For standard residential branch circuits using copper wire, use 14 AWG for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. These baseline values assume a 60°C temperature rating at the terminations, which is the legal limit for most standard breakers and receptacles rated under 100A, regardless of the wire's actual insulation rating. If you are pulling individual THHN wires in conduit for a subpanel, you can utilize the 75°C or 90°C columns for derating calculations, but your final overcurrent protection device (breaker) size remains capped by the lowest temperature rating of any connected component.
The Master Cable Size Amperage Chart (NEC Table 310.16)
This data-dense table is derived directly from NFPA 70 (NEC) Table 310.16. It assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. Bookmark this section; the most frequently queried residential and light-commercial rows are tagged with quick-jump IDs.
| Wire Size (AWG/kcmil) | Copper 60°C (140°F) | Copper 75°C (167°F) | Copper 90°C (194°F) | Aluminum 75°C (167°F) | Aluminum 90°C (194°F) |
|---|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | — | — |
| 12 AWG | 20A | 25A | 30A | — | — |
| 10 AWG | 30A | 35A | 40A | — | — |
| 8 AWG | 40A | 50A | 55A | 40A | 45A |
| 6 AWG | 55A | 65A | 75A | 50A | 60A |
| 4 AWG | 70A | 85A | 95A | 65A | 75A |
| 3 AWG | 85A | 100A | 115A | 75A | 85A |
| 2 AWG | 95A | 115A | 130A | 90A | 100A |
| 1 AWG | 110A | 130A | 145A | 100A | 120A |
| 1/0 AWG | 125A | 150A | 170A | 120A | 135A |
| 2/0 AWG | 145A | 175A | 195A | 135A | 150A |
| 3/0 AWG | 165A | 200A | 225A | 155A | 175A |
| 4/0 AWG | 195A | 230A | 260A | 180A | 205A |
If you are using NM-B (Romex) cable, you must use the 60°C column, per NEC 334.80. If you are using individual THHN/THWN-2 wires in conduit, the wire itself is rated for 90°C, but standard residential breakers and receptacles are only rated for 60°C (for circuits under 100A) or 75°C (for circuits 100A and above). You use the 90°C column strictly as a starting point to calculate derating, but your final breaker size cannot exceed the 60°C or 75°C termination limits.
How Derating and Installation Conditions Modify Base Ampacity
The values in the chart above represent a "best-case scenario." In the real world, wires generate heat. When you bundle multiple current-carrying conductors together in a single conduit, or when you run them through a hot attic, the wire cannot dissipate heat as efficiently. To prevent the insulation from melting or degrading, the NEC requires you to apply derating factors.
There are two primary derating modifiers you must calculate:
- Conduit Fill (Bundling): When you have 4 to 6 current-carrying conductors in a raceway, you multiply the base 90°C ampacity by 80%. For 7 to 9 conductors, the factor drops to 70%.
- Ambient Temperature: If the ambient temperature exceeds 30°C (86°F), you must apply a temperature correction factor based on the wire's insulation rating.
Worked Numeric Example: Conduit in a Hot Attic
Imagine you are running a 240V circuit for a workshop subpanel. You pull four 10 AWG THHN copper conductors (two hots, one neutral, one ground) through a conduit routed through an attic that reaches 40°C (104°F) in the summer. What is your actual allowable ampacity?
- Identify Base Ampacity: Looking at the 90°C copper column for 10 AWG, the base value is 40A.
- Apply Bundling Derating: The ground wire does not count as a current-carrying conductor. You have 3 current-carrying conductors (two hots, one neutral). Because you only have 3, the bundling derating factor is 100% (no penalty). *Note: If this were a multi-wire branch circuit with 4 current-carrying conductors, you would multiply by 0.80.*
- Apply Temperature Correction: According to NEC Table 310.15(B)(1), the correction factor for 90°C wire at 40°C ambient is 0.91.
- Calculate Final Derated Ampacity: 40A × 1.00 × 0.91 = 36.4A.
Because 36.4A is greater than the 60°C termination limit for 10 AWG (which is 30A), you are legally permitted to protect this circuit with a standard 30A breaker. However, if your attic reached 50°C (122°F), the temp correction factor would drop to 0.82. Your derated ampacity would be 40A × 0.82 = 32.8A. While still above 30A, you are getting dangerously close to the thermal limits, and an experienced electrician would upsize to 8 AWG to ensure long-term reliability and prevent nuisance tripping.
What This Chart Cannot Tell You: Voltage Drop and Termination Limits
A common mistake DIYers make is treating the cable size amperage chart as the final word on wire sizing. The chart only tells you the maximum current a wire can carry before its insulation fails. It tells you absolutely nothing about voltage drop over distance.
According to standard engineering reference data, all wire has inherent resistance. As circuit length increases, that resistance causes voltage to drop before it reaches the load. The NEC recommends (and many local codes mandate) keeping voltage drop under 3% for branch circuits and 5% overall from the service entrance to the furthest outlet.
The Voltage Drop Trap: A 100-Foot Run
Suppose you are wiring a 120V, 20A receptacle at the end of your driveway, exactly 100 feet from the panel. You look at the chart and see that 12 AWG copper is rated for 20A. You pull 12 AWG NM-B cable.
Here is the math on why that circuit will perform poorly:
- 12 AWG copper has a resistance of approximately 1.93 ohms per 1,000 feet.
- A 100-foot run requires 200 feet of total wire (hot and neutral returning).
- Total resistance = 1.93 × (200 / 1000) = 0.386 ohms.
- Voltage drop at a full 20A load = 20A × 0.386 ohms = 7.72 volts.
- Percentage drop = (7.72V / 120V) × 100 = 6.4%.
A 6.4% voltage drop exceeds the 3% recommendation. Your power tools will run hot, motors will struggle to start, and electronic devices may experience brownouts. To fix this, you must ignore the ampacity chart's minimum requirement and upsize to 8 AWG copper (or 6 AWG aluminum) purely to mitigate voltage drop, even though your breaker remains sized at 20A.
