Wire Gauge Calculator
Look up the ampacity, diameter, cross-section, and DC resistance of an American Wire Gauge conductor. Covers 14 AWG to 4/0 in copper and aluminum at 60, 75, and 90 °C insulation ratings, with the NEC Table 310.16 ampacity and the NEC 240.4(D) small-conductor overcurrent cap applied automatically.
Ampacity
20
- NEC Table 310.16 ampacity
- 25
- Conductor diameter (mm)
- 2.05
- Cross-section (mm²)
- 3.31
- Cross-section (kcmil)
- 6.53
- DC resistance at 20 °C (Ω / km)
- 5.21
12 AWG copper with 75 °C insulation (THW, THWN). Geometry from ASTM B258 (d = 0.005 × 92^((36 − n)/39) inches), resistance from material resistivity at 20 °C, ampacity from NEC 2023 Table 310.16 for a single conductor in raceway at 30 °C ambient with no more than three current-carrying conductors. NEC 240.4(D) caps the overcurrent device on this small conductor at 20 A, so the usable circuit rating is 20 A. Apply NEC 310.15 adjustment factors for higher ambient temperature or more than three conductors in a raceway.
How to use this calculator
Pick the wire size in AWG, the conductor material (copper or aluminum), and the insulation temperature rating printed on the cable jacket. The calculator returns the conductor diameter in millimetres, the cross-sectional area in both square millimetres and thousand circular mils (kcmil), the DC resistance per kilometre at 20 °C, and the NEC Table 310.16 ampacity for that combination. The headline result is the usable ampacity, which is the lower of the table value and the NEC 240.4(D) small-conductor overcurrent cap (15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper). If you are sizing a circuit in a hot ambient, or running more than three current-carrying conductors in the same raceway, apply the NEC 310.15 adjustment factors to the table value before sizing the breaker.
How the calculation works
AWG diameter follows ASTM B258: each successive gauge changes diameter by a fixed ratio of 92^(1/39), with 36 AWG fixed at 0.005 in and 4/0 AWG fixed at 0.46 in. The closed-form expression is d(inches) = 0.005 × 92^((36 − n)/39) where n is the AWG number — n is 0 for 1/0, −1 for 2/0, −2 for 3/0, and −3 for 4/0. Cross-section in square millimetres follows from the circular geometry, and the kcmil value is the square of the diameter in mils divided by a thousand. DC resistance per kilometre at 20 °C comes from the material resistivity (copper 1.724 × 10⁻⁸ Ω·m, aluminum 2.826 × 10⁻⁸ Ω·m) divided by the cross-section in square metres. Ampacity is read directly from NEC 2023 Table 310.16 for a single insulated conductor in raceway, 30 °C ambient, not more than three current-carrying conductors. The three insulation columns reflect the maximum continuous operating temperature of the insulation — 60 °C is old TW, 75 °C is THW or THWN, and 90 °C is modern THHN, THHW, or XHHW-2.
Worked example
12 AWG copper with 75 °C THHN insulation. The closed-form diameter is 0.005 × 92^((36 − 12)/39) = 0.005 × 92^(24/39) ≈ 0.0808 in, which is 2.053 mm. The cross-section is π × (2.053/2)² ≈ 3.31 mm², equivalently 6.53 kcmil. DC resistance at 20 °C is 1.724 × 10⁻⁸ / 3.31 × 10⁻⁶ × 1000 ≈ 5.21 Ω/km. NEC Table 310.16 lists 25 A for 12 AWG copper in the 75 °C column, but NEC 240.4(D) caps the overcurrent device at 20 A on 12 AWG copper regardless of insulation, so the usable circuit rating is 20 A. The same 12 AWG in the 90 °C column reads 30 A on the table; the 240.4(D) cap still pulls the usable rating to 20 A, but the higher 90 °C value matters for ampacity adjustment, where the adjusted ampacity (after temperature and conductor-fill derating) is compared against the table value before the 240.4(D) cap is applied.
Frequently asked questions
How do AWG numbers relate to wire thickness?
