Wire Gauge Selection Guide
// EVERYTHING YOU NEED TO KNOW ABOUT CHOOSING THE CORRECT WIRE GAUGE //
3-Step Wire Selection
Calculate Total Amperage
Add up all device currents on the circuit, include 125% safety margin for continuous loads
Measure Wire Run Length
Measure total distance from power source to load (include return path)
Use Our Calculator
Enter your values in our wire gauge calculator for instant recommendations
Understanding AWG System
The American Wire Gauge (AWG) system uses inverse numbering: smaller numbers = larger wires = higher capacity.
Common Applications:
- AWG 14 - 15A circuits, lighting (2400W @ 120V)
- AWG 12 - 20A circuits, outlets (2400W @ 120V)
- AWG 10 - 30A circuits, AC units (3600W @ 120V)
- AWG 8 - 40-50A, electric ranges
- AWG 6 - 55-65A, EV chargers, sub-panels
Voltage Drop Considerations
Voltage drop is the reduction in voltage along a wire due to resistance. NEC recommends ≤3% drop for branch circuits.
Why It Matters:
- Excessive drop causes motors to overheat
- Lights dim, especially during startups
- Reduced efficiency and wasted energy
- Shortened equipment lifespan
Solution: Use larger wire gauge for long runs (>50ft)
Ampacity & Temperature Ratings
Ampacity is the maximum current a wire can safely carry without overheating based on its insulation rating.
Insulation Types:
Copper vs Aluminum Wire
COPPER
- • Lower resistance
- • More flexible
- • Better conductivity
- • Smaller gauge needed
- • Residential standard
ALUMINUM
- • Lower cost
- • Lighter weight
- • Good for long runs
- • Service entrance
- • Large feeders
Note: Aluminum requires 2 gauge sizes larger than copper for same ampacity
Common Wire Sizing Mistakes to Avoid
✗DON'T:
- Ignore voltage drop on long runs
- Use undersized wire to save money
- Forget the 80% continuous load rule
- Mix copper and aluminum without proper connectors
- Exceed breaker size ratings
✓DO:
- Calculate based on actual load + safety margin
- Consider future expansion needs
- Follow NEC Table 310.16 for ampacity
- Use our calculator for accurate sizing
- Consult electrician for complex installations
Real-World Example: Sizing Wire for a Workshop
Scenario:
Installing a 240V, 30A circuit for a workshop 100 feet from the main panel.
Check NEC Ampacity Table
AWG 10 copper is rated for 30A at 60°C (NEC 310.16)
Calculate Voltage Drop
AWG 10 resistance: 1.0Ω/1000ft | Drop = (1.0 × 200ft × 30A) / 1000 = 6V | Percentage = (6V / 240V) × 100 = 2.5%
RESULT: AWG 10 Copper Wire
Meets both ampacity and voltage drop requirements. Use THHN/THWN-2 insulation rated for 90°C.
NEC Wire Sizing Requirements
The National Electrical Code (NEC) establishes minimum requirements for safe electrical installations. Understanding these requirements is essential for code-compliant wire sizing that passes inspection and ensures safety.
NEC Table 310.16
The primary reference for conductor ampacity. Lists maximum current for copper and aluminum conductors at different temperature ratings (60°C, 75°C, 90°C) based on 30°C ambient temperature.
| AWG | 60°C | 75°C | 90°C |
|---|---|---|---|
| 14 | 15A | 20A | 25A |
| 12 | 20A | 25A | 30A |
| 10 | 30A | 35A | 40A |
| 8 | 40A | 50A | 55A |
NEC 240.4(D) Overcurrent Protection
Small conductors must be protected at specific maximum breaker sizes, regardless of calculated ampacity. These are strict limits that cannot be exceeded.
NEC Voltage Drop Recommendations
While not mandatory requirements, the NEC provides informational notes recommending voltage drop limits for optimal equipment operation and energy efficiency.
3%
Branch Circuits
2%
Feeders
5%
Total Combined
When and How to Derate Wire Ampacity
Wire ampacity must be reduced (derated) under certain installation conditions. Failing to apply proper derating factors can result in overheated wires, insulation damage, and fire hazards.
