Wire Sizes Explained

Wire sizing questions come in a handful of shapes, and each one has a page on this site built for it. Find the row that matches what you already know.

American Wire Gauge (AWG)

AWG stands for American Wire Gauge, the standard system for sizing electrical conductors in North America. It numbers conductors backwards: a smaller number is a larger wire, so 6 AWG is thicker than 10 AWG and 14 AWG is thinner than both. The reason is mechanical. Wire was made by drawing it through progressively smaller dies, and the gauge number counted the passes — more passes, thinner wire, higher number.

The scale runs out at 1 AWG. Larger conductors continue into the aught sizes, written 1/0, 2/0, 3/0 and 4/0 and spoken "one-aught" through "four-aught," where more aughts mean more copper. Those sizes exist because service entrances and feeders outgrew the original scale, and 4/0 is the largest conductor AWG describes.

Circular Mils and kcmil

A circular mil is the area of a circle one-thousandth of an inch in diameter, and it is the unit that actually matters — gauge is only a proxy for cross-sectional area, and area is what sets resistance. Two conductors of equal circular-mil area have equal resistance per foot whatever their labels say.

Wire gauge sizes in circular mils climb steeply: 12 AWG is 6,530 circular mils and 4/0 is 211,600, a factor of thirty-two across what sounds like a handful of sizes. Above 4/0 the AWG scale stops and conductors are measured directly in thousands of circular mils — kcmil, formerly MCM — starting at 250 kcmil and running up through 1,000 kcmil for service and industrial feeders.

Metric Wire Sizes (mm²)

Most of the world sizes conductors in square millimetres of cross-sectional area rather than AWG, and the two scales do not line up exactly. A metric size is a true area; an AWG size is a step on a fixed ratio scale, so conversions land between sizes. Roughly, 2.5 mm² sits near 14 AWG, 4 mm² near 12 AWG, 6 mm² near 10 AWG, 10 mm² near 8 AWG, and 16 mm² near 6 AWG. An 8 mm² conductor falls just under 8 AWG, which is 8.37 mm², and is normally treated as 8 AWG.

With imported equipment, a minimum conductor given in mm² is a floor rather than a substitute for a NEC Table 310.16 check, and where a conversion lands between sizes the safe direction is up.

Identifying a conductor is a different job from understanding the scales. Telling what gauge an unmarked wire is comes down to measuring the bare conductor, not the insulation: 14 AWG is 0.064 in (1.63 mm), 12 AWG is 0.081 in (2.05 mm), and 10 AWG is 0.102 in (2.59 mm). A gauge wheel reads it directly; calipers work if the insulation is stripped back first.

Copper, Aluminum and Copper-Clad Aluminum

Aluminum conductors need roughly two sizes larger than copper for the same ampacity, a difference that shows up row by row in NEC Table 310.16. At equal gauge aluminum also loses more voltage over the same run, which compounds the difference on long feeders.

Aluminum remains the normal choice for service entrances and long feeders, at a fraction of copper's cost per foot and about a third of the weight. The cautions are at the terminations: lugs and breakers must be listed for aluminum, marked AL/CU or AL9CU, and the connection wants antioxidant compound and a torque wrench rather than a screwdriver. Copper-clad aluminum, an aluminum core with a copper skin, is sized as aluminum. The wire size chart lists both materials side by side at each temperature rating.

Solid and Stranded Conductors

Solid and stranded conductors of the same AWG size carry the same rated ampacity, but they are not identical: a stranded conductor has slightly more resistance per foot, because each strand spirals along the length and travels a little farther than the cable does. NEC Chapter 9 Table 8 lists solid and stranded resistance separately for that reason.

Solid is normal in residential branch circuits up to 10 AWG, where stiffness helps a conductor stay put behind a device. Stranded takes over above that and anywhere the wire must flex, pull through conduit or survive vibration — appliance cords, motor leads, marine wiring, and essentially all large feeders. Stranded is also slightly fatter overall at the same gauge, which matters for conduit fill.

Insulation Types and Temperature Ratings

The insulation printed on a conductor's jacket sets its temperature rating, and that rating decides which ampacity column the conductor may be sized from. It is the part of wire sizing skipped most often, and it changes the answer more than the material does.

The letters are a code: T is thermoplastic, H means heat-resistant to 75°C, HH means high-heat to 90°C, W means rated for wet locations, N is a nylon jacket that resists oil and gasoline, and X is cross-linked polyethylene. So THHN is 90°C in dry locations, THWN-2 is 90°C wet and dry, and XHHW-2 is the cross-linked equivalent. NM-B cable is the case worth remembering: its conductors are 90°C rated, but NEC 334.80 holds the cable to the 60°C column.

Reading the Markings on a Wire

Every conductor made in recent decades carries its specification printed along the insulation, and reading it answers most sizing questions before any measurement. The legend gives the gauge, the conductor material, the insulation type, the voltage rating and the listing marks.

