Solid vs stranded wire: construction, flexibility and the terminals rule

Open a residential wall box and you’ll find solid wire in one piece. Open the machine room of the same house and the terminations are nearly all stranded. Same copper, same mm², two constructions that split the electrical world by where they install.

Same gauge, different body

AWG numbers describe the copper’s cross-section area, so a 12 AWG solid and a 12 AWG stranded carry the same current by definition. What differs is the outer diameter and the mechanics:

Property	Solid	Stranded
Overall diameter	exact (2.05 mm for 12 AWG)	+10–25% (2.35–2.5 mm)
Flex life	~3 bends before fatigue	thousands of cycles
Termination	best in backstab/screw	risks strand escape (use ferrules!)
Corrosion ingress	none (solid body)	wicking risk in wet runs

Strand classes

Manufacturers grade stranded wire flexibility:

• Class B/C: 7–19 coarse strands, building wire stock (THHN/THWN-2 in multi-conductor cables like NM-B is actually solid, but service/feeder cables mostly stranded).
• Class G/H: hundreds of fine strands, welding cables and battery cable — “fine-strand” to electricians.
• Fine-strand Litz/film: for signal and RF, not power.

The practical effect: more, finer strands = droopier and take more abuse at the strain relief.

Terminals are the real divide

Solid wire pushes in and stays; the screw bites the whole cylinder. Stranded wire under the same set screw frays unless you:

1. Crimp ferrules (bootlace ends — standard practice in EU panels),
2. Use lugs lined for fine strand,
3. Or tin-lightly (north-American practice disfavors fully tinning fine-strand in vibration due to cold-flow).

That is exactly why residential walls are solid and industrial panels are stranded-with-ferrules: each construction pairs with a termination culture.

Metric labels stay construction-blind

Metric countries print the total copper mm² on the jacket regardless of construction: 4 mm² solid and 4 mm² fine-strand carry the same label. US AWG does this implicitly, but the packaging “type: 12/2 SF STR” tells you it’s stranded. When replacing a fixture, match construction first, then gauge — spring-back behavior is a safety issue at the terminal.

Converting diameters

If you’re measuring a stranded mystery cable with calipers, don’t compare against solid charts — the air gaps fool you. Count strands × area per strand instead, or use the stranded wire gauge chart to break down the equivalence; the copper chart has the resistances for verification.