For a standard 20 amp circuit, you need 12 AWG copper wire paired with a 20-amp breaker. This assumes THHN/THWN-2 or NM-B insulation, a 75°C terminal temperature rating, and an ambient temperature of 30°C (86°F). Never use 14 AWG on a 20A breaker; it violates NEC overcurrent protection rules and creates a severe fire hazard.
Baseline Assumptions for This Guide
- Material: Copper (Aluminum requires different sizing, covered below)
- Insulation: THHN/THWN-2 (in conduit) or NM-B (Romex)
- Temperature Column: 75°C (Standard for modern breakers and receptacles)
- Ambient Temperature: 30°C (86°F)
- Conduit Fill: Standard EMT/PVC with 3 or fewer current-carrying conductors
Note: This is NEC-style guidance; your local Authority Having Jurisdiction (AHJ) has final authority.
The Core Sizing Rule: Why 12 AWG and Not 14 AWG?
The National Electrical Code (NEC) dictates wire sizing based on ampacity—the maximum current a conductor can carry continuously without exceeding its temperature rating. According to NFPA 70 (NEC) Table 310.16, 14 AWG copper is only rated for 15 amps in the 60°C column and 20 amps in the 75°C/90°C columns.
However, NEC 240.4(D) enforces strict 'small conductor' overcurrent protection limits. It explicitly caps the breaker size for 14 AWG copper at 15 amps, regardless of the insulation's higher temperature rating. If you push 20 amps through 14 AWG wire, the wire will heat up, but the 20A breaker won't trip. Over time, this degrades the insulation and causes terminal meltdowns.
| AWG Size | 60°C Column (NM-B) | 75°C Column (THHN Terminals) | 90°C Column (THHN Wire) | Max Breaker (NEC 240.4(D)) |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 15 Amps |
| 12 AWG | 20A | 25A | 30A | 20 Amps |
| 10 AWG | 30A | 35A | 40A | 30 Amps |
When the Standard Answer Changes: Voltage Drop and Derating
While 12 AWG is the legal minimum for a 20A breaker, real-world physics often forces you to upsize to 10 AWG. This happens due to voltage drop and conduit bundling.
The 50-Foot Voltage Drop Check
NEC 210.19(A)(1) Informational Note No. 4 recommends that branch circuit voltage drop not exceed 3%. Let's run the math for a 120V circuit pulling a full 20A load over a 50-foot one-way run using 12 AWG copper (Circular Mil area = 6,530). Using the standard formula and a K-factor of 12.9 for copper, the drop is roughly 3.95V, which equals 3.29%.
Because this exceeds the 3% recommendation, a seasoned electrician will pull 10 AWG for any 20A branch circuit exceeding 50 feet to ensure motors don't overheat and electronics don't brown out. The Southwire Voltage Drop Calculator is an excellent bench reference for verifying these runs.
Conduit Bundling and the 90°C Derating Trick
If you pull multiple circuits through a single conduit, the wires heat each other up. NEC 310.15(C)(1) requires ampacity derating when you have more than three current-carrying conductors. Here is where the 90°C column saves you:
- 4 to 6 conductors: Derate to 80%. (12 AWG THHN 90°C base is 30A. 30A x 0.80 = 24A. Still above 20A, so 12 AWG is fine).
- 7 to 9 conductors: Derate to 70%. (30A x 0.70 = 21A. Still above 20A, so 12 AWG is fine).
- 10 to 20 conductors: Derate to 50%. (30A x 0.50 = 15A. Must upsize to 10 AWG).
Aluminum vs. Copper: A Critical Distinction
If you are using AA-8000 series aluminum alloy wire, the sizing changes completely. Aluminum has lower conductivity per volume than copper. A 12 AWG aluminum wire is only rated for 15 amps at 75°C. Therefore, for a 20-amp circuit using aluminum, you must step up to 10 AWG aluminum (rated 30A at 75°C).
In modern residential branch circuits, aluminum is rarely used below 8 AWG due to termination difficulties and historical oxidation issues. Stick to copper for 20A branch circuits unless you are pulling a heavy feeder where the cost savings of aluminum justify the larger physical size and specialized lugs.
When to Call an Engineer or the AHJ
Standard 12 AWG sizing applies to general lighting and receptacle circuits. You must involve a licensed professional or consult your local inspector under these conditions:
| Scenario | Code Impact | Required Action |
|---|---|---|
| Continuous Load (Runs 3+ hours) | NEC 210.20(A) requires 125% sizing. A 20A continuous load needs a 25A breaker. | Upsize to 10 AWG copper and a 25A or 30A breaker. |
| High Ambient Temperature (e.g., Arizona attic) | NEC 310.15(B)(1) requires temperature correction factors. At 113°F (45°C), THHN derates to 87%. | Calculate exact derating; likely requires 10 AWG minimum. |
| Motor Circuits (HVAC, Pumps) | NEC Article 430 allows breakers to be sized up to 250% of motor FLA for startup inrush. | Wire size is based on 125% of FLA, breaker is sized for inrush. Requires engineering calc. |
Frequently Asked Questions: 20 Amp Circuit Wire Sizing
Can I use 10 AWG wire on a 20 amp breaker?
Yes. The NEC establishes minimum wire sizes, not maximums. Upsizing to 10 AWG on a 20A breaker is perfectly legal and actually reduces voltage drop. The only practical hurdle is physical fitment: 10 AWG solid wire is stiff and can be difficult to fold into a standard single-gang device box, and some cheap receptacles have back-wire clamps that only accept up to 12 AWG. Always use the screw terminals torqued to the manufacturer's spec (usually 12-14 in-lbs) when terminating 10 AWG.
What wire for 20 amp circuit at 240 volts?
The wire size remains 12 AWG copper, but you will use a 2-pole 20A breaker and a 2-wire cable with a ground (like 12/2 NM-B). Because the voltage is doubled to 240V, the percentage of voltage drop is halved compared to a 120V circuit over the same distance. This means 12 AWG can comfortably run much further (up to roughly 100 feet) on a 240V/20A circuit before you need to upsize to 10 AWG for voltage drop mitigation.
Is 12/2 NM-B Romex good for a 20 amp breaker?
Yes. NM-B (Romex) is rated in the 60°C column, which lists 12 AWG at 20 amps. Because the base ampacity perfectly matches the NEC 240.4(D) overcurrent limit for 12 AWG, it is the standard, code-compliant choice for indoor residential 20A branch circuits like kitchen small-appliance receptacles and garage outlets.
What size ground wire do I need for a 20 amp circuit?
According to NEC 250.122, the minimum size equipment grounding conductor (EGC) for a 20A overcurrent device is 12 AWG copper. If you upsize your current-carrying conductors to 10 AWG to compensate for voltage drop over a long run, you are not strictly required by 250.122 to upsize the ground wire unless the circuit is fed from a larger breaker. However, many inspectors and the Copper Development Association recommend upsizing the ground proportionally to maintain a robust fault-current path.