A jacket reading "12 AWG THHN-2 CU 600V" is 12-gauge copper with 90°C insulation rated to 600 volts. Cable assemblies mark the assembly instead: "12-2 NM-B WITH GROUND" is two 12 AWG insulated conductors plus a bare equipment ground inside nonmetallic sheath. Where the print says AL rather than CU the conductor is aluminum and every ampacity figure changes. The UL or CSA mark matters as much as the numbers, since an unlisted conductor cannot be used in a compliant installation however it is sized.

• Ampacity

  Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating.

• Voltage Drop

  A conductor rated for the current can still be the wrong size, because voltage drop accumulates with length while ampacity does not.

• Derating

  Derating reduces a conductor's usable ampacity when the installation is hotter or more crowded than NEC Table 310.16 assumes.

Ampacity

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. It is set by the conductor size, the material and the temperature column the circuit is held to, and it is published in NEC Table 310.16 for the ordinary case of no more than three current-carrying conductors at 30°C ambient.

Ampacity is the first test a size has to pass, and the easiest to look up, which is why it gets treated as the whole answer. The wire size chart lists allowable ampacity for copper and aluminum from 14 AWG through 500 kcmil in the 60, 75 and 90°C columns, with the small-conductor limits marked.

Voltage Drop

A conductor rated for the current can still be the wrong size, because voltage drop accumulates with length while ampacity does not. The current arriving at the far end of a long run is the same; the voltage is not, and equipment fed low voltage runs hot, starts poorly or dims.

The figures to aim at are 3% for a branch circuit and 5% for branch circuit and feeder combined, from informational notes at NEC 210.19(A), Informational Note No. 4, and NEC 215.2(A), Informational Note No. 2. Both are recommendations rather than enforceable limits, so an inspector will rarely fail a job on voltage drop alone — the cost shows up in performance instead. The voltage drop calculator gives the percentage for a specific run, and drop by gauge, length and load moves predictably from there: it is proportional to both current and distance, so halving the run halves the drop.

Derating

Derating reduces a conductor's usable ampacity when the installation is hotter or more crowded than NEC Table 310.16 assumes. Two independent factors apply: ambient temperature correction under NEC 310.15(B) when the surrounding air is above 86°F, and conductor bundling adjustment under NEC 310.15(C)(1) when a raceway or cable carries more than three current-carrying conductors. Where both conditions exist, both factors apply.

This is the limit that quietly invalidates a correct chart lookup — an attic in summer, conduit on a sunlit roof, a raceway shared by several circuits. The correction and adjustment factors apply in a fixed order and multiply: Table 310.16 ampacity, then ambient correction from Table 310.15(B)(1), then the bundling adjustment from Table 310.15(C)(1). Both at once is ordinary — an attic in summer with four circuits in one conduit — and the two together can take a third off the printed figure.

The rule tying the three together: size the conductor to whichever limit demands the largest wire, because passing two of the three tests is failing. The wire sizing calculation is three lookups and one comparison: ampacity from Table 310.16 in the termination column NEC 110.14(C) sets, the 240.4(D) cap if the conductor is 14, 12 or 10 AWG, and the drop at your actual run length. Whichever demands the largest conductor is the answer.

Terminal Temperature Limitation (110.14(C))

The effect is that the highest number in a conductor's row is usually not the one you may use, and two circuits wired with identical THHN can have different allowable ampacity purely because of the devices at their ends. The 90°C rating still earns its keep as the starting point for derating.

Continuous Loads and the 125% Rule

The three-hour threshold is about heat rather than duty cycle. A conductor carrying steady current reaches its final temperature after a few hours, and the 25% margin keeps that temperature under the insulation's limit; a load that cycles, like a refrigerator or a well pump, never gets there and is not continuous. Where a circuit carries both kinds, the continuous portion takes the 125% factor and the rest is added at 100%.

How Breaker Size and Wire Size Relate

Conductors and breakers are sized as a pair: the breaker protects the conductor, so it can never be rated above what the conductor safely carries. Getting one right and the other wrong leaves a circuit that either trips constantly or, far worse, lets a conductor overheat without tripping at all.

Branch Circuits, Feeders and Services

Branch circuits, feeders and service conductors are sized on the same three limits but to different expectations. A branch circuit is the wiring from the last overcurrent device to the equipment, and it is where the 3% voltage-drop target applies. A feeder supplies another panel, and the recommendation is that feeder and branch circuit together stay under 5%.

Service-entrance conductors follow their own allowances. NEC 310.12 permits a dwelling service or main power feeder rated 100 to 400 amps and carrying the whole load to be sized at 83% of its rating, because the full calculated load of a house is diversified and rarely present at once. The same conductor can therefore be correct as a service, marginal as a feeder and wrong as a branch circuit.

