Building Wire: THHN, Romex, UF & Service Cable

THHN — single insulated conductors, usually pulled through conduit. The workhorse of commercial and industrial wiring. NM-B (Romex) — a flat or round outer sheath holding two or three insulated conductors plus a bare ground; the standard residential branch-circuit cable. UF-B — underground feeder, a tougher gray sheath rated for burial and wet locations. SE / SER (service entrance) — heavy cable running from the meter to the panel and to subpanels. All four are solid copper inside; they differ in construction, gauge, and where you find them.

• THHN: commonly 14, 12, 10, 8, 6, 4, 2 AWG and 1/0 through 4/0 — from branch circuits and panel work up to large feeders. Sources: commercial conduit pulls, panel upgrades, industrial tear-outs.
• NM-B / Romex: commonly 14, 12, 10, 8, 6 AWG. Sources: residential rewires, remodels, additions.
• UF-B: commonly 14 through 6 AWG. Sources: outdoor circuits, outbuildings, landscape lighting.
• SE / SER: commonly 8 AWG through 4/0. Sources: service upgrades, meter-to-panel replacements, subpanel feeders.

These are typical ranges — manufacturers and eras vary, so confirm against jacket print where you can.

They are different products and buyers treat them as different categories. THHN is single conductors — one insulated wire at a time, usually in conduit. Romex (NM-B) is a sheathed cable — multiple conductors plus ground inside one outer jacket. Published yard taxonomies list them as separate grades because the copper recovery differs: THHN insulation is thinner relative to the copper, while Romex carries the extra weight of its outer sheath. Sorting them into separate piles is one of the easiest ways to lift a load’s grade.

Heavy THHN and feeder-size conductors are often worth a hard look at stripping — thick copper, thin insulation. Romex and smaller building wire usually sell best insulated unless you have real volume. Either way, never burn insulation off: it is restricted or prohibited in many areas, the fumes are toxic, and it destroys the grade. Our strip-or-sell decision guide walks through the math wire by wire.

Photograph what you have — the wire, any jacket markings, and the overall quantity — and send it to Ironcrest for today’s offer. We buy nationally and coordinate pickup from your site.

Sell your copper to Ironcrest →  |  Call or text 312-880-8174  |  Send photos →

See also: Should I strip this wire?  |  How to read wire markings  |  Copper wire grades guide (canonical)

What is THHN wire?

THHN is a single-conductor building wire — thermoplastic insulation, high heat resistance, nylon coating — commonly pulled through conduit in commercial, industrial, and residential work. It is one of the most frequently scrapped wires because every panel upgrade and conduit pull produces offcuts.

What is the difference between THHN and Romex?

THHN is single insulated conductors, usually run in conduit. Romex (NM-B) is a sheathed cable holding two or three conductors plus a ground inside one outer jacket, used for residential branch circuits. Buyers grade them as separate categories.

Is Romex worth stripping?

Sometimes, with volume — but Romex carries an outer sheath plus conductor insulation, so the recovery per foot is lower than heavy THHN. Most sellers do best selling Romex insulated. Our strip-or-sell guide covers the decision.

What is UF wire?

UF-B (underground feeder) is a gray-sheathed cable rated for direct burial and wet locations — outdoor circuits, outbuildings, landscape lighting. Same copper inside as NM-B, tougher jacket outside.

Can I sell old service entrance cable?

Yes — SE/SER service cable is heavy insulated copper with strong recovery per foot, and service upgrades produce it in useful lengths. Photograph the markings and the coil; it is straightforward to quote.