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NM-B 14/2 cable (used for indoor residential electrical wiring) contains 3 wires:

NM-B 14/2

The cable contains:

  • (1) copper wire with black sheathing (to act as the hot wire)
  • (1) bare copper wire with no sheathing (to act as the ground wire)
  • (1) copper wire with white sheathing (to act as the neutral wire)

Are all three wires physically identical, in terms of the gauge of the current carrying conductors within it, and special properties of the cable itself?

Here's another way of phrasing my question: Could I (hypothetically) use, for example, a black wire as my neutral, a white wire as my ground, and the bare wire as my hot wire?

(I don't actually plan on doing this, since this is in obvious contradiction of protocol and is needlessly confusing. It is a theoretical question, as I try to understand if the wires truly differ in any way.)

If the answer to my question is "yes," then what role does the sheathing play, other than as a means of identification? Does the sheathing on the two sheathed wires differ in any way?

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    The -1 isn't from me, but I'm not sure how this is relevant to home improvement. Is there a specific problem you are trying to solve? If someone has wired the bare copper ground as anything other than ground, you've just lost an entire layer of insulation on a normally current-carrying conductor which poses a hazard and generally deviating from color coding standards also poses a hazard to anyone who expects wiring to be NEC-compliant (as they reasonably should be able). – statueuphemism Apr 22 '17 at 14:12
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    I don't get how a question trying to understand the make-up of electrical wire could be considered irrelevant to home improvement. There is no "specific problem" per se, but that does not mean that developing a fundamental understanding of a vital component of electrical work is not relevant to home improvement. – Fil Apr 22 '17 at 17:03
  • Fil your question amounts to asking a hypothetical on whether there is any reason why electrical wiring is done the way it is with the underlying implication that there are no good reasons and we will all say, "Gee, I never thought about it; like a sheep I just did what I was taught. But now that some bright young thing has questioned it, we all see what sheep we have been and of course we will all be freed from the yoke of pointless tradition (superstition, really) that we dim bulbs have struggled under all these weary years of our lives. We all thank you, etc., etc." Gentle kidding. – Jim Stewart Apr 23 '17 at 22:05
  • @JimStewart My question may have been phrased poorly, but I don't think it's a bad question. The question got bogged down with understanding the general role of insulation. But, now I know that all three wires are 100% identical, and the insulation found on the two insulated wires are 100% identical (save for color). Yes, I understand that insulation serves more than one purpose. But, the only reason that the insulation differs is for identification. That is why I tried to clean up my question by removing my tangential aside (the one that you quoted in your answer, published before my edit). – Fil Apr 25 '17 at 2:24
  • No, they're not 100% identical. The hot and neutral can be counted on as being identical; or in 3-conductor cable the hots will be identical. In larger sizes of cable, the ground is smaller, and in 3-conductor cable, the neutral may be smaller. That's because in large feeders, 100% neutral imbalance is unlikely. For instance 2-2-4-6 is a cable size; #4 neutral #6 ground. – Harper May 12 at 23:16
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No, they're not 100% identical. The hot and neutral can be counted on as being identical; or in 3-conductor +ground cable the hots will be identical. In larger sizes of cable, the ground is smaller, and in 3-conductor cable, the neutral may be smaller. That's because in large feeders, 100% neutral imbalance is unlikely. For instance 2-2-4-6 is a cable size; #4 neutral #6 ground.

But the hots (or hot+neutral in 2-conductor 3-wire cable) are the same size. If you were wiring a UK device with US wires, you might tape the white to be red (hot) and leave the black as neutral.

Ground, however, is special. It is not simply another conductor, it performs a very different fun than than neutral, and that is what makes it reasonable to not give it insulation.

The outer sheath and inner insulation serve many purposes more than you are giving them credit for.

The wire insulation's jobs

Aside from the obvious (insulation and color coding)... the insulation provides protection from abrasion on sharp edges, and when wires are stuffed into a junction box. It also must not burn easily, and must not emit toxic gases if it does. And it must not deteriorate over time (UV notwithstanding).

