Wire Size Calculator — Ampacity and Voltage Drop

A wire has to pass two checks. It must carry the current without overheating — its ampacity. And it must deliver enough voltage at the far end — its voltage drop. On short runs, ampacity usually decides the size. On long runs, voltage drop often forces a larger wire than ampacity alone would need. This wire size calculator runs both checks. Ampacity comes from NEC Table 310.16 (copper and aluminum, 60/75/90°C columns, 30°C ambient, no more than three current-carrying conductors). It applies the NEC 240.4(D) limits on 14, 12 and 10 AWG. Voltage drop uses the DC resistance values in NEC Chapter 9, Table 8, for the round trip. The recommendation is the larger of the two sizes. The temperature column matters. Under NEC 110.14(C), circuits of 100 A or less (or 14–1 AWG conductors) are limited to the 60°C column unless the equipment terminals are listed for 75°C. Larger circuits default to 75°C. The calculator follows that default unless you choose otherwise.

How to Calculate Wire Size

1. Start with the load current. For continuous loads (3 hours or more), multiply by 1.25. 2. Pick the temperature column your terminations allow: usually 60°C up to 100 A, 75°C above. 3. Find the smallest conductor in NEC Table 310.16 whose ampacity covers the design current. For 14–10 AWG, also respect the 240.4(D) overcurrent limits. 4. Calculate voltage drop at the load current: 2 × I × R × L ÷ 1000 for single-phase/DC, or √3 × I × R × L ÷ 1000 for three-phase. 5. If the drop exceeds your limit (3% is a common target), step up in size until it doesn't. Use the larger of the two results.

Formula

Design current   I_d = I × 1.25 (continuous) or I
Ampacity check   Table 310.16 ampacity(size, material, °C) ≥ I_d
                 and, for 14–10 AWG, 240.4(D) limit ≥ I_d

Voltage drop     VD = k × I × R × L ÷ 1000
                 k = 2 (single-phase, DC) or √3 (three-phase)
                 R = Ω per 1,000 ft (Chapter 9 Table 8, 75°C)
                 L = one-way length (ft)
VD %             = VD ÷ V × 100 ≤ limit

Recommended size = larger of (ampacity size, voltage-drop size)

Wire Size Examples

Ampacity (60°C): 12 AWG = 20 A ✓ (240.4(D) limit 20 A)
Drop on 12 AWG: 2 × 20 × 1.98 × 40 ÷ 1000 = 3.17 V = 2.6% ✓
Result: 12 AWG copper

12 AWG drop: 7.92 V = 6.6% ✗
10 AWG drop: 4.96 V = 4.1% ✗
 8 AWG drop: 3.11 V = 2.6% ✓
Result: voltage drop governs → 8 AWG copper

Design current: 40 × 1.25 = 50 A
Ampacity (75°C): 8 AWG = 50 A ✓
Drop on 8 AWG: 2 × 40 × 0.778 × 30 ÷ 1000 = 1.87 V = 0.78% ✓
Result: 8 AWG copper — if the charger and breaker terminals are listed for 75°C
(At 60°C the answer would be 6 AWG: 55 A, drop 0.49%)

Wire Sizing Notes

• › Check the temperature rating printed on the breaker and the equipment terminals. It, not the wire's insulation rating, usually sets the usable ampacity column.
• › Long runs to outbuildings, wells and EV chargers are where voltage drop bites. Check the run length before buying wire.
• › More than three current-carrying conductors in one raceway, or hot locations such as attics and rooftops, need adjustment factors this calculator does not apply. The wire must then be larger.
• › Need the current first? Convert watts, kW or horsepower with the amp calculator.

Ampacity and Resistance Table (NEC 310.16 and Chapter 9 Table 8)

Ampacity in amps, 30°C ambient, ≤3 current-carrying conductors. * Overcurrent protection limited by NEC 240.4(D): 14 Cu 15 A, 12 Cu 20 A, 10 Cu 30 A, 12 Al 15 A, 10 Al 25 A. Resistance: DC, 75°C, stranded, ohms per 1,000 ft.

Frequently Asked Questions