The lamp on my desk has a cord I have coiled and uncoiled a few hundred times since we moved in, and it still works fine. Ten feet away, inside the wall, a cable carrying the same 120 volts has not moved since 1978 and never will. Both are copper, and both are sized by the same gauge numbers. But look at a cut end of each and they have almost nothing in common: the cord is a soft rope of hair-thin wires you can bend around a finger, and the cable is three stiff conductors you could straighten out and use as a coat hanger. For years I filed that under manufacturing preference. It is not a preference. It is two different answers to two different questions, and the wire standards, the Code, and even the little square holes on the back of a receptacle all take a side.
One conductor or a rope of them
A solid conductor is exactly what it sounds like: one continuous cylinder of copper with insulation around it. A stranded conductor of the same gauge is a bundle of smaller wires twisted together, and the gauge number describes the copper in the bundle rather than any single strand. Seven strands is the standard building-wire arrangement, one in the middle and six around it, and you can see the pattern at a cut end under good light.
Cords go much finer. Where a stranded 12-gauge building conductor has seven strands, a cord conductor of similar size can have dozens.
The table where the difference is written down
If you want the difference in numbers rather than adjectives, it sits in Chapter 9 of the National Electrical Code, NFPA 70, in a reference table called Conductor Properties. It lists each gauge twice, once solid and once stranded, and the two rows are not identical.
A 14-gauge solid copper conductor measures 0.064 inches across, with a DC resistance of 3.07 ohms per thousand feet. The 7-strand version of that gauge measures 0.073 inches and 3.14 ohms. Twelve gauge runs the same way: 0.081 inches and 1.93 ohms solid, 0.092 inches and 1.98 ohms stranded. Those resistances are for uncoated copper at 75 degrees Celsius.
Two things fall out of those four numbers. The stranded version of a gauge is slightly fatter, because a bundle of round strands cannot pack without leaving gaps, which is why stranded wire fills a box faster than its gauge suggests. And it has slightly more resistance, because each strand spirals rather than running straight, so the copper is a little longer than the cable is. Neither difference matters on a twenty-foot run in a house, and both explain why an engineer sizing a long feeder will not treat the two as interchangeable.
Why a cord could never be solid
Copper does not mind carrying current. What it minds is being bent back and forth in the same spot. Each bend deforms the metal there, the deformed metal gets harder and less willing to deform again, and eventually the grain cracks. Bend a paperclip in one place enough times and it snaps without any heat or force worth mentioning. Same mechanism, same kind of metal.
A thin strand bends through a gentler radius than a thick one, so splitting the copper into many small strands spreads the movement over many small deformations instead of concentrating it in one. That is the entire reason a flexible cord is built the way it is. A solid 16-gauge lamp cord would work perfectly on day one and develop an invisible intermittent break behind the plug by the end of the first winter.
That failure mode is what the consumer safety guidance warns about at the plug end. The Consumer Product Safety Commission's leaflet on household extension cords and fires notes that hot plugs and sockets are often caused by deteriorated connections to the cord's wires, tells you to replace a worn cord rather than repair it, and sets a floor on size: extension cords should be at least 16 AWG, unless they are 18 AWG with fuse protection. Our own walk through the ratings printed on a cord covers the rest of that stamp.
Where the Code switches sides
Inside the walls the calculation inverts. Nothing behind the drywall is going to flex, so stranding stops paying for itself and the advantages of solid wire — cheaper, easier to strip, easier to hold under a screw without a strand escaping — take over. The nonmetallic-sheathed cable in most houses uses solid copper from 14 gauge through 10 gauge, and switches to stranded at 8 gauge and larger.
The Code pushes the same way once the wire gets big. Section 310.3(C) says conductors 8 AWG and larger installed in a raceway have to be stranded, except where the Code permits or requires solid somewhere else. The reasoning is mechanical rather than electrical: a solid conductor that size is hard to pull around a bend, and forcing it can damage the insulation on the way through. The one exception most homeowners will ever meet is the grounding electrode conductor running to the ground rod or water pipe, which may be solid or stranded either way.
The terminals care more than the wire does
Here is the part that surprised me most: the wire is rarely what decides. The termination is. A screw terminal, a lug, a wire connector, and a push-in hole are each tested with particular conductors.
The push-in holes on the back of an inexpensive receptacle are the tightest example. Under UL 498, the standard those devices are tested to, the push-in type of screwless terminal is restricted to 15-amp branch circuits and to 14 AWG solid copper wire. Not stranded 14. Not 12 of either kind. Not aluminum. The hole was shrunk years ago so 12-gauge would not fit, but nothing stops someone from working a few strands of the wrong wire in there, and a connection made that way has very little metal actually touching.
The same logic runs all the way up. Section 110.14 requires connectors and terminals to be identified for the conductors they hold, and where a conductor is more finely stranded than the ordinary building classes, the terminal has to be identified for that specific class. That is why a lug meant for welding cable lists a string of stranding classes on its label and a breaker lug does not. Fine-stranded wire in a lug never evaluated for it leaves strands outside the clamp and pressure on far less copper than the connection was designed around, which is the same story behind why panel screws get torqued to a published number.
What this changes when you are buying something
Almost none of this is a homeowner decision about house wiring; the cable type is settled by the installation. Where it reaches you is at the store.
- Anything that will be moved, coiled, or unplugged regularly needs a cord, not building wire. No amount of care makes solid wire acceptable in that role.
- Read the cord's stamp for the gauge rather than judging it by thickness, and remember the 16 AWG floor above.
- Retire a cord with a bulge, a stiff kink, a warm plug, or a section that has to be held at an angle to keep working. That last symptom is a broken strand bundle, and it does not heal.
- Hand an electrician the instruction sheet along with any device you bought. The conductor its terminals accept is printed there, not on the box.
- Never splice a cord to extend it, and never tape over damaged insulation. The safety guidance is blunt about that one.
What I took away is smaller than the detail suggests. Gauge tells you how much copper is in a conductor; construction tells you what that conductor was built to survive. Separate those two questions and the soft cord, the stiff cable, and the fussy little hole on the back of the outlet stop being trivia and start being one idea said three ways.