Electrical wire — types, sizes, insulation, and sizing

Size electrical wire — calculator

Once you know the wire type from the rest of this page, the calculator picks the right size — the smallest standard AWG or mm² that satisfies both NEC 310.16 ampacity (with derating) and your voltage-drop limit.

What is electrical wire?

Electrical wire is a single insulated conductor or an assembly of insulated conductors used to carry electric current between equipment. Each wire has three identifying parts: the conductor (the metal that carries current — copper or aluminium), the insulation (a polymer that prevents current from leaking out and rates the wire's temperature and environmental exposure), and, on cable assemblies, an outer jacket that bundles multiple conductors and protects them mechanically.

Each wire is designated by an alphanumeric code that encodes all three. For example, THHN means: T = thermoplastic insulation, H = high-heat-resistant (90°C), H again = even-higher-heat (now 90°C wet/dry on the -2 grade), N = nylon outer coating. NM-B means: N = nonmetallic-sheathed, M = multi-conductor cable, B = current 90°C insulated grade. UL and NEC standardise these codes; every wire sold in North America is marked with one.

Conductor size is given in AWG (American Wire Gauge) for sizes from very fine 30 AWG telephone wire up through 4/0 service-entrance cable. Above 4/0, conductors are sized in kcmil (thousand circular mils). Outside the US, conductors are sized in mm² per IEC 60228 — common electrical wire sizes in mm equivalents are: 14 AWG ≈ 2.5 mm², 12 AWG ≈ 4 mm², 10 AWG ≈ 6 mm², 8 AWG ≈ 10 mm², 6 AWG ≈ 16 mm². The most-installed AWG sizes in residential branch wiring are 14 AWG (15 A lighting), 12 AWG (20 A receptacles, kitchen and bath), and 10 AWG (30 A dryer / water heater); the smallest commonly stocked size for general-purpose wire is 18 AWG electrical wire (control circuits, fixture whips, doorbell, thermostat).

Common building wire types

The most common single-conductor types installed in residential and light-commercial work — by occurrence and by NEC-listed application.

Cable assemblies (jacketed multi-conductor)

Cable assemblies bundle 2 to 4 insulated conductors plus a grounding conductor inside an outer jacket or armour. They are pre-engineered for a specific NEC application and are usually pulled directly into framing or buried, without conduit.

Type NM cable shall be permitted to be used in the following: (1) One- and two-family dwellings and their attached or detached garages and their storage buildings. (2) Multi-family dwellings permitted to be of Types III, IV, and V construction. (3) Other structures permitted to be of Types III, IV, and V construction… Type NM cable shall not be permitted to be installed where exposed to corrosive fumes or vapors, embedded in masonry, concrete, adobe, fill, or plaster, or where subject to excessive moisture or dampness.

Conductor materials: copper vs aluminium

Two conductor materials dominate. Copper is the default for branch circuits and any circuit where space is tight or terminations are typically rated 75°C only. Aluminium is the standard for large feeders and service-entrance work, where its lower cost per amp-metre outweighs its larger size.

"Old aluminium" branch wiring — pure-Al solid conductors installed in US homes from 1965 to 1973 — is a known fire hazard because of cold-flow loosening at terminations. Modern AA-8000-series aluminium alloy (introduced in NEC 1987) is tested to a different specification and is reliable for service-entrance and feeder work, but is still not used for residential branch circuits.

Insulation temperature ratings

Three temperature ratings cover essentially all building wire, set by the polymer chemistry of the insulation.

The catch is NEC 110.14(C): even with 90°C wire, the termination determines the column you can actually use. For circuits ≤100 A, terminations are rated 60°C unless marked otherwise — many electricians use 75°C insulation as the practical floor and only rely on the 90°C column for derating math. This is why a 12 AWG Cu THHN-2 (90°C) wire is still on a 20 A breaker, not a 30 A one — the breaker terminal is the limit, not the wire.

Cord and flexible wire (SO, SOOW, SJOOW, SPT)

Cords are flexible cable assemblies for portable equipment and machine wiring (NEC Article 400). The letter coding describes the rubber compound and jacket.

Marine cord uses tin-plated finely-stranded copper to resist saltwater corrosion: BC-5W2 per UL 1426 / ABYC E-11 standards. Residential extension cords are SJTW; jobsite cords are SOOW. Never use ordinary building wire (THHN, NM-B) as a flexible cord — solid conductors fatigue and break with bending.

The wire-sizing formula in one line

Once cable type and material are picked, the AWG follows from one decision rule: the smallest standard size whose derated ampacity covers the load and whose voltage drop stays within the project limit. The drop side of the rule is a single equation.

For three-phase the leading factor becomes √3 and L·I gives line-to-line drop. The full sizing engine — including ampacity look-up, NEC 110.14(C) termination cap, derating for ambient and conductor count, and the small-conductor rule (NEC 240.4(D)) — runs on the wire size calculator. This page focuses on picking the right type; the calculator picks the right size.

Worked examples — three typical projects

Type, material, insulation, and AWG for three common installations under standard conditions (75°C terminations, 30°C ambient, ≤3 conductors in conduit).

Note how the conductor type changes with the application: NM-B for indoor branch, THHN-2 single conductors in conduit for the EV feed (because a single jacketed cable cannot meet the bend radius in tight garage runs), and SER aluminium for the service feeder where copper would double the cost. Run the live calculator to verify these against your actual length, ambient, and any continuous-load multipliers.

