Wire Size Calculator | Free Construction Calculators
Calculate the correct wire size (AWG) for electrical circuits based on amperage, distance, and voltage drop. Our professional wire gauge calculator helps electricians, contractors, and DIY enthusiasts select proper conductor size for safe, code-compliant electrical installations. Enter current load, wire run length, and system voltage (120V, 240V, 480V) to get instant wire size recommendations that meet NEC ampacity requirements and voltage drop limits. Essential for sizing wire for circuits, subpanels, generators, EV chargers, and long-distance power runs.
Proper wire sizing is critical for electrical safety and system performance. Undersized wire causes overheating, insulation breakdown, fire hazards, and NEC code violations, while oversized wire wastes money on unnecessary material costs. Wire size selection must satisfy two requirements: adequate ampacity to carry the load without overheating per NEC Table 310.16, and sufficient conductor cross-section to limit voltage drop within acceptable limits (3% for branch circuits, 2% for feeders). Our calculator evaluates both criteria simultaneously, recommending the minimum wire gauge that meets ampacity ratings and prevents excessive voltage loss across the circuit length.
Whether you're installing a new circuit, running power to a detached garage, sizing conductors for a subpanel, or troubleshooting voltage drop issues, accurate wire sizing ensures safe operation and optimal performance. Long wire runs require larger conductors to compensate for resistance, particularly for motor loads sensitive to voltage drop. Understanding the relationship between wire gauge (14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG), current capacity, conductor resistance, and circuit length allows proper wire selection that balances safety, performance, and cost. Our calculator simplifies this complex calculation, providing instant recommendations for copper and aluminum wire at various temperature ratings and voltages.
Dual Criteria Sizing
Calculates wire size based on both NEC ampacity requirements and voltage drop limits to ensure safe, efficient operation.
Copper & Aluminum Support
Provides accurate wire sizing for both copper and aluminum conductors with temperature-specific ampacity ratings.
Multiple Voltage Systems
Works with 120V, 240V, 208V, 277V, and 480V systems for residential, commercial, and industrial applications.
How to Use This Calculator
- Enter the current load in amps from your circuit calculations or equipment nameplate
- Input the one-way distance in feet from panel to load (calculator accounts for round trip)
- Select your system voltage (120V for outlets, 240V for appliances, 480V for industrial)
- Set the maximum voltage drop percentage (NEC recommends 3% for branch circuits, 2% for feeders)
- Choose wire temperature rating (75°C most common for residential THHN/THWN wire)
- Select wire material (copper standard for most applications, aluminum for service entrance)
- Choose phase type (single-phase residential, three-phase commercial/industrial)
- Click Calculate to see recommended wire size and verification of ampacity and voltage drop
Wire Sizing Formulas
Ampacity Requirement: Wire ampacity ≥ Load current (from NEC Table 310.16)
Voltage Drop (Single-Phase): VD = 2 × K × I × D ÷ CM
Voltage Drop (Three-Phase): VD = 1.732 × K × I × D ÷ CM
Where: K = resistance constant (12.9 copper, 21.2 aluminum), I = current (amps), D = distance (feet), CM = circular mils
Example: 30A load, 100 feet, 240V, 3% max drop
- Ampacity requirement: Minimum 10 AWG copper (35A rating at 75°C)
- Voltage drop with 10 AWG: 2 × 12.9 × 30 × 100 ÷ 10380 = 7.44V (3.1% - exceeds limit)
- Recommended size: 8 AWG copper (6.3V drop = 2.6% - acceptable)
- Final selection: 8 AWG copper wire for this circuit
About Wire Size Calculations
Wire sizing is one of the most critical aspects of electrical installation, directly affecting safety, equipment performance, energy efficiency, and code compliance. Proper wire gauge selection requires satisfying two independent criteria: adequate ampacity to carry the circuit load without exceeding conductor temperature ratings, and sufficient cross-sectional area to limit voltage drop within acceptable limits for the wire run distance. NEC Table 310.16 specifies minimum wire sizes based on ampacity: 14 AWG for 15A circuits, 12 AWG for 20A circuits, 10 AWG for 30A circuits, 8 AWG for 40A circuits, and 6 AWG for 55A circuits (copper at 75°C). These ampacity ratings prevent conductor overheating that degrades insulation, creates fire hazards, and violates electrical codes. However, ampacity alone is insufficient for long wire runs where resistance causes voltage drop that affects equipment operation.
