Wire Size Calculator
Your estimate
- Minimum for the current
- 12 AWG
- Minimum for voltage drop
- 8 AWG
- Voltage drop (%)
- 2.55%
The smallest wire that carries the current safely and keeps the voltage drop within your limit.
Updated 2026-10-20 · Based on: NEC Chapter 9 Table 8 and 310.16; DIN VDE 0100-520 for metric.
How to use the Wire Size Calculator
- Enter the current, the one-way distance and the voltage.
- Choose copper or aluminum and single or three phase.
- Keep 3 % as the maximum drop.
- Use the recommended size — the larger of the ampacity and voltage-drop sizes.
Formula
Ampacity: NEC 310.16 (75 °C column) with the small-conductor limits of 240.4(D): 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A copper.
Voltage drop: the smallest size whose drop is under your limit (see the voltage drop calculator).
Worked example
With the values preset in the calculator above:
- Material
- Copper
- One-way distance
- 100 ft
- Current
- 20 A
- Voltage
- 120 V
- Phase
- Single phase
- Max voltage drop
- 3 %
- Recommended wire size
- 8 AWG
- Minimum for the current
- 12 AWG
- Minimum for voltage drop
- 8 AWG
- Voltage drop (%)
- 2.55%
Wire size chart (copper, 75 °C)
- 14 AWG — 15 A
- 12 AWG — 20 A
- 10 AWG — 30 A
- 8 AWG — 50 A
- 6 AWG — 65 A
- 4 AWG — 85 A
- 2 AWG — 115 A
- 1/0 AWG — 150 A
- 2/0 AWG — 175 A
- 4/0 AWG — 230 A
Related guides
Frequently asked questions
What size wire for 20 amps at 100 ft?
12 AWG copper carries 20 A, but at 100 ft on 120 V the drop is too high — the calculator recommends 8 AWG.
What size wire for a 50 amp circuit?
8 AWG copper (6 AWG for long runs or aluminum).
Is this a substitute for an electrician?
No. It follows the NEC tables for typical conditions; ambient temperature, bundling and local code can change the size.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.