AC and DC Are Sized Differently

Low-voltage DC circuits need disproportionately larger conductors than AC circuits carrying the same current, because voltage drop is judged as a percentage of system voltage. A 3-volt loss is 1.25% of a 240-volt system and 25% of a 12-volt one, so the same absolute drop that nobody would notice on a range circuit will stall a 12-volt winch or brown out an inverter.

The practical consequence is that the order of the tests flips. On 12-volt and 24-volt circuits, voltage drop rather than ampacity almost always decides the size, and a conductor picked from an ampacity figure alone will be several sizes too small for any run of length. DC also has no reactance and no power factor, so only conductor resistance enters the calculation.

Conductor sizes for DC systems follow the same method with two differences. There is no reactance, so Table 8's direct-current resistance is the whole impedance rather than one term of it. And the low system voltage makes the percentage budget tiny: 3 percent of 12 volts is 0.36 volts, which is why a 12-volt or 24-volt circuit is almost always sized by drop rather than by ampacity.

Grounding and Neutral Conductors Are Sized Differently

An equipment grounding conductor is not sized from the ampacity table at all. It comes from NEC Table 250.122, against the rating of the overcurrent device protecting the circuit, which is why the ground wire in a cable is usually smaller than the circuit conductors — it carries fault current for milliseconds, not load current continuously.

Neutrals work the other way. A neutral carrying the unbalanced current of a multiwire circuit is a current-carrying conductor and is sized like one, and on circuits feeding electronic or LED loads the harmonic current in the neutral can exceed the current in any phase conductor. The equipment grounding conductor is sized from NEC Table 250.122 by the rating of the overcurrent device, not by the circuit conductor: 14 AWG copper up to 20 amps, 12 AWG to 60, 10 AWG to 100, 8 AWG to 200.

Motors and HVAC Follow Different Rules

Motor circuits are sized from the code's own current tables, not the motor nameplate. NEC Article 430 sets the method: conductors are sized at 125% of the full-load current listed in Tables 430.247 through 430.250 for that horsepower and voltage, while the nameplate amps are used for overload protection.

Inrush is why the rules separate. A motor draws several times its running current during a start, so the short-circuit and ground-fault device is sized well above the conductor's ampacity — an arrangement that would be a serious violation on an ordinary circuit. HVAC equipment follows a related path: NEC 440.4(B) requires the nameplate to carry a minimum circuit ampacity and a maximum overcurrent protection rating, both calculated by the manufacturer and taking precedence over any generic lookup.

Conduit Fill

A correctly sized conductor can still be illegal to install, because the raceway has a limit of its own. NEC Chapter 9 caps the share of a conduit's interior cross-section conductors may occupy — 53% for one conductor, 31% for two, and 40% for three or more. The reasons are heat and mechanical damage: tightly packed conductors cannot shed heat, and a full conduit cannot be pulled without scraping insulation.

Fill is calculated from overall diameter including insulation, so insulation type changes the answer. Fill also interacts with derating, since the conductor count that fills a raceway is often the count that triggers a bundling adjustment. Sizing up to satisfy voltage drop can push a run past the fill limit and force a larger conduit, so the two are worth calculating together.

The most frequent mistake is sizing from ampacity alone. A conductor that carries the current at 20 feet may lose too much voltage at 150. Anything past roughly 100 feet needs a voltage-drop check.

The second is using the 90°C column because the wire is 90°C rated. NEC 110.14(C) ties the circuit to the lowest-rated termination in it, and on nearly all residential equipment that is 60°C or 75°C.

Forgetting the 125% continuous-load factor comes next, most often on EV chargers and commercial lighting. Sizing a 32-amp charger as a 32-amp load produces a conductor and a breaker that are both one size small.

Measuring the wrong distance is a quieter error. Voltage drop is calculated from the one-way distance and the formula doubles it for the return path, so entering round-trip footage makes a run look twice as bad as it is and buys copper nobody needed.

Ignoring derating shows up in service calls rather than inspections: attic runs, rooftop conduit and shared raceways all cut usable ampacity, and none look different on a wiring diagram. The last is treating aluminum as a drop-in for copper, when it needs two sizes up and terminations listed for it.

Wire sizing in the United States is governed by the National Electrical Code, published as NFPA 70 by the National Fire Protection Association and revised every three years. The NEC is not law by itself. It becomes enforceable when a state or municipality adopts it, and jurisdictions adopt different editions on different schedules, so a 2023-edition state and a 2026-edition state can require different conductors for the same circuit.

Local amendments override the published code where they exist, and some are significant — a few jurisdictions restrict aluminum branch-circuit conductors, others add their own derating or raceway requirements. The authority having jurisdiction, meaning the inspector on your installation, has the final word on interpretation, and that word outranks any chart on any website. Figures on this site follow NFPA 70-2023. The 2026 edition is published and adoption is rolling out state by state, so confirm which edition your jurisdiction enforces before relying on any figure here. The code itself is available from the NFPA.