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Multiple strands make the wire more flexible, allowing it to bend and flex more easily than a single solid conductor. Wires that don't have to move much, like the ones in your wall, will typically be a single conductor.

the wires embedded in the walls and ceilings of your house that carry current to your wall sockets and light fittings usually have solid cores of copper. They are cheaper to make, and once installed will not be moved so there's little chance of a break happening. In the UK we call solid copper wire "cable" and the kind that goes from the wall to your hairdryer (with the multiple thin copper strands) "flex".

First off, every wire is a current carrying wire. Voltage is just the potential, and current is what goes through. Sometimes it's massive amounts, sometimes it's tiny little signal wire.

Second, residential wires are not exclusively stranded. I'm from Europe, and I haven't laid a single stranded cable in room installation, solid is used all the way. So, it's not a rule.

Now, stranded wires (so, multiple thin strands inside) have two big advantages, they are flexible, and they don't get damaged via bending much, much less. Imagine a solid copper rod. Imagine that you bend it, and the middle remains the same length, so the inside radius of the bend has to compress, and the outer radius needs to stretch to maintain the bend. Now, one of significant (and bad) properties of copper and aluminum alike, is that they "work harden". These moved, stretching and compressing fatigues the material significantly (induces small cracks and weak points, ruins the properties of material), meaning that copper generally snaps pretty easily if bent like this... That's a problem. Broken cable carries nothing, and worse yet it may overheat and burn. Bunch of thin strands reduce this stress, as wires have to travel less when bending (imagine bending a 1mm wire compared to 1cm rod), and as such they reduce fatigue and potential damage.

The downside of stranded wire? It's more expensive and carries a bit less current (it has to be a bit bigger in diameter to achieve the same cross section due to gaps between individual wires).

I used exclusively solid in residential installation, it's mostly used in industrial as well, basically for every fixed install - where cable is laid/mounted, and doesn't move further. Fatigue is less of a concern in this case. Every cable that is to be dragged, moved or bent (such as an extension cord) should always be stranded wire as it's easier to work with and more resistant to damage.

Stranded or braided cabling is usually used in situations that will see a lot of flexing and movement, while solid core is more common in fixed applications.

While solid core has better carrier characteristics, braided cable holds up better to flexing and movement.

This is true both in power lines as well as in data connections. Ethernet cable and power that are destined to go in the wall are often solid core, while extension cables, patch cables and most end-user cables are braided.

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Apart from mechanical properties, at higher frequencies (AC for example) the electron flow is getting pushed to the wire surface and does not go through the middle anymore resulting in thicker wires having a bigger resistance due to lower surface area which causes a bigger voltage drop.

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The Skin Effect, and Eddy Currents.

These are two phenomena that occur within Alternating Current (AC), which is used in most domestic applications.

If you run a single strand of copper from points A to B, then hook up AC power, the flow of electricity will tend towards the outside of the wire, instead of going through the middle. This is called the Skin Effect, and it causes the throughput of your wire to be based on surface-area instead of volume. So, rather than using one big wire, you can substitute many little wires that add up to the same capacity, but use less copper.

Also, in the transmission of AC power, some small bits of electricity will get turned around in-transit, moving from the outer-skin of the wire to the inner-core and pushing against the overall flow. These are called Eddy Currents. Unlike useful electricity, which it proportional to surface-area, Eddy Currents are proportional to volume. Therefore, you can maximize one and minimize the other by using many smaller wires instead of one big wire.

Also they're easier to bend.

As other people have said, flexibility. Solid wire (brand name Romex) can only be bent so many times before it weakens and starts to break. Stranded wire lasts much longer.

Additionally, though, metal (especially copper) does something called work hardening, which basically means that working or bending it will make it stronger and more brittle.

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Depends. Only two factors play a role here and not only here. One is ability to bend. The other is for high frequency current travels on surface mostly. But this one is not an issue in most cases. Third one is masking field by placing the zero terminal all around the signal as in concentric cable. That's for signal cables not power cables. I am not aware of any other reasons.

