why do current-carrying wires have multiple thin copper wires instead of a single thick copper wire?

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In domestic current-carrying wires, there are many thin copper wires inside the plastic insulation. Why is that so? Why can’t there be a single thick copper wire carrying the current instead of so many thin ones?

In: Engineering

Mostly because a stranded wire, being made of multiple thin wires, bends and flexes much more easily and so it’s a lot better if you have to move the wire

If you don’t have to move the wire, like for house wiring, solid wire is often used

A single thick copper wire is better and can carry more current than a comparably sized stranded wire and it’s cheaper to produce. That’s why they are what’s inside the walls of your house. Stranded wire is more flexible and designed to be repeatedly bent, that’s why it’s used on extension cords.

Single thick wires do exist. They are less flexible though and break when you bent them too often, at least when they are thicker than a certain size (and they need a certain size to not overheat when carrying some amps).

I’m not sure about the english terms but we call single wire ones “Bell wires” (YSTY) here.

Also for high frequency applications having more than one wire helps to prevent “whirl currents” where magnetic fields send electrons into spirals that cause extra losses. The thinner each wire the less space they have for these whirls.

Household wiring, the in-the-wall stuff, in the UK is typically single conductor.

It’s stiff, difficult to bend.

Appliance cords are typically multiple conductors, and are more flexible. I think the movement of the conductors slows this.

If you take a phone book, you can easily bend it in the middle, but only in one orientation: parallel to the spine of the book.

What happens if you try to bend it perpendicular to the spine? The spine resists bending, and you’re going to damage the spine if you do. That’s because the individual pages are glued together there and cannot move relative to each other.

A stranded wire with small individual wires behaves like a phone book that you’re bending parallel to the spine, and a solid wire with one large conductor inside behaves more like a phone book that you’re trying to bend perpendicular to the spine. It’s harder to do and you’re going to damage the conductor.

There are several reasons. Multistrand wire is much more flexible as the different strands can slide against each other when bent while a solid wire is harder to bend but stay the shape you bent it into. There are advantages and disadvantages to both of these but in general solid wires are used for fixed intallations and multistrand wires are used for flexible installations like extension chords. Secondly if the wire is thick enough or the frequency is high enough then you start getting skin effects. Essentially the current will mostly travel over the skin of the wire and not reach the inside at all. By using a multistrand wire you are increasing the surface area of the wire and reducing the skin effect. This is partly why high voltage wires are multistranded wires. The third reason is that copper is a very good conductor of electricity but is not good at supporting its weight. So when you have long stretches of copper wire the copper might get damaged from being stretched out. One of the ways to fix this is to reinforce it by using some strands of steel alongside the copper strands when making the wire. The steel is lighter and stronger then the copper so the steel reinforcement will make the cable able to support more weight before getting damaged.

The reason is,

if you take a big piece of metal, and you bend it, the outside is longer than the inside, which is stressful to the metal. That makes it harder to do the bend, and it produces wear and tear.

By contrast, if you take a bunch of parallel strands, this isn’t as much of an issue since the strands can just slide next to each other.

Now, I will note that some wires (particularly ones that don’t expect to be bent very often, like the wiring of a house) do use a single, thick piece of metal.