For a standard 20-amp breaker, you need 12 AWG copper wire. If using aluminum, you must step up to 10 AWG. The breaker protects the wire; therefore, the wire's ampacity must equal or exceed the breaker rating. Never use 14 AWG on a 20A circuit.
- Material: Copper (unless explicitly stated as aluminum)
- Temperature Column: 75°C (standard for THHN/THWN-2 and modern terminals)
- Ambient Temperature: 30°C (86°F)
- Conduit/Raceway: EMT, PVC, or NM-B cable with a maximum of 3 current-carrying conductors
The Baseline: 12 AWG Copper and the Termination Temperature Rule
When determining what size wire for 20 amp circuits, the National Electrical Code (NEC) dictates that the overcurrent protective device (the breaker) must be sized to protect the wire's insulation from melting. According to NEC Table 310.16, 14 AWG copper wire is only rated for 15 amps. If you pull 20 amps through 14 AWG wire, the resistive heating ($I^2R$) will degrade the insulation and start a fire long before a 20-amp breaker's thermal trip mechanism engages.
Therefore, 12 AWG is the absolute minimum. But which temperature column do you use? NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected termination, device, or conductor. Most modern 20-amp receptacles and breakers (like the Square D QO220 or Eaton BR220) are rated for 75°C. In the 75°C column, 12 AWG THHN copper is rated for 25 amps. However, for standard residential NM-B (Romex) cable, the 60°C column governs, which rates 12 AWG at exactly 20 amps. In both scenarios, 12 AWG safely handles a 20-amp breaker.
| Wire Size (AWG) | Insulation Type | 75°C Ampacity | Max Standard Breaker | Voltage Drop at 50ft (120V, 20A) |
|---|---|---|---|---|
| 14 AWG | THHN / NM-B | 20A / 15A | 15 Amp | 6.4V (5.3%) - FAIL |
| 12 AWG | THHN / NM-B | 25A / 20A | 20 Amp | 3.9V (3.2%) - PASS |
| 10 AWG | THHN / NM-B | 35A / 30A | 30 Amp | 2.5V (2.0%) - PASS |
| 8 AWG | THHN / NM-B | 50A / 40A | 40 Amp | 1.5V (1.2%) - PASS |
Voltage Drop: The Distance Factor
Ampacity tells you what size wire prevents a fire. Voltage drop tells you what size wire actually delivers usable power to your load. The NEC recommends (via Informational Notes in 210.19(A)) a maximum 3% voltage drop for branch circuits to ensure equipment operates efficiently and motors don't overheat.
Let's run a voltage drop check at a stated distance of 100 feet for a full 20-amp load on a 120V circuit. Using the standard formula $VD = \frac{2 \times K \times I \times D}{CM}$ (where K for copper is 12.9, I is 20A, D is 100ft, and CM for 12 AWG is 6530 circular mils):
$VD = \frac{2 \times 12.9 \times 20 \times 100}{6530} = 7.9V$
A 7.9V drop on a 120V circuit is a 6.58% drop. This vastly exceeds the 3% recommendation. Your 120V tool will only see 112V, which can cause premature failure in compressor motors and power supplies. If your 20-amp circuit run exceeds 75 feet, you must step up to 10 AWG copper (which drops the loss to roughly 4.1% at 100ft) or 8 AWG copper (which brings it down to a highly efficient 2.6%). Always use a dedicated voltage drop calculator for runs over 50 feet.
Derating and Material Swaps: What Changes the Sizing
The 12 AWG baseline assumes ideal conditions. Real-world jobsite factors will force you to upsize your wire. Here is what changes the answer:
1. Conductor Bundling (Derating)
When you pull more than three current-carrying conductors through a single raceway (like a conduit nipple or a long PVC run), the wires heat each other up. NEC 310.15(C)(1) requires you to apply a derating factor to the 90°C ampacity column of THHN wire.
- 4 to 6 conductors: Derate to 80%. (12 AWG THHN 90°C is 30A. $30 \times 0.8 = 24A$. Still safe for a 20A breaker).
- 7 to 9 conductors: Derate to 70%. ($30 \times 0.7 = 21A$. Still barely safe, but marginal).
- 10 to 20 conductors: Derate to 50%. ($30 \times 0.5 = 15A$. FAIL. You must step up to 10 AWG THHN, which derates to 20A).
2. Aluminum vs. Copper
Aluminum and copper are never interchangeable at the same gauge. Aluminum has higher electrical resistance and expands/contracts more under thermal load. For a 20-amp breaker using aluminum wire (like SER cable or XHHW), 12 AWG aluminum is only rated for 15A. You must use 10 AWG aluminum (rated 30A at 75°C) to safely feed a 20-amp breaker. Furthermore, you must use anti-oxidant paste (like Noalox) and torque aluminum terminations to exact manufacturer specs to prevent arcing.
If your 20-amp load will run for 3 hours or more (e.g., commercial lighting, server racks, or a continuously running heater), NEC 210.20(A) classifies it as a continuous load. You must multiply the load by 125%. A 16A continuous load requires wire rated for 20A (12 AWG) and a 20A breaker. However, a full 20A continuous load requires wire rated for 25A (10 AWG) and a breaker sized at 25A or 30A. Never put a 20A continuous load on a 20A breaker.
When to Call an Engineer or the AHJ
While the rules for standard residential branch circuits are straightforward, certain scenarios require professional oversight. You must consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) in the following cases:
| Scenario | Why It Requires Escalation | Action Required |
|---|---|---|
| Ambient temp exceeds 30°C (86°F) | Attics in summer or boiler rooms drastically reduce wire ampacity. | Apply NEC Table 310.15(B)(1) temperature correction factors. |
| Mixed insulation types in one conduit | The lowest rated insulation dictates the entire bundle's limit. | Redesign the run or upsize all conductors to match the highest temp rating. |
| Feeding subpanels or transformers | Feeder calculations involve diversity factors and complex load balancing. | Submit Article 220 load calculations to the AHJ for permit approval. |
Ultimately, understanding what size wire for 20 amp applications means respecting the physics of heat and resistance. Stick to 12 AWG copper for standard runs under 75 feet, upsize to 10 AWG or 8 AWG for long distances, and never compromise on termination torque. For specific breaker compatibility and torque values, always refer to the manufacturer's datasheet for your exact panel model.