The sheath's jobs

First is grouping. Grouping is vital to understanding how a circuit works - e.g. if you have a 3-way switch and 2 wires go into one cable and 1 wire into another, you know which ones are the messengers. It keeps neutrals paired with partner hots, which is mandatory and AFCI/GFCI breakers require this. Since currents should be equal, keeping the wires together assures that the wires' EMFs cancel each other out, and with nothing between the wires, it prevents inductive/eddy current heating.

Next is binding and strain relief, as the cable is clamped where it enters the junction box. Clamping the individual wires would create a stress point.

Also stopping nails. The usual rule is that cable is stapled or penetrates at the midpoint of a 4x4 joist, and one should use 1-1/2" or shorter nails so a nail isn't going to reach the staple or hole. But between holes and staples, the cable floats and can come within 1-1/2" of the wall. In that case you want the cable's skin to be tough enough that the nail moves the cable rather than penetrates it.

The cable also must hold the markings which identify the cable type, which decides where it is legal to use based on its characteristics. Color coding is often done - 14AWG being white, 12 AWG yellow, and 10 AWG red.

  • "The outer sheath and inner insulation serve many purposes more than you are giving them credit for." Can you explain the purposes of sheathing? – Fil Apr 22 '17 at 16:42
  • I will later. In the meantime think different about it; think in terms of all the practical problems that happen in real world wiring, i.e. the POV of someone doing forensics after a house fire or fatality. – Harper Apr 22 '17 at 17:07
  • Thanks for supplying more information. Is the sheathing on the hot wire and the sheathing on the neutral wire identical, except for the color? Also, can you explain why the ground wire does not benefit from insulation? I don't get why insulation wouldn't be useful on the ground wire to stop nails, to hold markings, for strain relief, etc. You wrote, "Ground, however, is special. It is not simply another conductor, it performs a very different fun[tion] than than neutral, and that is what makes it reasonable to not give it insulation." How so? – Fil Apr 23 '17 at 6:38
  • The wrapping on individual wires is not sheath, it is insulation. Reread what I said in that light, it'll make more sense. – Harper Apr 23 '17 at 14:23
  • Wires within cables are typically not marked and not listed for use as individual wires. Insulating the safety ground wire would be redundant and not aid its purpose. Not least, they don't want people misusing ground as a conductor (and thereby going with no ground). /3 cable is readily available. "What is the purpose of a ground wire" would be a good separate question. It may have already been asked. – Harper Apr 23 '17 at 14:48
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@Harper nailed the answer. I would just add that there are some fairly common circumstances where you would want to use a white normally-grounded wire as an energized wire. It's useful to know that the black and white wire are truly identical outside of color and therefore there is no reason to run a separate black wire instead of re-purposing the white one. Keep in mind that the white wire should be re-marked as black (or red, blue, yellow) to indicate that it may be energized.

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As I understand it the three wires are the same alloy and same diameter. They are in the same crystalline state. (Formerly the centre wire was smaller gauge, but not now.)

However, I disagree with this statement of yours

If the answer to the above question is "yes," then this would mean that the wire sheathing serves no purpose other than as a canvas to display colors as a means of identification, so that the user can have an immediate visual understanding of which wire should do what. (Otherwise, without a colored sheathing, each individual wire would have to be labeled by the handler in advance, or traced to the source, in order to determine which wire controls what.) Blockquote

The sheath actually performs some practical functions.

  • "The sheath actually performs some practical functions." Can you provide the practical functions of sheathing? – Fil Apr 22 '17 at 16:43
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    The sheath protects the wires inside from abrasion as the cable is pulled through holes in studs and joists during installation. The sheath associates a particular hot and a particular neutral so that the time varying magnetic fields surrounding a cable will be very low due to cancellation of the fields around the associated hot and neutral (same current value, opposite directions). The proper functioning of GFCI and AFCI breakers depends on not mixing neutrals. Cross connections and mixing of neutrals and hots is prohibited and having an associated pair of one hot and one neutral helps. – Jim Stewart Apr 22 '17 at 21:34

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