Wire sizing guide

What Gauge Wire for 20 Amps?

Short answer: 12 AWG copper. It is the standard minimum for 20 amp branch circuits and covers the majority of residential and light commercial installations. Read on for the details that affect your specific run.

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The baseline

12 AWG copper: the 20 amp standard

NEC Table 310.16 rates 12 AWG copper at 20 amps for 60°C insulation (NM-B / Romex) and 25 amps at 75°C (THHN/THWN). For a 20 amp breaker, 12 AWG meets the minimum ampacity requirement under standard conditions: no more than three current-carrying conductors, 30°C ambient, and typical residential installation.

This is the most common wire size in residential wiring. Kitchen counter circuits, bathroom receptacles, garage outlets, and general-purpose 20 amp branch circuits all use 12 AWG as the default.

Common installations

Where 20 amp circuits are used

  • Kitchen small-appliance circuits — NEC requires at least two 20 amp circuits serving kitchen countertop receptacles. These are typically short runs with 12/2 NM-B.
  • Bathroom receptacles — a dedicated 20 amp circuit is required. Usually a short run of 12/2 NM-B from the panel.
  • Garage and workshop outlets — 20 amp circuits handle most portable power tools. Runs to detached garages can be long enough for voltage drop to matter.
  • Outdoor and landscape circuits — weather-resistant receptacles, landscape lighting controllers, and pond pumps often run on 20 amp circuits with longer distances.
  • General-purpose branch circuits — code permits both 15 and 20 amp circuits for general lighting and receptacles, but 20 amp circuits offer more capacity for modern loads.
14 AWG warning

Why 14 AWG is not acceptable for 20 amps

14 AWG copper is rated for 15 amps at 60°C. Connecting it to a 20 amp breaker violates code and creates a genuine safety hazard: the breaker will not trip until 20 amps, but the wire can overheat at currents above 15 amps, especially under sustained load.

This is not a gray area. NEC 240.4(D) explicitly requires that 14 AWG copper be protected at no more than 15 amps. If you have a 20 amp breaker, the branch circuit wiring must be 12 AWG or larger — no exceptions.

Voltage drop

When 12 AWG is not enough

Ampacity is the floor. Voltage drop is often the real constraint on longer runs. At full 20 amp load with 12 AWG copper:

  • 120V source: the 3% drop limit (~3.6V) is reached at roughly 50 feet one-way.
  • 240V source: the same 3% threshold allows about 100 feet one-way.

Beyond these distances, step up to 10 AWG. The extra cost per foot is modest compared to the performance improvement: 10 AWG has about 60% of the resistance per foot of 12 AWG, giving you nearly twice the allowable run length.

Calculate voltage drop for your run
Quick reference

20 amp wire sizing at a glance

Scenario Wire gauge Notes
Short run (< 50 ft), 120V 12 AWG copper Standard case — voltage drop well within limits
Medium run (50–80 ft), 120V 10 AWG copper 12 AWG exceeds 3% drop at full load
Short run (< 100 ft), 240V 12 AWG copper Higher voltage gives more headroom on drop
Long run (100+ ft), 240V 10 AWG copper Voltage drop controls — verify with calculator
Any length, aluminum 10 AWG aluminum Minimum for 20A — use AL-rated terminals
Assumptions

Limitations of this guide

  • Sizes based on NEC Table 310.16, 60°C and 75°C columns, standard conditions.
  • Voltage drop estimates assume a simple two-conductor loop and nominal resistance values. Actual conditions may vary.
  • Derating for conduit fill, ambient temperature, or continuous loading is not included — apply NEC correction factors separately.
  • Always verify with local codes and the authority having jurisdiction (AHJ).
FAQ

20 amp wire sizing questions

What gauge wire do I need for 20 amps?

12 AWG copper for most installations. This is the NEC minimum for a 20 amp branch circuit under standard conditions.

Can I use 14 AWG wire on a 20 amp breaker?

No. 14 AWG is rated for 15 amps only. NEC 240.4(D) prohibits protecting it at more than 15 amps. Using it on a 20 amp breaker is a code violation and fire hazard.

How far can I run 12 AWG wire on a 20 amp circuit?

About 50 feet one-way at 120V, or 100 feet at 240V, before voltage drop exceeds the 3% recommendation at full load. Beyond that, use 10 AWG.

When should I use 10 AWG instead of 12 AWG for 20 amps?

When run length exceeds ~50 ft at 120V, when ambient temperature exceeds 30°C, or when conduit fill derating reduces 12 AWG below 20 amps effective capacity.