For a standard 20-amp residential branch circuit, use 12 AWG copper wire. For a 15-amp circuit, use 14 AWG. For a 30-amp circuit, use 10 AWG. While these are the baseline rules for standard NM-B (Romex) cable, precise sizing for larger loads, THHN in conduit, or subpanel feeders requires consulting the official amps and cable size chart derived from NEC Table 310.16. This chart dictates the maximum allowable ampacity based on wire gauge, insulation temperature rating, and installation method.
The Master Amps and Cable Size Chart (NEC Table 310.16)
The following data table is extracted from NFPA 70 (National Electrical Code) Table 310.16 (formerly 310.15(B)(16) in pre-2020 editions). It applies to copper conductors with up to three current-carrying conductors in a raceway, cable, or earth, at an ambient temperature of 30°C (86°F).
| Wire Size (AWG/kcmil) | 60°C Ampacity (NM-B / TW) | 75°C Ampacity (THHN / THWN-2) | Max Standard Breaker Size |
|---|---|---|---|
| 14 AWG | 15A | 20A | 15A (NEC 240.4(D)) |
| 12 AWG | 20A | 25A | 20A (NEC 240.4(D)) |
| 10 AWG | 30A | 35A | 30A (NEC 240.4(D)) |
| 8 AWG | 40A | 50A | 50A |
| 6 AWG | 55A | 65A | 60A |
| 4 AWG | 70A | 85A | 80A / 90A |
| 3 AWG | 85A | 100A | 100A |
| 2 AWG | 95A | 115A | 110A / 125A |
| 1 AWG | 110A | 130A | 125A |
| 1/0 AWG | 125A | 150A | 150A |
| 2/0 AWG | 145A | 175A | 175A |
| 3/0 AWG | 165A | 200A | 200A |
| 4/0 AWG | 195A | 230A | 225A / 250A |
Which Temperature Column Applies to Your Installation?
The most common mistake DIYers make is using the 75°C or 90°C column for an entire circuit. Under NEC 110.14(C), the ampacity of your wire is limited by the lowest temperature rating of any component in the circuit. This is known as the weakest-link rule.
Use the 60°C Column When:
- You are running NM-B (Romex) cable. The internal insulation of standard NM-B is rated for 60°C, regardless of what the outer jacket says.
- You are terminating into equipment, breakers, or lugs that are not explicitly marked with a temperature rating (common in older panels pre-1990s).
- You are wiring 14, 12, or 10 AWG conductors to standard residential breakers, as most residential breakers under 100A are only tested and rated for 60°C terminations.
Use the 75°C Column When:
- You are pulling individual THHN/THWN-2 conductors in conduit.
- Your panelboard, lugs, and breakers are explicitly stamped '75°C' (standard on modern 100A+ main breakers and commercial subpanels).
- You are sizing 8 AWG or larger conductors, provided the termination hardware is 75°C rated.
Bench Note: You can use the 90°C column for THHN wire, but only for applying ambient temperature derating corrections. Your final derated ampacity must still be equal to or greater than the breaker size, and the termination ampacity cannot exceed the 75°C (or 60°C) column limit. For a deep dive on terminal temperature rules, Electrical Construction & Maintenance (EC&M) provides excellent field-case breakdowns.
Derating: When the Base Chart Lies to You
The amps and cable size chart above assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) bundled together. When real-world conditions deviate, you must apply derating factors that reduce the base ampacity.
You are pulling four 10 AWG THHN current-carrying conductors through an attic where the ambient temperature reaches 120°F (49°C).
1. Base 90°C ampacity for 10 AWG THHN = 40A.
2. Ambient correction (113-122°F / 46-50°C) = 0.82 multiplier.
3. Bundling correction (4-6 CCCs) = 0.80 multiplier.
4. Final Ampacity = 40A × 0.82 × 0.80 = 26.24A.
Since 26.24A is below the 30A requirement for a standard 30-amp circuit, 10 AWG is no longer legal here. You must upsize to 8 AWG.
What counts as a Current-Carrying Conductor (CCC)?
- Hot wires (Black/Red/Blue): Always count.
- Neutral (White/Gray): Counts if it carries unbalanced current (standard 120/240V split-phase). Does not count in a balanced 240V-only circuit or a 3-phase wye where harmonics are negligible.
- Ground (Bare/Green): Never counts as a CCC for derating purposes.
Decision Path: Pick Your Wire and Breaker
Use this decision matrix to lock in your exact materials for standard residential and light commercial branch circuits. This path assumes copper conductors and standard 60°C/75°C terminations.
| If Your Continuous Load Is... | And Your Max Run Is... | Then Buy This Wire (Copper) | And Install This Breaker |
|---|---|---|---|
| Up to 12A (15A circuit) | < 50 ft | 14 AWG NM-B | 15A Single-Pole |
| Up to 16A (20A circuit) | < 50 ft | 12 AWG NM-B | 20A Single-Pole |
| Up to 24A (30A circuit) | < 50 ft | 10 AWG NM-B or THHN | 30A Double-Pole |
| Up to 40A (50A circuit) | < 60 ft | 6 AWG THHN in conduit | 50A Double-Pole |
| Up to 48A (60A subpanel) | < 75 ft | 4 AWG THHN (or 2 AWG Al) | 60A Main Lug/Breaker |
| Up to 80A (100A subpanel) | < 100 ft | 3 AWG THHN (or 1/0 Al) | 100A Main Lug/Breaker |
| Up to 160A (200A service) | < 100 ft | 2/0 AWG Copper (or 4/0 Al) | 200A Main Breaker |
What This Chart Cannot Tell You (And the Default Fix)
The NEC Table 310.16 amps and cable size chart only protects against fire (overheating insulation). It does not protect against voltage drop. If you run a 30-amp circuit to a detached garage 150 feet away using 10 AWG wire, the wire won't melt, but your table saw will bog down and burn out its motor because the voltage at the receptacle will drop below 110V under load.
The NEC recommends keeping voltage drop under 3% for branch circuits and 5% total from the utility transformer to the furthest outlet. For every doubling of wire length, resistance doubles.
If your one-way wire run exceeds 75 feet for a 120V circuit, or 150 feet for a 240V circuit, the base chart is no longer sufficient. Upsize your wire by one AWG step (e.g., use 10 AWG instead of 12 AWG for a 20A circuit) to compensate for voltage drop. If the run exceeds 150 feet at 120V, upsize by two steps or switch to a higher voltage (240V) at the source and step it down at the destination. Always calculate exact voltage drop using the formula: VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is amps, D is one-way distance, and CM is the circular mil area of the wire.
By anchoring your wire selection to the 60°C/75°C columns, applying bundling derating when pulling more than three wires in a conduit, and upsizing for runs over 75 feet, you will pass inspection and ensure your equipment operates at peak efficiency without nuisance tripping or motor burnout.