Voltage drop becomes the controlling factor for wire sizing when circuit length exceeds typical residential distances (50-100 feet). The National Electrical Code recommends limiting voltage drop to 3% maximum on branch circuits and 2% on feeders, with 5% total combined limit, to prevent motors from overheating, lights from dimming, and electronics from malfunctioning. A 20A circuit using 12 AWG wire at 100 feet experiences 3.2V drop (2.7% at 120V), which is acceptable. However, the same circuit at 200 feet suffers 6.4V drop (5.3%), requiring upsizing to 10 AWG to reduce drop to 2.0V (1.7%). Long runs to detached buildings, well pumps, or outdoor equipment frequently require wire sizes larger than ampacity alone would indicate. Motor loads are particularly sensitive to voltage drop - a 5% voltage reduction causes 10% reduction in motor torque and significant overheating.
Material selection between copper and aluminum significantly impacts wire sizing and installation economics. Copper offers superior conductivity, requiring smaller wire sizes for equivalent ampacity: 12 AWG copper equals 10 AWG aluminum for 20A circuits. Copper's smaller size means easier handling, smaller conduit requirements, and simpler terminations. However, aluminum costs 40-60% less than copper and weighs one-third as much, making it economical for large service entrance conductors and long feeder runs where labor savings offset larger conduit and termination costs. Aluminum requires special oxidation-inhibiting compound and proper termination techniques to prevent connection failures. Most residential wiring uses copper exclusively, while commercial and industrial installations strategically employ aluminum for large feeders and service conductors where its cost advantage overcomes handling complexities.
NEC Wire Ampacity Table (Copper, 75°C THHN/THWN)
- 14 AWG: 20A ampacity - Maximum 15A breaker (lighting, general outlets)
- 12 AWG: 25A ampacity - Maximum 20A breaker (kitchen, bathroom, appliances)
- 10 AWG: 35A ampacity - Maximum 30A breaker (water heaters, dryers, AC units)
- 8 AWG: 50A ampacity - Maximum 40A breaker (electric ranges, large appliances)
- 6 AWG: 65A ampacity - Maximum 50-60A breaker (subpanels, large loads)
- 4 AWG: 85A ampacity - Maximum 70-80A breaker (feeders, service conductors)
- 2 AWG: 115A ampacity - Maximum 100A breaker (main panels, large subpanels)
- 1/0 AWG: 150A ampacity - Maximum 125A breaker (service entrance conductors)
- Aluminum wire: Requires one size larger than copper for equivalent ampacity
Frequently Asked Questions
What wire size do I need for a 20-amp circuit?
For a 20-amp circuit, use 12 AWG copper wire minimum per NEC 240.4(D). This wire has 25A ampacity at 75°C, providing adequate margin for the 20A breaker. Never use 14 AWG wire (20A ampacity) with a 20A breaker - NEC requires wire ampacity to exceed breaker rating. For distances over 60 feet at full 20A load, consider upsizing to 10 AWG to reduce voltage drop below 3%. Kitchen, bathroom, and laundry circuits typically use 12 AWG with 20A breakers. Outdoor circuits or long runs to detached buildings may require 10 AWG even for 20A loads to maintain proper voltage.
How far can I run 12 AWG wire for a 20-amp circuit?
For a 20A circuit at 120V using 12 AWG copper, you can run approximately 60 feet before voltage drop exceeds 3% (3.6V) at full load. At 240V, the same wire can run 120 feet while maintaining 3% voltage drop. These distances assume continuous 20A load. For intermittent loads or circuits that rarely draw maximum current (most residential circuits), longer runs are acceptable. To extend distance while maintaining low voltage drop, upsize to 10 AWG (extends to 95 feet at 120V, 190 feet at 240V). Always calculate voltage drop for actual load and distance - our calculator determines exact wire size needed for your specific application.
