Wire Size Calculator (AWG and mm^2)
Enter your circuit current, supply voltage, wire length, and conductor material to find the correct American Wire Gauge (AWG) or metric cross-section. The calculator applies the standard voltage-drop formula and NEC ampacity limits so you get the safe minimum size for your run, plus the actual voltage drop and resistance for that gauge. Switch between copper and aluminum, DC or AC, metric or imperial units.
How to size a wire correctly
Selecting the right wire gauge requires satisfying two independent limits at the same time. The first is the voltage-drop limit: a wire that is too thin has enough resistance to waste a significant fraction of your supply voltage as heat, dimming lights and starving motors. NEC recommendations are 3% for branch circuits and a combined 5% for feeders plus branch circuits. The second is the ampacity limit: every wire gauge has a maximum safe current rating based on how much heat its insulation can handle before it degrades or becomes a fire risk. This calculator applies both limits and returns the larger gauge so you are always on the safe side of each constraint.
The voltage-drop formula
For DC and single-phase AC circuits the formula is: A (m^2) = 2 * rho * L * I / V_drop, where rho is the resistivity of the conductor in ohm-metres (1.72e-8 for copper, 2.65e-8 for aluminum), L is the one-way wire length in metres, I is the current in amperes, and V_drop is the allowable voltage drop in volts. The factor of 2 accounts for the round trip through both the supply and return conductors. For three-phase circuits the factor of 2 is replaced by the square root of 3, approximately 1.732, because the three phases partially cancel. This gives the minimum cross-sectional area in square metres, which is then converted to mm^2 and matched to the nearest standard AWG that meets or exceeds it.
Copper vs. aluminum wiring
Copper is the standard choice for household wiring because its resistivity is about 35% lower than aluminum, meaning a smaller conductor achieves the same resistance. Aluminum is lighter and cheaper per kilogram of metal, making it common for large service entrance conductors, overhead utility spans, and industrial feeders where the weight and cost savings are significant. Because aluminum expands and contracts more with temperature and forms an insulating oxide layer, aluminum connections require anti-oxidant compound, AL-rated lugs and connectors, and periodic torque checks - these requirements make aluminum unsuitable for most branch-circuit work without specialist attention. When the material is set to copper or aluminum, this calculator also checks the NEC ampacity table and sizes up if the ampacity limit is binding.
AWG numbering, diameter, and area
American Wire Gauge numbers run backwards: the smaller the number, the larger the wire. AWG 4/0 (written "0000") is the largest size in the standard table at 11.68 mm diameter and 107.2 mm^2 cross-section; AWG 40 at the other extreme is hair-thin at 0.08 mm. Each step of three AWG numbers approximately halves the cross-sectional area, and every 6 AWG steps halves the diameter. The diameter formula is D (inches) = 0.005 * 92^((36 - AWG) / 39) for integer AWG values. Above 4/0 the industry switches to kcmil (thousands of circular mils) sizes: 250, 350, 500, 750 and 1000 kcmil are common; very large feeders are also specified in square millimetres under IEC standards.
NEC AWG Ampacity Quick Reference
| AWG | Diameter (mm) | Area (mm^2) | Copper 60 C (A) | Copper 75 C (A) | Copper 90 C (A) | Aluminum 75 C (A) |
|---|---|---|---|---|---|---|
| 4/0 | 11.684 | 107.2 | 230 | 260 | 280 | 205 |
| 3/0 | 10.404 | 85 | 200 | 225 | 245 | 175 |
| 2/0 | 9.266 | 67.4 | 175 | 195 | 215 | 150 |
| 1/0 | 8.252 | 53.5 | 150 | 170 | 195 | 135 |
| 1 | 7.348 | 42.4 | 130 | 150 | 170 | 115 |
| 2 | 6.544 | 33.6 | 115 | 130 | 145 | 100 |
| 3 | 5.827 | 26.7 | 100 | 110 | 125 | 85 |
| 4 | 5.189 | 21.2 | 85 | 95 | 110 | 75 |
| 6 | 4.115 | 13.3 | 65 | 75 | 90 | 60 |
| 8 | 3.264 | 8.37 | 40 | 50 | 55 | 40 |
| 10 | 2.588 | 5.26 | 30 | 35 | 40 | 30 |
| 12 | 2.053 | 3.31 | 20 | 25 | 30 | 20 |
| 14 | 1.628 | 2.08 | 15 | 20 | 25 | - |
| 16 | 1.291 | 1.31 | 13 | 13 | 18 | - |
| 18 | 1.024 | 0.823 | 10 | 10 | 14 | - |
NEC Table 310.15(B)(16) ampacities for copper conductors in conduit (not more than three current-carrying conductors, 30 C ambient). Ampacity 0 means that gauge is not listed in NEC for that insulation class.
Frequently asked questions
What wire gauge do I need for a 20-amp circuit?
A 20-amp 120 V branch circuit in the United States requires at minimum AWG 12 copper wire with 60 C or 75 C insulation. At 75 C the NEC ampacity for AWG 12 copper is 25 A, giving 5 A of headroom above the 20 A breaker. However, the actual gauge you need also depends on the circuit length: a long run can require a step up to AWG 10 or larger to keep the voltage drop within the 3% NEC guideline. Use this calculator with your actual run length to confirm.
What is the difference between ampacity and the voltage-drop result?
Ampacity is the maximum continuous current a wire can carry without overheating its insulation, as defined by the NEC based on conductor material, cross-section, insulation type, and installation conditions. Voltage drop is a separate electrical performance criterion: a wire may safely carry the current without overheating but still drop enough voltage over a long run to impair the equipment at the end of the circuit. Both limits must be satisfied. For short runs, ampacity usually governs; for long runs at lower currents, voltage drop often requires a larger gauge.
Why does the three-phase formula use sqrt(3) instead of 2?
In a balanced three-phase circuit the return current is shared among all three phases, so the effective round-trip factor is the square root of 3 (approximately 1.732) rather than 2. This means three-phase wiring is inherently more efficient than single-phase for the same conductor cross-section, which is one of the main reasons large industrial loads use three-phase power.
How much voltage drop is acceptable?
The NEC recommends no more than 3% voltage drop on any single branch circuit and no more than 5% total drop from the service entrance through any branch circuit (feeders plus branch). These are recommendations, not absolute code requirements in all jurisdictions, but they are widely adopted as best practice. Low-voltage DC systems such as 12 V solar or automotive circuits are often designed to 2% or even 1% because the absolute voltage lost is much more significant relative to the supply voltage.
Can I use aluminum wire for branch circuits?
In most jurisdictions you can legally use aluminum for branch circuits if you use AL-rated devices, anti-oxidant compound at all terminations, and check for aluminum-compatible ratings on outlets and switches. In practice, most electricians avoid aluminum for 15 and 20 A branch circuits because the connection maintenance requirements and the history of early solid-aluminum wiring failures make it a liability. Aluminum is standard and uncontroversial for service entrance conductors of AWG 1/0 and larger.
What is AWG and how does it relate to mm^2?
AWG stands for American Wire Gauge, a standardized system for specifying conductor diameters. Smaller AWG numbers mean thicker wire. The IEC standard used in most of the world specifies wire by cross-sectional area in square millimetres instead. Common conversions: AWG 14 is approximately 2.1 mm^2, AWG 12 is about 3.3 mm^2, AWG 10 is about 5.3 mm^2, AWG 8 is about 8.4 mm^2, and AWG 6 is about 13.3 mm^2. This calculator shows both to help with international wiring projects.