How to Read the Printing on Electrical Wire (And What Every Letter Means)

Every legitimate roll of building wire has its markings printed or embossed right on the outer jacket, repeating every couple of feet along the run. On a spool or reel, you'll also find the same information on the reel tag or box label — sometimes clearer than the jacket print, which can be faint on old stock.

The markings always follow the same pattern: size, then conductor count, then the type letters, then the voltage and temperature ratings, then the listing mark (UL), the manufacturer, and a footage number. Miss the printing entirely — no letters, no numbers, nothing legible — and you're holding either very old wire or something made outside the listing system. Either way, that matters.

Gauge (AWG) is the wire's size — American Wire Gauge. The counterintuitive part: a bigger number means a smaller wire. 14 AWG is thinner than 12 AWG, which is thinner than 10 AWG. The sizes homeowners and shops actually touch every day are 14, 12, 10, 8, and 6 AWG.

Conductor count comes next, and this is where people misread markings. 12-2 means two 12 AWG insulated conductors — plus a bare ground, which is always there and never counted. So 12-2 Romex has three physical wires inside (black, white, bare) but only two insulated current-carrying conductors. 12-3 adds a red conductor: three insulated, plus ground. On a single conductor like THHN you'll just see 12 AWG, sometimes with a strand count like "19 STR" — 19 strands making up the conductor.

600V is the voltage rating stamped on virtually all building wire. It means the insulation is rated for circuits up to 600 volts — which covers every residential and most commercial application. It's not the voltage the wire carries; it's the maximum the insulation is rated to handle safely.

Temperature ratings show as 60°C, 75°C, or 90°C, sometimes split: "90°C dry / 75°C wet." These feed directly into how much current the wire can carry (ampacity). A higher temperature rating doesn't automatically mean you can load the wire harder — terminations and code rules usually cap you at the 60°C or 75°C column — but it's the first thing an inspector checks. Want the full ampacity table? Here's the wire gauge chart — what size wire for every amperage.

The jacket tells you the conductor metal too. CU means copper. AL means aluminum — usually the 8000-series aluminum alloy used in modern feeder and service wire, which is fine when terminated properly.

The tricky one is copper-clad aluminum (CCA): an aluminum core with a thin copper skin. It looks copper on the outside and is sometimes marketed in language that blurs the line. Legitimate CCA is marked — look for "COPPER-CLAD ALUMINUM," "CU-CLAD," or similar wording on the jacket. But cheap or unlisted wire sometimes skips the marking, which is why the physical check matters.

The file-a-strand test: take an exposed end or cut conductor and lightly file or scrape a strand with a pocketknife or file. Solid copper shows copper all the way through. Copper-clad aluminum shows a silver-gray aluminum core under the copper skin. That's the whole test — one scrape tells you. Do not use a flame or lighter test. Burning insulation gives off toxic fumes and tells you less than the file does.

Why this matters: CCA is not the same as copper. It carries less current for the same gauge, it's more brittle, and scrap buyers price it as aluminum — not copper. If the jacket says AL and the outside looks copper, check before you assume.

Example 1 — Romex

12-2 CU TYPE NM-B 600V 90°C (UL) E123456 — SOUTHWIRE — 000342 FT

12-2 — two 12 AWG insulated conductors (black and white), plus an uncounted bare copper ground. CU — copper conductors. NM-B — nonmetallic-sheathed cable with 90°C-rated conductor insulation. 600V — standard building-wire voltage rating. 90°C — the conductor insulation rating (used at 60°C ampacity per code). (UL) — UL listed, with E123456 as the UL file number identifying the manufacturer. SOUTHWIRE — the manufacturer ("Romex" is Southwire's brand name for NM-B). 000342 FT — a sequential footage mark: this point on the jacket is 342 feet from the start of the run, used for measuring what's been pulled.

Example 2 — THHN building wire

12 AWG CU TYPE THHN/THWN-2 600V 90°C DRY / 90°C WET — GASOLINE AND OIL RESISTANT II — MTW — (UL)

12 AWG — 12-gauge single conductor. CU — copper. THHN/THWN-2 — dual rated: T = thermoplastic insulation, H = heat resistant, H = high heat (90°C), N = nylon jacket, W = wet rated, and the -2 means 90°C in wet locations too. 600V — standard rating. 90°C dry / 90°C wet — full rating in both conditions thanks to the -2. GASOLINE AND OIL RESISTANT II — passes oil and gasoline exposure testing (Category II). MTW — also rated as machine tool wire for industrial control panels. (UL) — listed.

Example 3 — MC cable

6 AWG 3/C CU TYPE MC 600V 90°C DRY (UL)

6 AWG — 6-gauge conductors. 3/C — three conductors. CU — copper. TYPE MC — metal-clad cable: the conductors ride inside flexible interlocked metal armor. 600V — standard. 90°C dry — rated 90°C in dry locations only (no W, so not a wet-location cable). (UL) — listed.

Example 4 — UF-B underground feeder

10-2 CU TYPE UF-B 600V SUNLIGHT RESISTANT DIRECT BURIAL (UL)

10-2 — two 10 AWG insulated conductors plus bare ground. CU — copper. UF-B — underground feeder with 90°C-rated conductor insulation. 600V — standard. SUNLIGHT RESISTANT — the gray jacket won't break down under UV exposure above ground. DIRECT BURIAL — rated to be buried in the earth without conduit. (UL) — listed.

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