I know it's probably a little far fetched but it is possible. I have...no had 2 circuits like this in my house. Do I have OCD or what? Residential electricians...what do you think?

Reply to
Beeper

Someone wrote:

Err? Different 'Legs' might be a better term? The point about peak currents and being close to unity power factor seem very valid comments.

In an industrial setting (or in a large apartment building) they MAY happen be two 'phases'. But in most individual residence situations the two wires which have 230 volts between them are, sort of, plus 115 volts and minus 115 volts to neutral. They are (Not quite true because we are talking AC here) usually the two outer ends of a 230 volt 'single phase' which has a centre tapped neutral to create the two 115 volt 'legs'. These legs are often mistakenly called 'Phases'. They are typically Leg A (Say Black) and Leg B (Say Red). Thus if there is load on both 'legs' it will tend to cancel out any load in the neutral middle conductor. However; think about this ................; if there is load on only one side (leg) of the circuit, the neutral will carry the same current as the hot lead on that side; and the other hot lead will be carrying nothing; right? It is a common mistake to think of the neutral as NOT carrying current; but all our basic circuit training tells us that current has to flow from a supply, through a load and return! That return IS the neutral conductor and it better be intact and in good shape! The fact that a good neutral is almost or only a volt or two above ground potential means that it is doing it's job of returning the current to the low voltage (neutral) side of the supply panel with little loss due to the resistance of the conductor. An open neutral is bad news and may results in electrical current trying to return to the supply panel through whatever is available. For example: Those so called GFIs (Ground Fault Interupters) actually work on the currents in the hot lead and the neutral being 'balanced'. If unbalanced due to defective neutral or a fault to ground the GFI disconnects the outlet/s to potentially save life. It is also bad to think of the ground wire as "Being the same as a neutral". It is not. The ground wire is there for safety in case something goes wrong. I would not like to have the grounded frame of my fridge being used as the return wire for the electric current operating the fridge compressor! Have fun, safely!

Reply to
Terry

Beeper: I have duplex outlets in my kitchen wired this way. the upper socket is wired, say to the red, and the lower socket of the duplex to the black. This is not to get 230 volts between upper and lower but does allow two 115 volt loads to be plugged in, effectively doubling the current capacity of the outlet. With 14 AWG that circuit should be fused/breakered at 15 amps. A double pole breaker should be used to disconnect both sides 'legs' of the supply to that circuit simultaneously. Terry

Reply to
Terry

Yes, they are 180 degrees out of phase "in phase." Tom

Reply to
Tom Biasi

As I said, to see if they're in phase, merely connect them together and see what happens. Report back and I'll interpret the experiment for you.

John

Reply to
John Larkin

Reply to
Beeper

Then why does the power company run them in on separate wires?

It can't.

Take the A and B signals from a residential 120/240 circuit. Use a time-interval counter to measure the delay from identical points and slopes on the A and B signals, both referenced to N. You will find the delay to be very close to 8.333 milliseconds.

Repeat experiment with a phase meter. Repeat with an oscilloscope. All will indicate a 180 degree phase difference.

The only sensible way to refer to such a power system is "two phase."

Sorry.

John

Reply to
John Larkin

If you run two separate circuits with 14-3, I don't see the neutral sharing aspect. Each 14-3 leg has it's own neutral. They should have used 12-3 anyway. And if you add circuits, run new lines from the breaker panel with a new breaker.. Don't add to existing wiring.

Reply to
Rodney Kelp

No, phase is meaningless for DC.

John

Reply to
John Larkin

If two circuits are run in a single 14/3 cable, the breakers feeding that cable _must_ be mechanically interlocked, so that they will switch on or off together, whether or not you are using them as a 220 V supply. The reason for this is so that if you turn off one circuit to work on it, you won't get burned by the other, which will terminate in the same box.

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Peter Bennett VE7CEI 
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Reply to
Peter Bennett

On Wed, 23 Feb 2005 13:38:01 -0330 in sci.electronics.basics, "Terry" wrote msg :

You're absolutely right about this. It is not right to share neutral circuits. Think of the implications of doing wiring repairs - if a worker disconnects a neutral wire splice while working on a disconnected circuit, the currents carried by a shared circuit will start looking for a place to go. The worker will likely get a little spark surprise at the same time. I'm surprised that an electrical inspector would allow it, is such practice really allowed by code?

--
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Reply to
Kitchen Man

In some cases, the Canadian electrical code _requires_ that two circuits be run in 14/3 (or 12/3).

Duplex outlets on a kitchen counter must be split, with one of the two sockets fed from one phase, and the other from the other phase. They must be fed by 14/3 or 12/3 cable from a two-pole breaker.

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Peter Bennett, VE7CEI  
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Reply to
Peter Bennett

Ok, how about this? Two duplex outlets in one double box. Do you?

  1. Run two 14-2/G to the box, one for each outlet.

or

  1. Run one 14-3/G to the box, and connect the red wire to one outlet, the black to the other, then connect the white wire and ground to both outlets? As long as the current in the neutral doesn't add, there is less current in the neutral than either supply wire.

Method one uses more material, more labor and wires can be mixed between the devices.

Method two uses only what is needed, takes less labor t install, and the wiring is rather obvious to anyone capable of doing a later repair.

If it doesn't meet code the inspector can't allow it.

Think about it.

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Michael A. Terrell
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Reply to
Michael A. Terrell

On Sat, 26 Feb 2005 23:05:03 GMT in sci.electronics.basics, "Michael A. Terrell" wrote msg :

Maybe I've misunderstood the discussion, because I don't see a problem with what you're saying, and possibly what I'm asking isn't clear. I've had experience where three separate circuits, using three separate circuit breakers, shared neutral paths via wire-nut node boxes throughout a building. It was my impression that safe practice dictated that each circuit (defined by a common termination in the breaker box) should have its neutral line isolated until it also is terminated in the breaker box.

I don't have a copy of the NEC, and I know that the situation I've described caused unpredictable and, to my eyes, unsafe conditions within the circuitry. I'd appreciate any insight anyone has to offer.

--
Al Brennan
http://www.xmission.com/~tiger885/motorbike/NART/nart.html
Reply to
Kitchen Man

OK, ground the center tap and add the signals from each end. You get zero, not twice the original signal. Hence they are not the same signal.

Causality doesn't matter. The two signals behave exactly the same as if you'd used two allpass networks of 90 degrees each. Show me how you would distinguish between two sets of sinusoidal signals, one set antiphase as the result of a delay circuit, and the other from a center tapped transformer. It's only when signals are non-repetitive that you can perhaps tell.

IMO. it's the use of the word "shift" that's confusing things. Use "difference", and things get clearer.

"Antiphase" is not "in phase", by definition.

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Reply to
Fred Abse

I don't have a current copy of the NEC codebook. It got to expensive for me after I became disabled.

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Michael A. Terrell
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Michael A. Terrell

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