What AWG wire do I need for a 20 amp circuit?

A 20-amp branch circuit requires minimum 12 AWG copper wire under NEC Table 310.12. This is the minimum -- if the run is longer than roughly 50 feet at 120V or 100 feet at 240V, voltage drop may push you to 10 AWG. Many electricians use 10 AWG as a default for 20-amp circuits serving garages, workshops, or any run over 75 feet. The breaker size must match the wire's ampacity: a 20-amp breaker on 14 AWG wire is a code violation because 14 AWG is rated for 15 amps.

What is the 3% voltage drop rule?

The NEC recommends keeping voltage drop to 3% or less on branch circuits (conductors from the panel to the outlet), with a combined feeder-plus-branch maximum of 5%. For a 120V circuit, 3% equals 3.6V of allowable drop; for 240V, it equals 7.2V. To stay within 3%, calculate the voltage drop using VD = (2 x K x I x L) / CM, where K is 12.9 for copper, I is current in amps, L is one-way run length in feet, and CM is the wire's circular mil area from NEC tables. Oversizing wire to meet the 3% limit on long runs is a routine part of circuit design.

Why does aluminum wire require a larger gauge?

Aluminum has approximately 61% of the electrical conductivity of copper, so it requires a larger cross-sectional area to carry the same current with equivalent resistance. NEC Table 310.12 reflects this: a 15-amp circuit requires 14 AWG copper or 12 AWG aluminum; a 20-amp circuit requires 12 AWG copper or 10 AWG aluminum. Aluminum is also more susceptible to oxidation and requires anti-oxidant compound at connections. It is commonly used for service entrance conductors and large feeder runs where its cost and weight advantage over copper is significant, but it requires aluminum-rated connectors and devices.

How do I derate wire ampacity for bundled cables?

When more than three current-carrying conductors share a conduit or are bundled together, NEC Table 310.15(C)(1) requires ampacity derating: 4-6 conductors = 80% of rated ampacity; 7-9 conductors = 70%; 10-20 conductors = 50%. A 12 AWG THHN rated at 30 amps (90 degree C column) in a bundle of 6 must be derated to 30 x 0.80 = 24 amps, and then limited to the 20-amp terminal temperature rating. Bundles in hot attics require combined derating for both bundling and ambient temperature. This is a common calculation error in multi-circuit conduit fills.

When should I choose aluminum wire over copper?

Aluminum makes economic sense for service entrance conductors, main feeders, and large sub-panel runs where conductor size is 4 AWG or larger. At 4/0 AWG and above, aluminum costs roughly 60-70% less than copper by weight for equivalent conductance. Aluminum conductors must use aluminum-rated (AL/CU marked) lugs and connectors, and connections require anti-oxidant paste (Noalox) to prevent corrosion. Aluminum should not be used for 15 or 20 amp branch circuits due to connection reliability issues at standard outlets and switches. All residential service drops from the utility are aluminum.

What is the difference between THHN and NM-B cable?

THHN is a single-conductor wire with thermoplastic insulation rated for 90 degrees C in dry locations and 75 degrees C in wet locations, typically used in conduit. NM-B (non-metallic sheathed cable, brand name Romex) bundles two or more THHN-equivalent conductors plus a bare ground inside a plastic outer jacket, rated for dry indoor residential use. NM-B cannot be used in commercial buildings, exposed outdoor locations, or embedded in concrete. The conductors inside NM-B are rated 90 degrees C, but NEC limits their allowable ampacity to the 60 degree C column due to heat buildup within the jacket -- this is why NM-B 12 AWG is limited to 20 amps despite the conductor's higher thermal rating.

NEC Wire Size Calculator

Find minimum AWG by NEC ampacity and voltage drop for copper or aluminum branch circuits, with conduit derating included.

100% client-side. Inputs stay in your browser (ons-wire-size-inputs).

Inputs

Load (amps)Circuit voltagePhaseWire materialTemperature ratingOne-way length (ft)Max voltage drop (%)Current-carrying wires in conduitConduit type

Results

Recommended AWG

8 AWG

Min. for ampacity

12 AWG

Min. for voltage drop

8 AWG

Voltage drop

2.55% (3.06 V)

Ampacity at size

50 A (50.0 A derated)

Based on NEC 2023, Table 310.16 (ampacity) and Chapter 9 Table 8 (resistance). Verified 2026-05-25.

This calculator is for reference only. All electrical work must comply with local codes and be inspected by a licensed electrician.