How to choose the right electrical wire, step by step

1. Choose the cable type by location and code. Indoor dry residential branch → NM-B (Romex). Conduit in commercial occupancies → THHN/THWN-2 single conductors. Direct-buried or wet locations → UF-B or USE-2. Healthcare branch → MC. Service entrance → SE / SEU / SER / USE-2. The location and the NEC chapter that governs it determine the cable type before anything else.
2. Pick the conductor material. Copper for branch circuits and any termination tighter than 100 A. Aluminium for service entrance, large feeders ≥ 100 A, and overhead transmission. Mixed installations are common — Al feeder to the panel, Cu branches. Never use copper-clad aluminium (CCA) for 15/20 A residential circuits.
3. Pick the insulation rating. New work uses 90°C (THHN/THWN-2 or XHHW-2). 75°C is acceptable for repair stock. 60°C is legacy only. NEC 110.14(C) caps your effective ampacity at the termination rating — usually 75°C — so the 90°C rating is mostly used as a starting point for derating math, not for direct sizing.
4. Verify the cable is allowed in your location. NM-B is banned in commercial occupancies over 3 storeys, in wet or corrosive locations, and exposed where it could be damaged (NEC 334.10/334.12). MC and AC are allowed almost everywhere. UF-B is the only NM-equivalent allowed in wet or buried locations. Check your local jurisdiction's NEC adoption — some cities have stricter rules.
5. Size the conductor with the calculator. Once cable type, material, and insulation are decided, the AWG (or mm²) follows from load, length, and derating. Use the embedded calculator above — it applies all NEC 310.16 ampacity, voltage drop, and EGC rules automatically and emits a PDF report.
6. Confirm the OCPD and grounding match the conductor. Round the load up to the next standard breaker per NEC 240.6(A): 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A. Look up the minimum equipment grounding conductor (EGC) per NEC Table 250.122 from the breaker rating. NEC 240.4(D) caps 14/12/10 AWG copper at 15/20/30 A regardless of insulation.

Related calculators and references

Frequently asked questions

What is the difference between THHN and THWN wire?

Both are PVC-insulated single conductors with a nylon jacket. THHN = Thermoplastic, High-Heat-resistant, Nylon-coated, rated 90°C dry. THWN adds W = Wet — the same insulation rated 75°C wet (or 90°C dry). Modern wire is usually marked THHN/THWN-2: 90°C dry and 90°C wet, the universal building wire. NEC 310.16 ampacity at 75°C is what most installations are limited by because terminations are typically rated 75°C.

What is Romex / NM-B cable used for?

NM-B ("Romex" is the Southwire trade name) is a flat, multi-conductor jacketed cable used for residential branch wiring inside dry indoor walls. NM-B contains 2, 3, or 4 THHN-grade conductors plus a bare copper grounding conductor under a PVC jacket. NEC 334 forbids NM-B in wet locations, conduit underground, exposed runs in commercial occupancies (over 3 storeys), or where it could be physically damaged.

When do I use UF-B instead of NM-B?

UF-B is "Underground Feeder" — a solid PVC-encased cable rated for direct burial in earth, in wet locations, and outdoors. Use it for garage feeders, well pumps, post lights, and anywhere underground or wet. UF-B costs more than NM-B and is harder to strip, but NEC 340 allows it in places NM-B is banned. Above grade in conduit you can use individual THWN-2 conductors instead.

What is MC vs AC cable?

Both are flexible metallic-armoured cables. MC (Metal-Clad, NEC 330) has insulated conductors plus a separate equipment grounding conductor inside an aluminium spiral armour. AC (Armoured Cable, NEC 320, "BX") has insulated conductors and an aluminium bonding strip — the armour itself acts as part of the equipment ground. AC is older; MC is more common in modern commercial work, including healthcare branch circuits where NM-B is not permitted.

Service entrance — SE, SER, SEU, USE — what is the difference?

SE is the umbrella for service-entrance cable (NEC 338). SEU is "service entrance, unjacketed" — used from the service drop or meter to the panel above grade. SER is "service entrance, round" — three or four insulated conductors in a round PVC jacket, used as a panel feeder. USE ("Underground Service Entrance") is a single-conductor or multi-conductor cable rated for direct burial.

Solid vs stranded — when to use which?

Solid is one solid conductor; preferred for 14/12/10 AWG residential branch circuits because it grips screw terminals firmly and is easier to push through device boxes. Stranded is multiple small wires twisted together; preferred for 8 AWG and larger and for any flexible application like cord and machine wiring. NEC requires stranded for sizes 8 AWG and larger in raceway.

What insulation rating do I need?

Three temperature ratings: 60°C (TW, UF) for older or specialised low-temp applications; 75°C (THW, RHW, USE) for general use; 90°C (THHN/THWN-2, XHHW-2) for modern building wire. Most equipment terminals are listed only to 75°C, so even with 90°C wire NEC 110.14(C) caps your effective ampacity at the 75°C column. The 90°C insulation is still useful for derating math.

Should I buy CCA (copper-clad aluminium) wire?

No, not for branch circuits. CCA has a steel or aluminium core with a thin copper skin; resistance per AWG is much higher than solid copper, and NEC does not list it for general residential wiring. CCA shows up in budget speaker cable and some online "Romex-style" listings — never use it on a 20 A circuit just because the label reads "12 gauge."

Sources and methodology

1. NFPA. National Electrical Code (NEC) NFPA 70, 2023 Edition. Articles 110.14, 240.4(D), 250.122, 310.16, 320, 330, 334, 338, 340, 400.
2. UL. UL 83 — Thermoplastic-Insulated Wires and Cables.
3. UL. UL 854 — Service-Entrance Cables.
4. UL. UL 1581 — Reference Standard for Electrical Wires, Cables, and Flexible Cords.
5. ASTM. ASTM B258 — Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes.
6. ABYC. E-11: AC and DC Electrical Systems on Boats, 2018 Edition.