A
Please enter a valid current in amps.
Full-load current of the circuit
m
Please enter a valid length.
Distance from panel to load (one way)
Minimum Wire Size
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⚠️ Disclaimer: This calculator provides general guidance based on NEC 2023 Table 310.16 and standard voltage drop formulas. Actual wire sizing must comply with your local electrical code and be verified by a licensed electrician. Factors including ambient temperature, conduit fill, circuit type, and local amendments may require a larger conductor.
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Sources & Methodology
Ampacity from NEC 2023 Table 310.16 (NFPA 70). Voltage drop formula per NEC 210.19(A) commentary. Resistivity from NIST.
NFPA 70 (NEC 2023) — Table 310.16 Conductor Ampacity
Authoritative US source for conductor ampacity ratings for insulated conductors in conduit at 30°C ambient. NEC 210.19(A) and 215.2(A)(1) commentary recommends 3% maximum voltage drop on branch circuits.
NIST — Electrical Resistivity of Copper
Copper resistivity ρ = 1.724×10⁻⁸ Ω·m at 20°C. Aluminium ρ = 2.82×10⁻⁸ Ω·m. Used for calculating DC resistance per metre of each AWG size.
Methodology: Step 1: Size for ampacity — find smallest AWG whose NEC 310.16 ampacity ≥ (I × load factor). Step 2: Size for voltage drop — find smallest AWG whose resistance satisfies VD = 2×I×R×L ≤ (VD% × V / 100). Step 3: Use the larger (lower AWG number) of the two requirements. Resistivity: Cu = 1.724×10⁻⁸ Ω·m; Al = 2.82×10⁻⁸ Ω·m. AWG area: A = πd²/4 where d(mm) = 0.127×92ˆ((36−AWG)/39).
⏱ Last reviewed: April 2026
How to Choose the Right Wire Size
Wire sizing has two independent requirements: the wire must carry the current without overheating (ampacity), and it must not drop too much voltage over its length (voltage drop). Both must be checked; use whichever gives the larger wire.
Two-Step Wire Sizing Method
Step 1 (ampacity): ampacity ≥ I × 1.25 (continuous loads)
Step 2 (voltage drop): VD = 2 × I × R × L ≤ VD_max
Example: 20A continuous load, 240V, 20 m run, copper, 3% VD limit.
Step 1: Design current = 20 × 1.25 = 25A. AWG 10 (30A rated) needed for ampacity.
Step 2: VD_max = 0.03 × 240 = 7.2V. Need R ≤ 7.2 / (2 × 20 × 20) = 0.009 Ω/m = 9 mΩ/m.
AWG 10 R = 3.28 mΩ/m ✓. AWG 12 R = 5.21 mΩ/m ✓. AWG 14 R = 8.28 mΩ/m ✓.
Result: AWG 10 required (ampacity governs for this run).
Step 1: Design current = 20 × 1.25 = 25A. AWG 10 (30A rated) needed for ampacity.
Step 2: VD_max = 0.03 × 240 = 7.2V. Need R ≤ 7.2 / (2 × 20 × 20) = 0.009 Ω/m = 9 mΩ/m.
AWG 10 R = 3.28 mΩ/m ✓. AWG 12 R = 5.21 mΩ/m ✓. AWG 14 R = 8.28 mΩ/m ✓.
Result: AWG 10 required (ampacity governs for this run).
Quick Wire Size Guide (Copper, NEC 310.16)
| AWG | Ampacity (60°C) | Typical Use | Max Continuous |
|---|---|---|---|
| 14 | 15A | 15A lighting & outlet circuits | 12A |
| 12 | 20A | 20A kitchen, bathroom, laundry | 16A |
| 10 | 30A | 30A dryer, A/C circuits | 24A |
| 8 | 40A | 40A range, large A/C | 32A |
| 6 | 55A | 50A sub-panel, EV charger | 44A |
| 4 | 70A | 60–70A sub-panel feeder | 56A |
| 2 | 95A | 100A sub-panel, service | 76A |
| 1/0 | 125A | 100A service entrance | 100A |
| 2/0 | 145A | 150A service | 116A |
| 4/0 | 195A | 200A service entrance | 156A |
💡 Long-run tip: For runs over 30 m (100 ft), voltage drop often governs wire sizing. At 30 m on 120V with 15A load, a 3% drop limit requires AWG 10 or better, even though AWG 14 would be adequate for ampacity alone. Always check both criteria. The NEC 3% voltage drop rule is a recommendation, not a hard requirement, but good practice.
