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I thought that the wire size depends on the amperage that the current passes. However in a 3 conductor cable, the size of the neutral wire is the same as the hot wire; which puzzles me. If both hot wires carry 15A current, the neutral wire will carry 30A current.

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6 Answers 6

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A simplified representation of a multi-wire branch circuit, would look something like this.

Circuit

If each part of the circuit had a 120 Watt light bulb installed, it would look like this.

Circuit with 2 120 Watt light bulbs

If the switch on L1 was closed, you'd see 1 ampere on the circuit.
Ohm's law I=P/E

Circuit with 2 120 Watt light bulbs. Switch 1 closed
L1 = 1A, N = 1A, L2 = 0A

If both switches were closed, you'd still see 1 ampere on the circuit. However, in this case the current flows line to line, and there will be no current on N.

Circuit with 2 120 Watt light bulbs. Both switches closed
L1 = 1A, N = 0A, L2 = 1A

If we replace one of the 120 Watt bulbs with a 240 Watt bulb, you'll see that the unbalanced load flows on N.

Circuit with 1 120 Watt bulb and 1 240 Watt bulb. Both switches closed
'L1' = 2A, N = 1A, L2 = 1A

So as you can see, the neutral (N) will never carry more than either line (L). At least not in a properly wired circuit. This is because of the nature of alternating current, and the properties of a split-phase system.

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When it is said that the current cancels each other out, it means that the neutral of a multi-wire branch circuit only carries the imbalance of the current between the two circuits of a properly wired MWBC.

So a load of 5A on one leg, and 15A on the other leg, will result in a load of 10A on the neutral.

For a typical MWBC it is extremely rare that both legs will be 100% balanced, so the neutral will almost certainly carry some current. This is why a straight 240V circuit (which is a line-to-line circuit as opposed to a line-to-neutral) requires no neutral wire. It is a 100% balanced line-to-line circuit.

See?

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"This is why a straight 240V circuit requires no neutral wire. It is a 100% balanced line-to-line circuit." - This statement is confusing. If there were a neutral wire, it wouldn't carry a current, but it's not because both legs are balanced; it's because the neutral wouldn't be connected to anything (which is why both legs are balanced...) –  BlueRaja - Danny Pflughoeft Feb 14 at 16:51
    
Well, it would not be connected to anything, true, because it is not needed. –  Speedy Petey Feb 14 at 16:52

No, because the two hot legs are out of phase. So 15 amps on one side will cancel out the 15 amps on the other side.

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3 conductor cable is usually used for one of two purposes: An additional/dedicated hot (i.e. feeding a switched light and an always-on receptacle), or a multi-wire branch circuit. In the case of a multi-wire branch circuit, the two hot legs come from different legs of your supply (+120V, -120V), so they actually cancel each other out when properly balanced.

You are correct in that you would not want to return 2 15 amp circuits on the same leg via a single neutral.

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Isn't longneck's answer below correct? –  bib Feb 13 at 19:57
    
yes, Longneck is correct. The two legs are 180 degrees out of phase. there is no - or + leg, they both supply 120 volts of alternating voltage both + and -, but reversed polarity from eachother, 60 times a second. –  shirlock homes Feb 13 at 22:06
    
correct me if i'm wrong, but residential supply is not two phases but rather one phase stepped down via a center tap transformer, unlike 3-phase supply where your phases are 120 degrees apart –  Steven Feb 13 at 23:48
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@Steven 120/240V Split-phase systems supply a single 240 circuit line to line, but since the neutral is a center tap on the transformer you end up with two 120V circuits line to neutral. Since it's alternating current, when line1 is "pushing", line2 is "pulling". Because of all this line1 and line2 appear to be 180 degrees out of phase. While in fact it's just the nature of an alternating current circuit. –  Tester101 Feb 14 at 1:33

As others have said the 3 conductor cable could be used for feeding two circuits on split phases. Transformer on the pole (or in the ground) is used as shown in Tester101's diagrams. However, you must be sure that the breakers in the panel are on two different phases. Generally breakers that are adjacent to one another are on different phases but not always. Some panels have alternating phases at the top of the panel and some at the lower end adjacent breakers are the same phase. Also a dual breaker that fits in a single slot would not be on separate phases.

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The exception to this rule is harmonics. Triplet harmonics will SUM in the neutral, in a 3 phase supply. Triplets come from single phase rectifiers (eg most electronic power supplies for computers, TV's, microwaves, etc). In years gone by when there were less electronic consumer products, half size neutrals were used in 3P supplies. Today the minimum is same size neutrals, and even double size in some installations, depending on the nature of the expected load.

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