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Wire Gauge Calculator

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Enter your circuit's current, one-way length, and voltage to find the smallest wire gauge that keeps voltage drop within your target percentage.

Wire Gauge Calculator

Live
Recommended gauge
8 AWG
Actual drop at this gauge
2.34%
Assumes copper conductor. This finds the minimum gauge for voltage drop only - confirm it also meets ampacity (current-carrying capacity) requirements for your application.

The formula

For each gauge, starting from the smallest: check if (2 x 12.9 x I x L) / CM stays within your target voltage drop Use the smallest (thinnest) gauge that satisfies the target
Example

20A load, 75 ft one-way, 120V circuit, targeting 3% max drop: 10 AWG alone isn't quite enough, so the calculator recommends 8 AWG, which keeps the actual drop around 2.34%.

Step-by-step guide

  1. Enter the current the circuit will carry.
  2. Enter the one-way distance from source to load.
  3. Set your target maximum voltage drop percentage - 3% is common guidance for a branch circuit.
  4. Read the recommended gauge, then confirm it also meets your local code's ampacity requirements - voltage drop and ampacity are two separate checks.

Voltage drop vs. ampacity - two different checks

This calculator solves specifically for voltage drop. But wire sizing has a second, separate requirement: ampacity, meaning the wire must also be rated to safely carry the expected current without overheating, based on your electrical code's tables (which factor in insulation type, ambient temperature, and how many conductors are bundled together). A wire can pass one check and fail the other - always confirm both, and remember that code-required ampacity is a safety minimum, while voltage drop is a performance guideline.

Note: This tool estimates wire sizing for voltage drop only, for planning purposes. Actual electrical installations must meet your local electrical code's full requirements and should be installed by a qualified electrician.

Common mistakes

Sizing wire for voltage drop alone and skipping the ampacity check - a wire adequate for voltage drop on a very short run might still be undersized for safely carrying the current.
Sizing for today's load without margin for future additions - it's often cheaper to run one size larger during initial installation than to re-run wire later for added load.

Frequently asked questions

Does a smaller AWG number mean thicker or thinner wire?

Thicker - AWG (American Wire Gauge) numbering runs backward from what feels intuitive: a smaller number means a larger diameter wire, which carries more current and has less resistance.

Why does a longer run need a thicker wire for the same load?

Resistance increases with wire length, so a longer run drops more voltage for the same current - a thicker wire (more circular mils) has lower resistance per foot, offsetting that extra length.

Should I always oversize my wire "just in case"?

A reasonable safety margin is common practice, but wire also gets significantly more expensive and harder to work with at larger gauges - balance future-proofing against practical cost and installation difficulty.

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