Wire Size Calculator
Calculate the correct wire gauge (AWG) for your electrical installation based on current, distance, and allowable voltage drop. Proper wire sizing ensures safety and efficiency. See also our Voltage Drop Calculator and Ohm's Law Calculator.
How to Determine Wire Size
Selecting the correct wire size involves two primary considerations: ampacity (the maximum current a wire can safely carry without overheating) and voltage drop (the loss of voltage over distance due to wire resistance). The National Electrical Code (NEC) requires that voltage drop not exceed 3% for branch circuits and 5% for combined feeder and branch circuits.
Wire size is measured in American Wire Gauge (AWG), where smaller numbers indicate larger wire. The gauge system is logarithmic — every 3 gauge decrease doubles the cross-sectional area and halves the resistance. For example, 10 AWG has twice the area of 13 AWG and half the resistance per foot.
Wire Size Formula
Circular Mils Required:
CM = (2 × K × I × D) / Vdrop_allowed
Where:
K = resistivity constant (copper: 10.4, aluminum: 17.0)
I = current in amperes
D = one-way distance in feet
Vdrop_allowed = system voltage × (% drop / 100)
Voltage Drop:
Vdrop = 2 × I × R × D / 1000
Where R = resistance per 1000 feet (Ω/kft)
Example Calculation
Determine wire size for a 20A, 240V circuit running 100 feet with maximum 3% voltage drop using copper:
Allowable voltage drop = 240V × 3% = 7.2V
CM needed = (2 × 10.4 × 20 × 100) / 7.2 = 5,778 circular mils
Nearest AWG: 12 AWG (6,530 CM) — meets requirement
Actual drop with 12 AWG: 2 × 20 × 1.588 × 100/1000 = 6.35V (2.65%)
Also verify ampacity: 12 AWG copper rated for 20A (60°C) ✓
AWG Wire Size Reference Table
| AWG | Circ. Mils | Cu Ω/kft | Al Ω/kft | Amps (60°C) | Amps (75°C) |
|---|---|---|---|---|---|
| 14 | 4,107 | 2.525 | 4.148 | 15A | 20A |
| 12 | 6,530 | 1.588 | 2.607 | 20A | 25A |
| 10 | 10,380 | 0.999 | 1.64 | 30A | 35A |
| 8 | 16,510 | 0.628 | 1.03 | 40A | 50A |
| 6 | 26,240 | 0.395 | 0.648 | 55A | 65A |
| 4 | 41,740 | 0.249 | 0.408 | 70A | 85A |
| 3 | 52,620 | 0.197 | 0.323 | 85A | 100A |
| 2 | 66,360 | 0.156 | 0.256 | 95A | 115A |
| 1 | 83,690 | 0.124 | 0.203 | 110A | 130A |
| 1/0 | 105,600 | 0.098 | 0.161 | 125A | 150A |
| 2/0 | 133,100 | 0.078 | 0.128 | 145A | 175A |
| 3/0 | 167,800 | 0.062 | 0.101 | 165A | 200A |
| 4/0 | 211,600 | 0.049 | 0.08 | 195A | 230A |
Frequently Asked Questions
What wire size do I need for 20 amps?
For a 20-amp circuit, the NEC requires minimum 12 AWG copper wire for short runs. For longer distances, you may need 10 AWG or larger to keep voltage drop within 3%. Always check both ampacity and voltage drop requirements — the larger wire size wins.
What is the 80% rule for wire sizing?
The NEC 80% rule (Article 210.20) states that continuous loads (running 3+ hours) should not exceed 80% of the circuit's rated ampacity. A 20A breaker on 12 AWG wire should carry no more than 16A continuously. This provides a safety margin for heat buildup during extended operation.
Copper vs aluminum wire — which should I use?
Copper has lower resistance (better conductivity), is more durable, and easier to work with. Aluminum is lighter and cheaper but requires larger gauge for the same ampacity and needs special connectors to prevent oxidation and loose connections. Aluminum is common for service entrance cables and large feeders; copper is standard for branch circuits.
Why does distance matter for wire sizing?
Wire has resistance proportional to its length. Longer runs mean more resistance, which causes more voltage drop. A wire that is perfectly adequate for a 25-foot run may cause excessive voltage drop at 100 feet. The formula accounts for round-trip distance (×2) since current must travel to the load and back.
What happens if wire is too small?
Undersized wire causes excessive heat buildup (fire hazard), voltage drop (equipment malfunction), energy waste, and potential insulation damage. Motors may overheat or fail to start due to low voltage. The NEC wire sizing requirements exist specifically to prevent these dangerous conditions.
How do I account for conduit fill and ambient temperature?
When multiple current-carrying conductors share a conduit, ampacity must be derated (NEC Table 310.15(C)(1)): 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%. High ambient temperatures also require derating. These factors may require upsizing wire beyond the basic calculation. Always consult NEC tables for your specific installation conditions.
Safety Considerations
Wire sizing is a critical safety calculation. Undersized wires are a leading cause of electrical fires. Always follow the National Electrical Code (NEC) or your local electrical code, which may be more restrictive. Consider these additional factors: ambient temperature derating, conduit fill derating, continuous vs. intermittent loads, terminal temperature ratings, and future load growth. When in doubt, size up — the cost difference between wire gauges is minimal compared to the safety margin gained. All electrical work should be performed by or inspected by a licensed electrician.
Practical Applications
- Residential circuits: 14 AWG for 15A lighting, 12 AWG for 20A outlets, 10 AWG for 30A dryers
- Sub-panels: Size feeder wire for total sub-panel ampacity plus voltage drop over distance
- EV chargers: Level 2 chargers (40-50A) typically require 6 AWG copper
- Solar installations: Size wire for maximum panel current with voltage drop under 2%
- Well pumps: Long underground runs often require oversized wire for voltage drop
- Outbuildings: Detached garages and shops need careful wire sizing for the distance involved