Understanding Wire Sizing
Proper wire sizing ensures safety and efficiency. Undersized wires can overheat, while oversized wires waste money. Consider both ampacity and voltage drop.
Wire Size Calculator
Our wire size calculator helps electricians, engineers, and DIY enthusiasts select the appropriate wire gauge for any electrical project. Enter your circuit parameters to get recommended wire sizes that meet electrical code requirements while minimizing voltage drop and power loss.
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Updated: November 2025
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Wire Size Calculator calculator
A
m
Recommended Wire Size
10
AWG
5.26 mm² cross-section
Wire Specifications
Voltage Drop
2.5%
Actual Drop
6.0 V
Wire Resistance
0.15 Ω
Ampacity
30 A
Power Loss
60 W
Common AWG Sizes
14
15A
12
20A
10
30A
8
40A
cable AWG Ampacity (Copper)
14 AWG
15A
12 AWG
20A
10 AWG
30A
8 AWG
40A
6 AWG
55A
warning NEC Voltage Drop
- 📏 Branch circuits: Max 3%
- 🔌 Feeder + Branch: Max 5%
- 🔧 Aluminum needs 1-2 sizes larger
How to Use the Wire Size Calculator
1
Enter Current
Input the maximum current (amps) your circuit will carry
2
Select Voltage
Choose your system voltage (120V, 240V, etc.)
3
Enter Distance
Input one-way distance from panel to load
4
Choose Material
Select copper or aluminum conductor
5
Review Results
See recommended wire size and voltage drop analysis
The Formula
Wire size is determined by ampacity (current carrying capacity) and acceptable voltage drop. NEC recommends max 3% voltage drop for branch circuits and 5% total for feeder + branch combined.
Voltage Drop = (2 × L × I × R) / 1000
lightbulb Variables Explained
- L One-way wire length in meters
- I Current in amperes
- R Wire resistance in Ω/km
tips_and_updates Pro Tips
1
Always size wire for both ampacity AND voltage drop requirements
2
Use the larger wire size if ampacity and voltage drop calculations differ
3
Copper has better conductivity but aluminum is lighter and cheaper for large runs
4
Derate ampacity for bundled wires or high ambient temperatures
5
NEC allows max 3% voltage drop for branch circuits
6
For long distances, consider increasing wire size to reduce power loss
7
Always follow local electrical codes and permit requirements
Selecting the correct wire gauge is one of the most critical decisions in electrical installation — undersized wire overheats, risks fire, and violates the National Electrical Code (NEC), while oversized wire wastes copper and increases material costs. The American Wire Gauge (AWG) system, used throughout North America, assigns smaller numbers to larger wire diameters: 14 AWG (1.628 mm diameter) is rated for 15-amp circuits, 12 AWG for 20 amps, 10 AWG for 30 amps, and 8 AWG for 40-50 amps at standard 75°C insulation ratings per NEC Table 310.16. Wire sizing must account for three factors: ampacity (current-carrying capacity), voltage drop over distance, and ambient temperature derating. NEC Article 210 limits voltage drop to 3% on branch circuits and 5% total from service entrance to the farthest outlet. For a 20-amp, 120-volt circuit running 100 feet, 12 AWG wire produces approximately 3.2% voltage drop — borderline acceptable — while upgrading to 10 AWG reduces it to about 2%. Long runs for outbuildings, well pumps, and EV chargers frequently require upsizing one or two gauges beyond the minimum ampacity rating to stay within voltage drop limits. Conduit fill, bundling derating, and continuous load factors (requiring 125% ampacity for loads running over 3 hours) further influence the final wire size selection.
AWG Wire Gauge Chart
Common sizes: 14 AWG (15A), 12 AWG (20A), 10 AWG (30A), 8 AWG (40A), 6 AWG (55A). Aluminum requires 1-2 sizes larger than copper for the same ampacity.
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Data sourced from trusted institutions
All formulas verified against official standards.