Is 3% voltage drop a code requirement or recommendation?
The 3% voltage drop limit is an NEC recommendation, not a mandatory code requirement. NEC Fine Print Note 215.2(A)(3) recommends 3% maximum on branch circuits and 2% on feeders (5% combined total) for proper equipment operation and energy efficiency. While not legally required, exceeding these limits causes motors to overheat and fail prematurely, lights to dim noticeably, electronics to malfunction, and appliances to perform poorly. Many local jurisdictions adopt the 3% recommendation as mandatory code, and professional electricians follow it to prevent equipment problems and callbacks. Critical motor loads (well pumps, HVAC compressors, machine tools) should be limited to 2% or less voltage drop for optimal performance and longevity.
Can I use aluminum wire instead of copper to save money?
Aluminum wire is acceptable for many applications and can reduce material costs 40-60%, but requires larger sizes and special installation techniques. Aluminum conductivity is 61% of copper, requiring one wire size larger: use 10 AWG aluminum where 12 AWG copper would suffice. Aluminum requires special anti-oxidant compound at all terminations, compatible connectors rated for aluminum (CO/ALR switches and receptacles), and proper torque to prevent thermal cycling failures that caused house fires with old aluminum wiring in the 1960s-70s. Modern aluminum alloy (AA-8000) is safe when properly installed. It's commonly used for service entrance conductors, large feeders, and overhead runs where weight savings matter. Residential branch circuits almost universally use copper due to easier installation and superior reliability.
What happens if I use undersized wire?
Undersized wire creates multiple serious problems: conductor overheating exceeding insulation temperature ratings (degrades insulation leading to short circuits and fires), breaker nuisance tripping under normal loads, excessive voltage drop causing equipment malfunction and premature failure, NEC code violations preventing inspection approval, and potential liability for fires or equipment damage. A 30A circuit on 14 AWG wire (20A rating) experiences dangerous overheating that can ignite surrounding materials before the breaker trips. Even if wire ampacity matches breaker size, undersizing for voltage drop causes motors to overheat (5% voltage drop reduces motor torque 10% while increasing current 11%), lights to dim, and electronics to malfunction. Always size wire for BOTH ampacity and voltage drop requirements using proper calculations.
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Professional Wire Sizing Tips
- Size for both criteria: Wire must satisfy BOTH ampacity and voltage drop requirements - use larger size
- Account for continuous loads: Size wire for 125% of continuous loads (operating 3+ hours)
- Consider conduit fill: More than 3 current-carrying conductors require derating ampacity
- Plan for expansion: Oversizing one wire gauge provides flexibility for future load increases
- Use voltage drop tables: Pre-calculated tables speed up wire selection for common scenarios
- Verify equipment nameplate: Use actual full-load amperage from equipment ratings, not estimates
- Temperature derating: Ambient temperatures exceeding 86°F require ampacity reduction
- Check local amendments: Some jurisdictions require larger wire sizes than NEC minimums
Wire Ampacity Quick Reference (NEC Table 310.16)
| Wire Size | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 75°C |
|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A | - |
| 12 AWG | 20 A | 25 A | 30 A | 20 A |
| 10 AWG | 30 A | 35 A | 40 A | 30 A |
| 8 AWG | 40 A | 50 A | 55 A | 40 A |
| 6 AWG | 55 A | 65 A | 75 A | 50 A |
| 4 AWG | 70 A | 85 A | 95 A | 65 A |
| 2 AWG | 95 A | 115 A | 130 A | 90 A |
| 1/0 AWG | 125 A | 150 A | 170 A | 120 A |
| 2/0 AWG | 145 A | 175 A | 195 A | 135 A |
| 3/0 AWG | 165 A | 200 A | 225 A | 155 A |
| 4/0 AWG | 195 A | 230 A | 260 A | 180 A |
Note: Breaker size must not exceed conductor ampacity. Use 75°C column for most residential/commercial installations (THHN/THWN wire). Derating required for conduit fill >3 conductors or ambient temperature >86°F.