Wire Gauge Calculator
Size AWG copper wire by load, distance and voltage drop - based on the NEC
Usage Notes & Disclaimer
- This calculator is for reference/estimating only and is not professional electrical design
- Actual wiring must comply with the NEC (NFPA 70), local amendments, and be done or inspected by a licensed electrician
- Always use the actual nameplate rating of the equipment; values vary by brand
- This site assumes no liability for the accuracy of results or consequences of use
Recommended Wire Gauge
💡 Practical Recommendation
Recommended actual size:
AWG 10
Upsize one gauge for ~30% headroom for future loads
Voltage Drop Analysis
Input Parameters
Advanced (voltage drop / wiring / temperature / conductor count / continuous load)
Shortcut: Ctrl+S Save
Wire Ampacity Chart (NEC 310.16)
Data source: NEC Table 310.16, copper conductors. Ampacity shown for 60°C / 75°C / 90°C insulation at 30°C (86°F) ambient, ≤3 current-carrying conductors. Sizing here uses the 75°C column; small conductors (14/12/10 AWG) are limited by NEC 240.4(D) to 15/20/30 A breakers.
| AWG | 60°C (A) | 75°C (A) | 90°C (A) | Circular Mils | Notes |
|---|---|---|---|---|---|
| 14 | 15 | 20 | 25 | 4,110 | 15A max breaker |
| 12 | 20 | 25 | 30 | 6,530 | 20A max breaker |
| 10 | 30 | 35 | 40 | 10,380 | 30A max breaker |
| 8 | 40 | 50 | 55 | 16,510 | — |
| 6 | 55 | 65 | 75 | 26,240 | — |
| 4 | 70 | 85 | 95 | 41,740 | — |
| 3 | 85 | 100 | 115 | 52,620 | — |
| 2 | 95 | 115 | 130 | 66,360 | — |
| 1 | 110 | 130 | 145 | 83,690 | — |
| 1/0 | 125 | 150 | 170 | 105,600 | — |
| 2/0 | 145 | 175 | 195 | 133,100 | — |
| 3/0 | 165 | 200 | 225 | 167,800 | — |
| 4/0 | 195 | 230 | 260 | 211,600 | — |
| 250 kcmil | 215 | 255 | 290 | 250,000 | — |
| 300 kcmil | 240 | 285 | 320 | 300,000 | — |
| 350 kcmil | 260 | 310 | 350 | 350,000 | — |
EMT Conduit Fill Reference (NEC Chapter 9)
Maximum copper THHN/THWN-2 conductors per EMT trade size at 40% fill (NEC Chapter 9, Tables 1, 4 & 5). For 3 or more conductors the fill limit is 40%.
| EMT Trade Size | 40% Fill Area (in²) | #12 THHN | #10 THHN | #8 THHN | #6 THHN |
|---|---|---|---|---|---|
| 1/2" | 0.122 | 9 | 5 | 3 | 2 |
| 3/4" | 0.213 | 16 | 10 | 5 | 4 |
| 1" | 0.346 | 26 | 16 | 9 | 6 |
| 1-1/4" | 0.598 | 44 | 28 | 16 | 11 |
| 1-1/2" | 0.814 | 61 | 38 | 22 | 16 |
| 2" | 1.342 | 100 | 63 | 36 | 26 |
Tip: These are code maximums at 40% fill. Upsize the conduit one trade size for easier pulling and future adds.
⚠️ Safety Reminders
- Reference data only. Actual installation must comply with the current NEC and local code
- All electrical work should be performed by a licensed electrician
- Ambient temperature, wiring method, and conductor count all affect ampacity
- Reserve a 20-30% safety margin
About / How to use / Formulas / FAQ / Example
About This Wire Gauge Calculator
What is Wire Gauge Sizing?
Wire gauge sizing determines the correct copper conductor (AWG) for a circuit based on the load current, the run length, and the allowable voltage drop. Sizing to the NEC (NFPA 70) keeps the conductor within its safe ampacity so it does not overheat, and keeps voltage drop low enough that equipment runs properly - while avoiding the cost of oversized wire.
How to Use This Calculator
- Enter the load current: read the nameplate, or calculate it from power ÷ voltage (I = P / V)
- Enter the wire length: the one-way distance from the panel to the load, in feet
- Select the voltage: 120V or 240V single-phase, or 208V / 480V three-phase
- Set the allowable voltage drop: NEC recommends ≤3% on a branch circuit, ≤5% total
- Choose the wiring method: raceway/cable per NEC Table 310.16
- Set the ambient temperature: hot locations derate ampacity
- Read the recommended AWG, breaker size, and EMT conduit
Calculation Formulas
1. Voltage Drop (NEC Chapter 9 method)
Single Phase: VD = (2 × K × I × L) / CM
Three Phase: VD = (1.732 × K × I × L) / CM
Where K = 12.9 (copper resistivity constant, Ω·cmil/ft at 75°C), I = current (A), L = one-way length (ft), CM = conductor circular mils. This resistance-only method ignores conductor reactance and power factor; for long runs, large conductors, or low-PF/inductive loads (motors, compressors), the real voltage drop is higher - use the full impedance method. Ampacity follows NEC Table 310.16, breakers NEC 240.6 / 240.4(D), and conduit fill NEC Chapter 9.
2. Voltage Drop Percentage
Voltage Drop (%) = (VD / nominal voltage) × 100%
3. Power Loss
Power Loss (W) = I² × R (round trip)
FAQ
Q1: Why can't voltage drop be too large?
Excessive voltage drop causes: (1) low voltage at the equipment, so it may not run correctly (2) dim lighting (3) hard motor starting or overheating (4) wasted energy as heat. The NEC recommends keeping voltage drop to ≤3% on a branch circuit and ≤5% for branch + feeder combined (NEC 210.19 and 215.2 informational notes).
Q2: What is ampacity?
Ampacity is the maximum current a conductor can carry continuously without exceeding the temperature rating of its insulation. Exceeding it overheats the wire, degrades the insulation, and can start a fire. Ampacity comes from NEC Table 310.16 and is derated for ambient temperature (310.15(B)(1)) and for more than three current-carrying conductors (310.15(C)(1)).
Q3: What do AWG numbers mean?
AWG (American Wire Gauge) is the US standard for wire size. The scale is inverse: a smaller AWG number means a thicker (higher-ampacity) wire. Below AWG 1 the sizes continue as 1/0, 2/0, 3/0, 4/0, then kcmil (thousands of circular mils). Example: 14 AWG (15A) < 12 AWG (20A) < 10 AWG (30A) < 8 AWG (50A).
Q4: Why is temperature correction needed?
Table 310.16 ampacity is based on a 30°C (86°F) ambient. When the surroundings are hotter, the conductor cannot shed heat as well, so its allowable current is reduced (derated). For example, at 40°C (104°F) the 75°C-column factor is 0.88, so ampacity drops to about 88% of the table value.
Q5: What matters during the actual installation?
1. Follow the current NEC and any local amendments 2. Use a licensed electrician 3. Confirm the equipment nameplate rating 4. Account for continuous loads at 125% 5. Derate for high ambient temperature and for more than three conductors in a raceway 6. Verify terminal temperature ratings (60°C vs 75°C).
Usage Example
Example: 240V Appliance Circuit
- Load: 30 A appliance / subpanel feeder
- Distance: 80 ft from the panel
- Voltage: 240V single-phase
- Allowable voltage drop: 3%
- Wiring: copper in EMT, 3 conductors
- Result: 10 AWG copper, actual voltage drop ~2.5%, 30 A breaker, 1/2" EMT - meets NEC ampacity and voltage-drop limits with margin.
