how come equiptment or appliances CAN share a neutral in a 3 phase system, but you CANT do that in residential 240 volt single phase/ split phase systems?. why cant a 14/3 feed a fridge and dishwasher?. the main service shares one neutral..so whats up?
But when you have three balanced loads, one on each phase of a three-phase system, the currents cancel out at the neutral, so there is very little current flowing in the neutral conductor back to the supply (I'll ignore issues with switching-power supplies and high harmonic loads).
With the residential circuit, if the two loads are balanced and fed from the opposite 'hot legs', the same thing can happen, the neutral current cancels out.
If somebody screws up and wires to circuits from the *same* hot leg to a common neutral, then obviously the neutral conductor is carrying the sum of the two load currents and may overload.
There are also problems if you go to work on one of the circuits, open the neutral leg and there is a load on the other circuit (that now also has an open neutral).
If the two circuits are run in metal conduits and they are not in the *same* conduit, the mis-match between 'hot leg' and 'neutral' current will induce currents in the metal conduit, causing heating and losses.
For these reasons, the NEC does not allow this sort of connection in residential circuits. There is a type of multi-branch circuit that allows this in commercial installations, but I'd have to look it up.
Really?? Where does the NEC forbid residential multiwire branch circuits? Article 100 defines them and Article 210.4 gives the rules for multiwire branch circuits and there is nothing there that forbids them in residential occupancies. The explanatory material in the 2005 NEC handbook specifically references 240/120 Volt single phase three wire circuits and gives several examples of allowed usage.
As long as the original poster's fridge and dishwasher are "line to neutal loads" article 210.4 seems to allow a multiwire branch circuit, subject of course to the provision that both breakers are in the same panelboard (and not on the same phase!) and there are situations where a double pole breaker would be required (likely not a good idea in this scenario...).
If there is a section of the NEC that disallows multiwire branch circuits in a residential occupancy, kindly provide the reference.
Multiwire Branch Circuits are allowed for residential circuits. It should be noted that a GFCI Circuit Breaker cannot be used to protect a multiwire branch circuit. However, GFCI receptacles work just fine on multiwire circuits. Also, by 300.13(B) pigtails are usually required for the grounded conductor at outlets.
2005 NEC REF: I. General Provisions
210.1 Scope. This article covers branch circuits except for branch circuits that supply only motor loads, which are covered in Article 430. Provisions of this article and Article
430 apply to branch circuits with combination loads.
210.4 Multiwire Branch Circuits. (A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard or similar distribution equipment. FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral currents. (B) Devices or Equipment. Where a multiwire branch circuit supplies more than one device or equipment on the same yoke, a means shall be provided to disconnect simultaneously all ungrounded conductors supplying those devices or equipment at the point where the branch circuit originates. (C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads. Exception No. 1: A multiwire branch circuit that supplies only one utilization equipment. Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device. FPN: See 300.13(B) for continuity of grounded conductor on multiwire circuits.
300.13 Mechanical and Electrical Continuity - Conductors. (B) Device Removal. In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.
Yes you can. And it is typical to find the Dishwasher and the Disposal on a split receptacle sharing the neutral in a 12/3 cable under the kitchen sink. And you do put it on a 2-pole breaker or use a "Handle-Tie" to join them at the panel.
No, you got me. I was thinking there was an explicit ban for residential, but cannot find it.
It does mention if multi-wire circuits supply the same 'yoke', then provisions must be made to disconnect both ungrounded conductors at the same time. This is usually done, as you said, with a double-pole breaker.
Sorry about that, I should have looked closer. But multi-wire does require some additional precautions, sometimes pigtails in the neutral line are needed; and if run in conduit must all be in the same run.
300.13 Mechanical and Electrical Continuity ? Conductors. ... (B) Device Removal. In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.
The current on one half cancels the current on the other half. Any difference in current flows on the neutral. Say 10 amps is on one half, and 3 amps is on the other - the net current on the neutral is
A DC example may help you: Say you have a 12 volt battery and 3 twelve volt bulbs, each drawing 1 amp:
Now to understand what happens to the current flow: First we will stipulate that current flow is from positive to negative. (Depending on who you ask, it is positive to negative, or negative to positive. It doesn't matter for this example, as long as we are consistent, thus the stipulation: we'll use positive to negative.) Arrows will show the direction:
+---->-----+---->----+---->----+ ^ + v v v Battery Bulb Bulb Bulb ^ - v v v +----------+ ^ + v v v Battery Bulb Bulb Bulb ^ - v v v +---------+ ^ + v v v Battery Bulb Bulb Bulb ^ - v v v +---->
If you have six outlets on a single pole circuit, and six more on a second single pole circuit and they share *one* neutral return, then the current in the neutral (groundED) conductor could be almost double the circuit breaker trip rating if both single pole circuits are from the same 'hot'.
When both circuits are from the same side of the line (same 'hot'), then the neutral currents from all the loads will *add* and overload the neutral conductor. If the two single pole circuits are from opposite 'hot' legs, then their load currents will cancel and there will be almost no current in the neutral returning to the source.
Get it right, you save the cost of a conductor, get it wrong you could overload the neutral conductor.
"if you have 6 outlets on the same single pole circuit and each outlet has an appliance plugged into it, all with different loads. whats happening on that common neutral wire?"
Now, see where he said, 'on the same single pole circuit'?? What part of that sounds like multi-wire??
The way he phrased the question, the 'commonn neutral wire' he's asking about is the one feeding the 6 outlets. That's a common wire back to the source, not a 'common neutral' for a multi-wire circuit.
Yes, most of 'us' were discussing 'multi-wire branch circuits'. But then 'matt' jumped in with a question about a single circuit connection. And some folks just went right on and answered him as if he *did* ask about multi-wire circuits. I was pointing out that if you read matt's question carefully, you'd have to give 'matt' a different answer.