About the Wire Size Calculator
This wire size calculator recommends the smallest conductor that satisfies two tests at once: it must carry the current safely (ampacity) and it must keep voltage drop within your target over the length of the run. Enter the load current, the one-way distance, the supply voltage and the maximum drop you will accept, and it returns the recommended AWG or kcmil size with its metric area in mm².
It is aimed at homeowners planning a subpanel, garage or shed feed, electricians double-checking a long run, and anyone wiring a well pump, EV charger, heat pump or RV and solar DC system. The results table shows every size with its ampacity and resulting drop, so you can see exactly why a size was chosen.
Ampacities follow the 75 °C column of NEC Table 310.16 (no more than three current-carrying conductors, 30 °C ambient) with the small-conductor limits of NEC 240.4(D). Continuous loads are sized at 125%. Many residential cables (NM-B) are limited to the 60 °C column, and bundling or hot locations need derating — always confirm against your local code or an electrician.
How to use the wire size calculator
- 1Select DC, single-phase or three-phase and enter the voltage.
- 2Enter the load in amps and switch on "continuous" if it runs 3 hours or more.
- 3Enter the one-way distance from the panel to the load.
- 4Pick copper or aluminum and your maximum voltage drop (3% is typical).
- 5Use the recommended size and review the table to compare alternatives.
Formula and method
The calculator checks every standard size from 14 AWG up to 500 kcmil. The first test is ampacity: the conductor’s rating from the 75 °C column of NEC Table 310.16 (capped at 15/20/30 A for 14/12/10 AWG copper by NEC 240.4(D)) must be at least the design current. Continuous loads are multiplied by 1.25 first.
The second test is voltage drop using the K method: K = 12.9 for copper or 21.2 for aluminum, I is the actual load current, L the one-way length in feet and CM the conductor area in circular mils; single-phase and DC use a factor of 2 and three-phase uses √3. The recommendation is the larger of the two minimum sizes.
- I
- Load current in amps
- K
- Resistivity constant (Ω·cmil/ft) at 75 °C
- L
- One-way run length in feet
- CM
- Conductor area in circular mils
- V
- Supply voltage
Worked examples
30 A at 240 V over 100 ft (copper)
10 AWG copper is rated for 30 A, but its drop would be 2 × 12.9 × 30 × 100 ÷ 10,380 ≈ 7.46 V (3.11%). 8 AWG cuts that to 4.69 V (1.95%), so 8 AWG is recommended.
20 A at 120 V over 25 ft
12 AWG copper is allowed on a 20 A circuit, and over 25 ft the drop is 2 × 12.9 × 20 × 25 ÷ 6,530 ≈ 1.98 V, or 1.65% — both tests pass.
40 A continuous EV charger, 50 ft at 240 V
A continuous 40 A load is sized at 40 × 1.25 = 50 A, which 8 AWG copper (75 °C) meets exactly. The drop at 40 A is 2 × 12.9 × 40 × 50 ÷ 16,510 ≈ 3.13 V, just 1.3%.
100 A three-phase at 480 V, 300 ft of aluminum
1 AWG aluminum is the smallest size rated 100 A. Its three-phase drop is 1.732 × 21.2 × 100 × 300 ÷ 83,690 ≈ 13.16 V, or 2.74% — under 3%, so no upsizing is needed.
Frequently asked questions
What size wire do I need for 30 amps?+
For a short run, 10 AWG copper is the minimum for a 30 A circuit. Over roughly 80–100 ft at 240 V, voltage drop pushes the answer to 8 AWG copper.
What size wire for a 50 amp circuit?+
6 AWG copper is the usual choice for 50 A. 8 AWG copper is rated 50 A in the 75 °C column but is only 40 A as NM-B cable, so check your cable type and terminals.
Does distance affect wire size?+
Yes. Longer runs have more resistance and therefore more voltage drop. Past a certain distance, the drop test — not the ampacity test — decides the size, which is what this calculator checks.
Is aluminum wire safe?+
Modern AA-8000 aluminum conductors are widely used for feeders and services. They must be about two sizes larger than copper for the same current and need terminals rated for aluminum (marked AL/CU).
What is the difference between AWG and mm²?+
AWG is the American wire gauge where smaller numbers mean thicker wire; metric sizes give the cross-sectional area in square millimetres. For example 12 AWG ≈ 3.31 mm² and 10 AWG ≈ 5.26 mm².