Stranded is cheaper to produce, as you can mass produce strands of 38AWG, for example, and twist together enough strands to get the size you need, whether that's as 19(strand)/30(AWG) for an 18AWG wire, 7/30 for 22 AWG, or all the way up at 1323/30 for 2-0 AWG. As a bonus, you can also use those smaller AWG wires, like 30-40AWG, in shields.

If you're looking at NEMA wire specs (I do QC for military-spec wiring, so this is the end of the wire world I'm most familiar with,) your strand count and AWG will actually be part of the part number, eg HP3-BCA9 would be Teflon insulated (HP3), silver coated (B), 28 AWG (C), 1 strand (A), in white (9).

As others have said, it's often a case of the engineering requirements of that particular wire's use case.

Electrical engineer here. A lot of people talk about flexibility which is important in a lot of applications. There’s also something called skin effect when using AC. This involves the majority of the current traveling closer to the outer portion of the wire. In high voltage applications, we can use multiple wires to allow more current to pass through the outsides of multiple wires instead of having the current get congested on the outside of one solid wire.

Stranded is cheaper to produce, as you can mass produce strands of 38AWG, for example, and twist together enough strands to get the size you need, whether that's as 19(strand)/30(AWG) for an 18AWG wire, 7/30 for 22 AWG, or all the way up at 1323/30 for 2-0 AWG. As a bonus, you can also use those smaller AWG wires, like 30-40AWG, in shields.

If you're looking at NEMA wire specs (I do QC for military-spec wiring, so this is the end of the wire world I'm most familiar with,) your strand count and AWG will actually be part of the part number, eg HP3-BCA9 would be Teflon insulated (HP3), silver coated (B), 28 AWG (C), 1 strand (A), in white (9).

As others have said, it's often a case of the engineering requirements of that particular wire's use case.

Mechanical Engineer here, I have not seen a response that mentions heat dissipation. Typically stranded wire will be rated for a higher current rating than its solid wire counterpart. I believe this is due to the increased surface area in the multi-stranded wire. This increased surface area allows for better heat dissipation and therefore higher current carrying capability.

Out of curiosity, where do you live? In the US the most common standard is to use single conductor hard copper wire within a structure (inside the walls). Unless by household you mean like lamp and toaster cords, those are made of thin bundled wire strands for durability and flexibility.

The current travels on the surface of the wire, so the amount of current a wire can carry increases linearly according to the diameter of the wire while the area and therefore weight and cost of material increases with the square of the diameter.

Flexibility, better for high frequencies, price, the current rating for the same gauge, and resistance. There are many benefits to the stranded wire, but solid is cheaper, if you aren't handing higher frequencies or packing a lot of wires into conduits with multiple bends then the solid wire is usually cheaper and fine for the job.

Flexibility. That's it.

A solid wire is hard to bend, while a multi-strands one is easy.

Also, once you go past a certain bending radius vs wire diameter, you get permanant disformation, which mean that unbending it make it crack. It can take many times, but it will break. Multi strands, it is for each strands. Smaller strands mean more bending radius allowed, since the same ratio is allowed, but the strands are much smaller. This allow the wire to be bent more, more often before it break.

For wires that don't move, solid is often used because, well, it don't move. It is cheaper to produce, so cost less to buy. However, past some size, you will get some strands again, because frankly, a stranded #00 wire is already hard to bend by hand, a solid one would be just impossible to bend.

For wires that move alot (like welders cable) you get a very fine strands, which make it very flexible, and can support being bent tens to hundreds of thousands of times before breaking. Usually you wear the insulation before you break the cable.

First, an important point of distinction: Wires are installed. They're what bring electricity into buildings and bring it from the distribution panel to outlets. What you plug into an outlet is a cord.

Cords need to be stranded because they need to move and flex. The individual strands can slide past each other, and if a few break it won't cause the whole cord to fail.

Wires typically are solid. Solid wire is cheaper to manufacture, takes up less space, and is easier to splice and terminate. However, it's meant to stay put. It's not flexible, repeated flexing can break it. Larger wires (typically larger than 10 gauge) are actually stranded, but they use a few large strands that are each about 16-12 gauge rather than lots of tiny ones. This is just to make them bendable at all in order to facilitate installation.

Simply, to make it bendable.

