This wire size calculator finds the smallest copper or aluminum conductor that keeps voltage drop within your limit and, for AWG sizes, carries the load current according to the NEC 75 °C ampacity table. Enter the current, voltage and distance and the recommended size appears instantly.
How the wire size is chosen
- Ampacity (AWG): the wire must carry the design current. For continuous loads (running 3 hours or more) the NEC requires conductors sized at 125% of the load (NEC 210.19(A)(1) and 215.2(A)(1)).
- Terminal temperature (NEC 110.14(C)): for circuits of 100 A or less, conductors are sized from the 60 °C ampacity column unless every termination is listed and marked 75 °C; above 100 A the 75 °C column is used. The calculator applies this automatically and also shows the 75 °C answer when it differs.
- Voltage drop: the calculator checks each size from smallest to largest and picks the first one whose drop is at or below your limit (3% by default).
- Parallel sets: for large feeders, several conductors per phase share the current. NEC 310.10(G) allows parallel conductors from 1/0 AWG and larger.
For mm² cables the calculator sizes for voltage drop only, because IEC current ratings depend heavily on the installation method (conduit, tray, buried, grouped). Check the result against IEC 60364-5-52 or BS 7671 Appendix 4 tables for your method of installation.
Worked examples
Example 1: 20 A, 120 V, 100 ft, 3% maximum
A 20 A circuit needs at least 12 AWG copper for ampacity. But the allowed drop is only 3% of 120 V = 3.6 V, so the maximum resistance is 3.6 ÷ (2 × 20 × 0.1) = 0.90 Ω per 1000 ft. That rules out 12 AWG (1.93) and 10 AWG (1.21); 8 AWG (0.764) gives 3.1 V (2.55%). If you accept a 5% drop, 10 AWG (4.0%) is enough.
Example 2: 30 A, 240 V single-phase, 150 ft, 3% maximum
The allowed drop is 3% of 240 V = 7.2 V. The maximum resistance is 7.2 ÷ (2 × 30 × 0.15) = 0.80 Ω per 1000 ft. The first copper size below that is 8 AWG (0.764 Ω/1000 ft), giving a drop of 6.9 V (2.9%). 8 AWG copper is rated 40 A even in the 60 °C column, so ampacity is also satisfied.
Example 3: 100 A, 480 V three-phase, 300 ft, 3% maximum
Allowed drop = 14.4 V. Maximum resistance = 14.4 ÷ (1.732 × 100 × 0.3) = 0.277 Ω/1000 ft, so 3 AWG copper (0.245) passes on voltage drop (2.65%). For ampacity, a 100 A circuit uses the 60 °C column unless terminals are marked 75 °C: 3 AWG is only 85 A at 60 °C, so the answer is 1 AWG (110 A). With 75 °C-rated terminals, 3 AWG (100 A) is enough. If the load is continuous, the design current becomes 125 A, the 75 °C column applies, and the answer is 1 AWG (130 A).
Wire ampacity quick reference (NEC 310.16)
| Wire size | Copper 60 °C | Copper 75 °C | Aluminum 60 °C | Aluminum 75 °C | Max breaker (NEC 240.4(D)) |
|---|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | – | – | 15 A |
| 12 AWG | 20 A | 25 A | 15 A | 20 A | 20 A Cu / 15 A Al |
| 10 AWG | 30 A | 35 A | 25 A | 30 A | 30 A Cu / 25 A Al |
| 8 AWG | 40 A | 50 A | 35 A | 40 A | – |
| 6 AWG | 55 A | 65 A | 40 A | 50 A | – |
| 4 AWG | 70 A | 85 A | 55 A | 65 A | – |
| 3 AWG | 85 A | 100 A | 65 A | 75 A | – |
| 2 AWG | 95 A | 115 A | 75 A | 90 A | – |
| 1 AWG | 110 A | 130 A | 85 A | 100 A | – |
| 1/0 AWG | 125 A | 150 A | 100 A | 120 A | – |
| 2/0 AWG | 145 A | 175 A | 115 A | 135 A | – |
| 3/0 AWG | 165 A | 200 A | 130 A | 155 A | – |
| 4/0 AWG | 195 A | 230 A | 150 A | 180 A | – |
| 250 kcmil | 215 A | 255 A | 170 A | 205 A | – |
| 500 kcmil | 320 A | 380 A | 260 A | 310 A | – |
Ampacities apply to no more than three current-carrying conductors in a raceway at 30 °C ambient. Apply the adjustment and correction factors in NEC 310.15 for hotter locations or more conductors, and use the 60 °C column for circuits of 100 A or less unless every termination is marked 75 °C (NEC 110.14(C)).
Frequently asked questions
What size wire do I need for 30 amps?
For a 30 A circuit, 10 AWG copper is the minimum under NEC 240.4(D). On long runs, voltage drop may require 8 AWG or larger. For example, a 30 A load at 240 V over 150 ft needs 8 AWG copper to stay under 3% drop.
What size wire do I need for 50 amps?
For a 50 A breaker, use 6 AWG copper (55 A in the 60 °C column). 8 AWG copper (50 A at 75 °C) is allowed only when the breaker and the equipment terminals are both rated 75 °C. In aluminum, use 4 AWG. Long runs may need a larger size for voltage drop.
What size wire for a 20 amp 120 V circuit?
12 AWG copper is the minimum for a 20 A circuit (NEC 240.4(D)). At a full 20 A load, 12 AWG stays within 3% voltage drop only up to about 45 ft one way on 120 V; for longer runs use 10 AWG, or 8 AWG beyond about 75 ft.
How does distance affect wire size?
Voltage drop increases with distance, so the longer the run, the larger the wire needed to keep the drop within 3–5%. Beyond roughly 100 ft, voltage drop rather than ampacity usually decides the size.
Why is the 125% rule used?
The NEC requires conductors and breakers for continuous loads (running 3 hours or more) to be sized at 125% of the continuous load, so the conductor and terminations do not overheat during long operation.
Can I use this calculator for mm² cables in Pakistan or the UK?
Yes. Select mm² (IEC). The calculator sizes for voltage drop; then confirm the current-carrying capacity for your installation method using IEC 60364-5-52, BS 7671 or your cable manufacturer’s tables.
Related calculators and guides
These calculators give engineering estimates for planning and learning. Final designs must follow your local code (NEC, IEC/BS 7671, PEC or the authority having jurisdiction) and be checked by a qualified electrical engineer or licensed electrician.