Ambient Temperature Correction
NEC Table 310.16 is based on 30°C (86°F) ambient temperature. For higher temperatures, apply correction factors from NEC Table 310.15(B)(1).
| Ambient (°C) | 60°C Wire | 75°C Wire | 90°C Wire |
|---|---|---|---|
| 31-35 | 0.91 | 0.94 | 0.96 |
| 36-40 | 0.82 | 0.88 | 0.91 |
| 41-45 | 0.71 | 0.82 | 0.87 |
| 46-50 | 0.58 | 0.75 | 0.82 |
Conduit Fill Adjustment
When multiple current-carrying conductors share a raceway, heat buildup requires ampacity reduction per NEC Table 310.15(C)(1).
| Conductors | Adjustment Factor |
|---|---|
| 1-3 | 100% |
| 4-6 | 80% |
| 7-9 | 70% |
| 10-20 | 50% |
| 21-30 | 45% |
Example: Combined Derating
Scenario: 10 AWG THHN (90°C) in conduit with 6 conductors at 40°C ambient
Base ampacity: 40A (90°C column)
Temperature factor: 0.91
Conduit factor: 0.80
Adjusted ampacity: 40 × 0.91 × 0.80 = 29.1A
Wire Sizing by Application Type
Different applications have unique considerations beyond basic ampacity and voltage drop calculations. Understanding these specific requirements helps ensure optimal performance and code compliance.
Residential Circuits
- • 15A circuits: 14 AWG minimum
- • 20A circuits: 12 AWG minimum
- • Kitchen/bathroom: 20A GFCI required
- • Laundry: Dedicated 20A circuit
- • Consider future load growth
Industrial/Commercial
- • Motor circuits: Size for FLA + 25%
- • HVAC: Check manufacturer specs
- • 3-phase: Different calculations apply
- • Demand factors may reduce size
- • Consider harmonic loads
EV Charging
- • Level 2: 40-80A typical
- • 100% continuous load rating
- • 6 AWG for 50A, 4 AWG for 60A
- • Plan for longer cable runs
- • Consider load management
Solar PV Systems
- • DC circuits: Different ampacity rules
- • String sizing affects wire size
- • Conduit in sun: Temperature derating
- • PV wire vs THHN requirements
- • NEC Article 690 compliance
Shop/Garage
- • Welder: Check duty cycle rating
- • Air compressor: Motor starting current
- • Size for largest single load
- • Sub-panel may be needed
- • Voltage drop critical for motors
Low Voltage/Data
- • 12V systems: Voltage drop critical
- • Landscape lighting: 12-16 AWG typical
- • Speaker wire: 14-16 AWG for distance
- • POE: Cat6 cable ratings
- • DC power: Lower voltage = larger wire
Understanding Voltage Drop Calculations
Voltage drop is the reduction in voltage along a conductor due to its inherent resistance. For long wire runs, voltage drop often becomes the determining factor in wire size selection, requiring larger wire than ampacity alone would suggest.
Single-Phase Voltage Drop Formula
K = K = Resistivity constant (12.9 for Cu, 21.2 for Al)
I = I = Current in amperes
D = D = One-way distance in feet
CM = CM = Circular mils of conductor
The factor of 2 accounts for current traveling both ways (to load and back).
Three-Phase Voltage Drop Formula
1.732 = √3 (three-phase factor)
K = Resistivity constant
I = Line current in amperes
D = One-way distance in feet
Three-phase systems have lower voltage drop for the same power due to the √3 factor.
When Voltage Drop Matters Most
HIGH IMPACT SCENARIOS:
- • Long runs (>50ft for 120V, >100ft for 240V)
- • Motor loads (sensitive to voltage)
- • Low voltage systems (12V, 24V)
- • High current circuits
- • Sensitive electronic equipment
LOWER IMPACT SCENARIOS:
- • Short runs under 25 feet
- • Higher voltage systems (480V+)
- • Resistive loads (heaters, lights)
- • Modern switching power supplies
- • Low current applications
Wire Selection Decision Process
Follow this systematic approach to ensure you select the correct wire size for any application. Each step builds on the previous to arrive at a safe, code-compliant, and efficient installation.
Determine Load Requirements
Calculate total connected load in amperes. For continuous loads (3+ hours), multiply by 1.25. For motor loads, use full-load amperage (FLA) from nameplate, not running amps.
Size for Ampacity (NEC 310.16)
Select wire size based on temperature rating of insulation and terminals. Most residential uses 75°C column. This is your minimum wire size based on current.
Apply Derating Factors
Reduce ampacity for ambient temperature above 30°C and for more than 3 conductors in a raceway. If derated ampacity is below required load, size up.
Calculate Voltage Drop
For runs over 50 feet, verify voltage drop is within 3% for branch circuits. If drop exceeds limit, increase wire size until compliant.
Select Final Wire Size
Choose the LARGER of: ampacity-based size (after derating) OR voltage drop-based size. This ensures both safety and performance requirements are met.
Wire Sizing Calculators & Tools
Use our suite of electrical calculators to simplify wire sizing decisions. Each tool handles specific calculations automatically while following NEC guidelines.
READY TO SIZE YOUR WIRE?
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