How this tool works

We check two limits separately. Ampacity uses NEC Table 310.16 for your wire metal and temperature rating, then applies conduit fill derating from NEC 310.15(B)(3) when more than three current-carrying conductors share the raceway. Voltage drop uses Chapter 9 Table 8 resistance per thousand feet with single-phase formula 2 × length × amps × R/V, or √3 × length × amps × R/V for three-phase. The recommended gauge is whichever requirement needs the larger wire (smaller AWG number).

Worked example

A 20 amp, 120 volt copper branch circuit at 75°C with three conductors in EMT and a 100-foot one-way run needs 12 AWG for ampacity (25 A rated) but about 6.4% drop on 12 AWG. To stay under 3% drop you need 8 AWG copper. This tool reports 8 AWG as the final size so both ampacity and voltage drop pass.

Frequently asked questions

What AWG wire do I need for a 20 amp circuit?
A 20-amp branch circuit requires minimum 12 AWG copper wire under NEC Table 310.12. This is the minimum -- if the run is longer than roughly 50 feet at 120V or 100 feet at 240V, voltage drop may push you to 10 AWG. Many electricians use 10 AWG as a default for 20-amp circuits serving garages, workshops, or any run over 75 feet. The breaker size must match the wire's ampacity: a 20-amp breaker on 14 AWG wire is a code violation because 14 AWG is rated for 15 amps.
What is the 3% voltage drop rule?
The NEC recommends keeping voltage drop to 3% or less on branch circuits (conductors from the panel to the outlet), with a combined feeder-plus-branch maximum of 5%. For a 120V circuit, 3% equals 3.6V of allowable drop; for 240V, it equals 7.2V. To stay within 3%, calculate the voltage drop using VD = (2 x K x I x L) / CM, where K is 12.9 for copper, I is current in amps, L is one-way run length in feet, and CM is the wire's circular mil area from NEC tables. Oversizing wire to meet the 3% limit on long runs is a routine part of circuit design.
Why does aluminum wire require a larger gauge?
Aluminum has approximately 61% of the electrical conductivity of copper, so it requires a larger cross-sectional area to carry the same current with equivalent resistance. NEC Table 310.12 reflects this: a 15-amp circuit requires 14 AWG copper or 12 AWG aluminum; a 20-amp circuit requires 12 AWG copper or 10 AWG aluminum. Aluminum is also more susceptible to oxidation and requires anti-oxidant compound at connections. It is commonly used for service entrance conductors and large feeder runs where its cost and weight advantage over copper is significant, but it requires aluminum-rated connectors and devices.
How do I derate wire ampacity for bundled cables?
When more than three current-carrying conductors share a conduit or are bundled together, NEC Table 310.15(C)(1) requires ampacity derating: 4-6 conductors = 80% of rated ampacity; 7-9 conductors = 70%; 10-20 conductors = 50%. A 12 AWG THHN rated at 30 amps (90 degree C column) in a bundle of 6 must be derated to 30 x 0.80 = 24 amps, and then limited to the 20-amp terminal temperature rating. Bundles in hot attics require combined derating for both bundling and ambient temperature. This is a common calculation error in multi-circuit conduit fills.
When should I choose aluminum wire over copper?
Aluminum makes economic sense for service entrance conductors, main feeders, and large sub-panel runs where conductor size is 4 AWG or larger. At 4/0 AWG and above, aluminum costs roughly 60-70% less than copper by weight for equivalent conductance. Aluminum conductors must use aluminum-rated (AL/CU marked) lugs and connectors, and connections require anti-oxidant paste (Noalox) to prevent corrosion. Aluminum should not be used for 15 or 20 amp branch circuits due to connection reliability issues at standard outlets and switches. All residential service drops from the utility are aluminum.
What is the difference between THHN and NM-B cable?
THHN is a single-conductor wire with thermoplastic insulation rated for 90 degrees C in dry locations and 75 degrees C in wet locations, typically used in conduit. NM-B (non-metallic sheathed cable, brand name Romex) bundles two or more THHN-equivalent conductors plus a bare ground inside a plastic outer jacket, rated for dry indoor residential use. NM-B cannot be used in commercial buildings, exposed outdoor locations, or embedded in concrete. The conductors inside NM-B are rated 90 degrees C, but NEC limits their allowable ampacity to the 60 degree C column due to heat buildup within the jacket -- this is why NM-B 12 AWG is limited to 20 amps despite the conductor's higher thermal rating.

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