Frequently Asked Questions
Check both ampacity and voltage drop. For ampacity: the wire’s NEC rating must be ≥ load current × 1.25 (continuous loads). For voltage drop: VD = 2×I×R×L ≤ 3% of supply voltage. Use the larger wire (lower AWG) from both calculations. Always follow your local electrical code.
Minimum AWG 12 copper for 20A (NEC 240.4). For continuous 20A load: design current = 20×1.25 = 25A, requiring AWG 10 (rated 30A). For 15A: AWG 14. For 30A: AWG 10. For 40A: AWG 8. For 50A: AWG 6. Long runs may require a larger gauge to meet voltage drop limits.
VD = 2 × I × R × L. I = current (A), R = resistance per metre (mΩ/m) ÷ 1000, L = one-way length (m). For AWG 12 at 20A over 15 m: VD = 2 × 20 × 0.00521 × 15 = 3.13 V = 2.6% at 120V = 1.3% at 240V. NEC recommends ≤3% voltage drop on branch circuits.
NEC 210.19(A) recommends (not mandates) ≤3% on branch circuits. NEC 215.2(A)(1) recommends ≤3% on feeders. Combined total ≤5%. These are recommendations in NEC FPN (Fine Print Notes), not enforced violations. However, most jurisdictions follow them as best practice, and sensitive equipment or motors may need 1–2% limits.
NEC 210.19(A) requires that branch circuit conductors serving continuous loads (on >3 hours) be sized at 125% of the load, equivalent to the load being limited to 80% of the wire’s ampacity. For a 20A circuit with AWG 12: max continuous load = 20 × 0.80 = 16A. This is the same as the 125% derating rule: 16A × 1.25 = 20A design current.
Aluminium is permitted for feeders AWG 1 and larger and for service entrance. For branch circuits, aluminium is allowed at AWG 6 and larger with aluminium-rated (AL-CU) terminals. Aluminium requires approximately 2 AWG sizes larger than copper for the same ampacity: AWG 4 Al ≈ AWG 6 Cu for ampacity. Use AA-8000 series aluminium alloy conductors for any new aluminium wiring.
AWG 6 copper minimum for 50A at 60°C. For continuous loads at 50A: design current = 50 × 1.25 = 62.5A, requiring AWG 4 (rated 70A). Common 50A applications: large EV chargers (NEMA 14-50), ranges, and sub-panels. For aluminium: AWG 4 Al for 50A non-continuous. Always verify with your local electrical code.
AWG 10 copper minimum for 30A (NEC 422.11). Use a 30A breaker. For the dryer itself (non-continuous load per NEC convention for household appliances), AWG 10 rated 30A is correct. Wire as 10/3 (two hots plus ground, no neutral needed for newer 4-prong NEMA 14-30 installations per NEC 250.140). Aluminium equivalent: AWG 8.
NEC Table 310.15(B)(1) temperature correction: at 40°C ambient, multiply ampacity by 0.91. At 50°C: 0.82. At 60°C: 0.71. For AWG 12 (20A rated) in a 45°C attic: corrected ampacity = 20 × (0.91+0.82)/2 ≈ 17.3A. Hot locations like attics, boiler rooms, and rooftop conduit require using next larger wire size to maintain full ampacity.
AWG 3 copper or AWG 1 aluminium for a 100A feeder (NEC 310.16). AWG 3 Cu is rated 100A at 75°C in conduit. For 125% safety margin on a 100A continuous feeder: design current = 125A, requiring AWG 1 Cu (rated 130A at 75°C) or AWG 2/0 Al. Service entrance to a 100A panel typically uses AWG 2 Cu or AWG 1/0 Al.
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