AWG is inverse: smaller numbers mean thicker wire. The series is defined so that consecutive gauges differ in diameter by a factor of 92^(1/39) ≈ 1.0226, which means every six gauge steps roughly doubles the diameter and every three steps roughly doubles the cross-section. Once you go thicker than 1 AWG the notation switches to aught sizes — 1/0, 2/0, 3/0, 4/0 — and above 4/0 it switches again to thousands of circular mils (250 kcmil, 500 kcmil, etc.) because the geometric series breaks down. 14 AWG, 12 AWG, and 10 AWG are by far the most common residential branch-circuit sizes; 6 AWG and 8 AWG appear on range and dryer circuits; 4/0 and kcmil sizes show up on service entrances and feeders.
Why is the 12 AWG ampacity 25 A but breakers are 20 A?
NEC Table 310.16 lists the conductor ampacity — the current the conductor can carry indefinitely without exceeding its insulation temperature rating. NEC 240.4(D) then adds a separate "small-conductor rule" that caps the overcurrent device for 14, 12, and 10 AWG copper at 15, 20, and 30 amps respectively, regardless of the table value. The intent is to protect against the higher fault risk on small conductors that may be installed in less controlled environments, like residential branch circuits. The headline result on this calculator applies the cap automatically; the raw table ampacity is shown alongside it so you can see both numbers.
What does the 60 / 75 / 90 °C column mean?
It refers to the maximum continuous operating temperature of the conductor insulation, not the ambient temperature. 60 °C is old TW insulation, rare in new work. 75 °C is THW or THWN — the standard 1980s-90s residential wiring. 90 °C is modern THHN, THHW, or XHHW-2, which is what most cable sold today is rated for. The catch is NEC 110.14(C) terminations: even if the conductor is 90 °C-rated, the terminations on the breaker or device may only be rated for 60 or 75 °C, and you have to use the lower-rated column for sizing. As a rule of thumb, circuits ≤ 100 A use the 60 °C column for terminations; circuits > 100 A use the 75 °C column; the 90 °C column is mainly used as the starting point for ampacity adjustment under NEC 310.15.
Why is aluminum two gauge sizes larger than copper?
Aluminum has roughly 64 % the conductivity of copper, so for the same ampacity the aluminum conductor needs about 56 % more cross-section. Two AWG sizes corresponds to a cross-section ratio of about 1.59, which is close enough to that ratio that the rule of thumb works: 4/0 aluminum carries about the same current as 2/0 copper, 2 AWG aluminum about the same as 4 AWG copper, and so on. Aluminum requires AL- or CU/AL-rated terminations and an antioxidant compound at lugs to avoid the high-resistance oxidation that caused the bad reputation of 1960s-70s solid-aluminum branch wiring; modern stranded AA-8000-series aluminum on properly rated terminations is reliable and widely used on services and feeders where the cost saving on heavy gauges is significant.
Does this account for derating?
No — the values here are the base NEC Table 310.16 ampacities at 30 °C ambient with no more than three current-carrying conductors in a raceway. NEC 310.15(B) adds temperature-correction factors for higher ambient temperatures (multiply by 0.91 at 31-35 °C, 0.82 at 36-40 °C, 0.71 at 41-45 °C, etc.) and 310.15(C) adds adjustment factors for more conductors in the raceway (multiply by 0.80 for 4-6 conductors, 0.70 for 7-9, and so on). Both factors compound. The standard workflow is to start from the 90 °C column, apply the adjustment factors, then compare the result against the 60 / 75 °C termination rating from NEC 110.14(C) and take the lower of the two as the usable ampacity, before finally applying the 240.4(D) small-conductor cap.
Is the resistance value DC or AC?
The Ω/km figure here is DC resistance at 20 °C derived from the conductor cross-section and the material resistivity. For typical 60 Hz residential and light-commercial branch circuits the inductive reactance is negligible and the DC value is what every published voltage-drop calculation uses (and what NEC Chapter 9 Table 8 tabulates). For long high-current feeders at 1/0 AWG and larger, conductor reactance becomes a noticeable contributor and the NEC Table 9 effective Z values (which fold in reactance and conduit material) give a more accurate answer; the difference is in the few-percent range and matters most for utility-style feeders and long motor branch circuits. For sizing residential branches and short feeders, the DC value here is fine.