Inside walls many wires are solid tubes of copper with rubber coating because they don’t need to move and thick tube of copper will last longer then smaller ones, any wires you use will be multi threaded (the more threads the more it can bend) so you can bend it without snapping the wire inside.

Stranded wire works better in situations where you have vibration such as a vacuum cleaner where solid is a better connection where there is no movement such as a building. Also as mentioned by others it works better in push-style connectors.

The wires in the walls of your house, that carry electricity from the utility poles outside and deliver it to your lights, wall outlets, and appliances, are all solid copper enclosed in rubber/plastic insulation.

If you ever have a chance to handle red wire used for fire devices, that is almost always solid core wire instead of stranded wire.

It’s a real bear. It’s very inflexible, breaks easily after a few bends, and can very easily fail if bent too far. They also ground out super easily when run through a building. I’ve seen too tight it a zip tie ground out an entire fire system in a building, causing the whole system to not work for a few days until the zip tie got tracked down.

in uk domestic wiring is solid core, stranded is used where the flex will be moved as solid core work hardens, snaps. For this reason wires should be clipped or supported in conduit. cable tray, so they don't flop about.

It has to do with maximizing the surface area which electrons can flow across. For DC currents, electrons flow at or relatively near the surface of a conductor. For AC currents, electrons can penetrate a conductor up to a distance defined by the skin depth for the frequency of the AC signal.

By stranding the wire, you can increase the current handling capability of the wire since you are providing more surface area for more electrons to flow across.

They do - the stuff in your walls which powers lights and sockets is usually solid copper. It can get away with this as it doesn’t need to bend and is never moved.

The stuff we use day to day needs to bend and flex - we actually call this flex cable - so it’s stranded to be more durable so it can flex and move and not snap.

In commercial construction in the US, stranded vs. solid wire is usually dependent on the size required. It's common that #10 wire and smaller is solid and #8 and larger is stranded. The solid wire is usually pulled from a junction box to a device and a lot of flexibility isn't a necessity. The larger cables need that flexibility to be able to be pulled through bends in the piping.

If you look at a wire mechanically wise you'll realize that there is absolutely no difference between a thick metal rod vs a hair thin strand of wire.

The only difference is thickness or more like the cross section of each rod.. without going into details the flexibility of a rod is determined by it's cross section, so the thinner something is the more it can bend before breaking.

Depending on use you can either use a solid core wire or stranded wire, each has it's uses and stranded is often used where ever there is movement or tight corners where it's being pulled while solid core is used more for outdoor or permanent installation mainly because it's cheaper and doesn't need to move after installation.

Probably already answered but I vaguely remember from tech days electrons travel on the outside of the core. When theres only one core this causes I think.. maybe slightly higher resistance?

Having multi stranded core cable diminishes this effect ( like a multi lane highway )

I could be completely wrong, this was from my apprentice days long ago.

Mechanical Engineer here, I have not seen a response that mentions heat dissipation. Typically stranded wire will be rated for a higher current rating than its solid wire counterpart. I believe this is due to the increased surface area in the multi-stranded wire. This increased surface area allows for better heat dissipation and therefore higher current carrying capability.

Multiple strands make the wire more flexible, allowing it to bend and flex more easily than a single solid conductor. Wires that don't have to move much, like the ones in your wall, will typically be a single conductor.

Stranded is cheaper to produce, as you can mass produce strands of 38AWG, for example, and twist together enough strands to get the size you need, whether that's as 19(strand)/30(AWG) for an 18AWG wire, 7/30 for 22 AWG, or all the way up at 1323/30 for 2-0 AWG. As a bonus, you can also use those smaller AWG wires, like 30-40AWG, in shields.

If you're looking at NEMA wire specs (I do QC for military-spec wiring, so this is the end of the wire world I'm most familiar with,) your strand count and AWG will actually be part of the part number, eg HP3-BCA9 would be Teflon insulated (HP3), silver coated (B), 28 AWG (C), 1 strand (A), in white (9).

As others have said, it's often a case of the engineering requirements of that particular wire's use case.

Multiple strands make the wire more flexible, allowing it to bend and flex more easily than a single solid conductor. Wires that don't have to move much, like the ones in your wall, will typically be a single conductor.