For standard residential copper wiring (NM-B or THHN in conduit with up to three current-carrying conductors at an ambient temperature of 86°F/30°C), the baseline matches are: 14 AWG to 15A, 12 AWG to 20A, 10 AWG to 30A, 8 AWG to 40A, and 6 AWG to 55A (typically protected at 50A or 60A depending on specific NEC small conductor rules). However, pulling the right wire and breaker pair requires understanding temperature ratings and derating factors, not just memorizing a list.
The Master Wire Size Breaker Size Chart (NEC Table 310.16)
This chart synthesizes data from NFPA 70 (National Electrical Code) Table 310.16 for ampacities and Article 240.4 for overcurrent protection limits. How to read this table: The 60°C column applies to NM-B (Romex) cable and terminals on equipment rated 100A or less. The 75°C column applies to THHN/THWN wires in conduit and terminals on equipment rated over 100A. The 'Max Breaker' column reflects standard NEC 240.4(B) and 240.4(D) limits for branch circuits.
| AWG Size | Copper 60°C Ampacity | Copper 75°C Ampacity | Aluminum 60°C Ampacity | Aluminum 75°C Ampacity | Max Standard Breaker (Cu) |
|---|---|---|---|---|---|
| 14 AWG | 15A | 20A | -- | -- | 15A* |
| 12 AWG | 20A | 25A | 15A | 20A | 20A* |
| 10 AWG | 30A | 35A | 25A | 30A | 30A* |
| 8 AWG | 40A | 50A | 30A | 40A | 40A / 50A |
| 6 AWG | 55A | 65A | 40A | 50A | 60A |
| 4 AWG | 70A | 85A | 55A | 65A | 70A / 80A |
| 3 AWG | 85A | 100A | 65A | 75A | 100A |
| 2 AWG | 95A | 115A | 75A | 90A | 100A / 110A |
| 1 AWG | 110A | 130A | 85A | 100A | 125A |
| 1/0 AWG | 125A | 150A | 100A | 120A | 150A |
| 2/0 AWG | 145A | 175A | 115A | 135A | 175A |
| 3/0 AWG | 165A | 200A | 130A | 155A | 200A |
| 4/0 AWG | 195A | 230A | 150A | 180A | 225A |
*NEC 240.4(D) Small Conductor Rule strictly limits 14 AWG to 15A, 12 AWG to 20A, and 10 AWG to 30A, overriding the standard 'next size up' allowance.
Applying the Chart: Columns, Derating, and Blind Spots
Which Column Applies to Your Installation?
If you are running NM-B (non-metallic sheathed cable, commonly called Romex) through wall cavities, you must use the 60°C column, even if the wire's printed jacket says 90°C. NEC Article 334.80 mandates this because the bundled cable cannot dissipate heat as effectively as individual wires. If you are pulling individual THHN/THWN-2 conductors through EMT or PVC conduit, you can use the 75°C column for determining termination ampacity, provided your breakers and lugs are rated for 75°C (which almost all modern residential gear is).
How Derating Modifies the Base Value
The chart assumes an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors in a raceway. If you bundle 4 to 6 current-carrying conductors in a single conduit, NEC Table 310.15(C)(1) requires you to derate the ampacity to 80% of the wire's 90°C base value.
What the Table Cannot Tell You
This chart dictates thermal limits (ampacity), but it ignores voltage drop and conduit fill. If you are running a 20A circuit to a detached garage 150 feet away, 12 AWG wire will suffer nearly a 10% voltage drop under full load, which can damage motor-driven tools. For runs over 100 feet, you must calculate voltage drop (aiming for <3% on branch circuits) and likely bump up to 10 AWG or 8 AWG, regardless of the breaker size. Furthermore, Chapter 9 of the NEC limits how many wires physically fit inside a specific conduit diameter; always check conduit fill tables before pulling.
Bookmark-Friendly Quick-Jump Rows for Common DIY Circuits
- 15A Lighting Circuits: 14 AWG Copper. (Standard for bedrooms, living rooms, and hallways. Do not use for kitchen or bathroom receptacles).
- 20A Receptacle Circuits: 12 AWG Copper. (Required for kitchen small-appliance circuits, bathroom GFCI outlets, and garage/workshop general use).
- 30A Heavy Appliance: 10 AWG Copper. (Standard for electric dryers, 120V window AC units on dedicated circuits, and some smaller tankless water heaters).
- 40A Range / EVSE: 8 AWG Copper. (Used for standard electric ranges and Level 2 EV chargers configured for 32A continuous draw).
- 50A Subpanel or Hot Tub: 6 AWG Copper or 4 AWG Aluminum. (Note: 6 AWG Copper has a 55A ampacity at 60°C, but NEC 240.4(B) allows the next standard breaker size up, which is 60A, for feeders not subject to the 240.4(D) small conductor rule).
Frequently Asked Questions
What wire size do I need for a 20 amp breaker over a long distance?
For a standard 20A breaker, 12 AWG copper is the minimum. However, if the one-way distance from the panel to the outlet exceeds 90 feet, you should bump up to 10 AWG copper to keep voltage drop under the recommended 3% threshold. If the run exceeds 150 feet, step up to 8 AWG copper. The breaker remains 20A; you are simply using thicker wire to reduce resistance over distance.
Can I use aluminum wire instead of copper for a 50 amp breaker?
Yes, but you must increase the gauge. To safely carry 50A on a 75°C rated termination, you need 4 AWG aluminum (which has a 65A ampacity at 75°C). Never use aluminum wire on terminals not explicitly marked 'CO/ALR' or 'AL/CU', and always apply an antioxidant paste (like Noalox) to the stripped ends to prevent galvanic corrosion and high-resistance heating over time.
Why does my 6 AWG copper wire have a 55A ampacity but I see it on 60A breakers?
This is due to NEC 240.4(B), the 'next standard size up' rule. Standard breaker sizes are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, etc. Since there is no 55A breaker, the code allows you to round up to the next standard size (60A) to protect a 55A-rated conductor, provided the load is not continuous (running for 3+ hours). However, if the load is continuous, you must size the breaker at 125% of the load, which would restrict a 60A breaker to a 48A continuous load.
Does the ground wire count when derating conductors in conduit?
No. According to NEC 310.15(C)(1), equipment grounding conductors (the bare copper or green wire) are not considered 'current-carrying conductors' for the purpose of derating. They only carry current during a fault condition. Therefore, if you pull three hot/neutral wires and one ground wire through a conduit, you only count the three current-carrying wires, meaning no derating penalty applies. For more on grounding and bonding safety, refer to OSHA's electrical safety guidelines.
