When sizing conductors for a residential or commercial branch circuit, the ampacity chart for copper wire (based on NEC Table 310.16) is your single source of truth. For standard home wiring using NM-B (Romex) cable, your baseline picks are: 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, and 6 AWG for 55A (protected by a 60A breaker). These values assume an ambient temperature of 30°C (86°F), copper conductors, and no more than three current-carrying conductors in a raceway or cable.
How to Read the NEC Ampacity Chart for Copper Wire
The most common mistake DIYers and junior apprentices make is looking at the 90°C column and assuming they can push that much current through the wire. You cannot. The National Electrical Code (NEC) dictates that the allowable ampacity is limited by the lowest temperature rating of any connected termination, device, or conductor in the circuit (NEC 110.14(C)).
Here is how to determine which column applies to your installation:
- 60°C Column: Use this for NM-B (Romex) cable, UF-B cable, and any circuit terminating on devices rated only for 60°C. Even though the individual THHN wires inside NM-B are technically rated for 90°C, NEC 334.80 strictly limits the overall cable ampacity to the 60°C column.
- 75°C Column: Use this for THHN/THWN-2 wires in conduit terminating on standard modern breakers, lugs, and receptacles rated for 75°C. This is your standard column for most commercial and residential conduit runs.
- 90°C Column: You can only use this column as a starting point for derating (adjusting for high ambient temperatures or bundling multiple wires). The final derated ampacity must still be compared to the 60°C or 75°C termination limits, and the lower value wins.
Complete Copper Wire Ampacity Table (NEC Table 310.16)
The following data is extracted from NFPA 70 (NEC) Table 310.16 for copper conductors with an ambient temperature of 30°C (86°F).
• 15A Lighting/Receptacles: 14 AWG (60°C col: 15A)
• 20A Kitchen/Bath/Standard: 12 AWG (60°C col: 20A)
• 30A Dryer/Water Heater: 10 AWG (60°C col: 30A)
• 60A Subpanel/EV Charger: 6 AWG (60°C col: 55A → next size up 60A breaker)
| AWG / kcmil | 60°C (140°F) NM-B, UF-B, TW |
75°C (167°F) THWN, XHHW, Terminations |
90°C (194°F) THHN, THWN-2 (Derating Base) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
| 4 AWG | 70A | 85A | 95A |
| 3 AWG | 85A | 100A | 110A |
| 2 AWG | 95A | 115A | 130A |
| 1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
| 3/0 AWG | 165A | 200A | 225A |
| 4/0 AWG | 195A | 230A | 260A |
Decision Path: Picking the Right AWG and Breaker
Use this decision tree to terminate your sizing process with a concrete part pick. The fundamental rule is that the overcurrent protective device (breaker) must protect the weakest link in the circuit, which is almost always the 60°C termination or the NM-B cable jacket.
| If Your Continuous + Non-Continuous Load Is... | And Your Wiring Method Is... | Then Pick This Wire Size... | And Protect With This Breaker |
|---|---|---|---|
| ≤ 12A (15A max circuit) | NM-B (Romex) | 14 AWG Copper | 15A Single-Pole |
| ≤ 16A (20A max circuit) | NM-B (Romex) | 12 AWG Copper | 20A Single/Double-Pole |
| ≤ 24A (30A max circuit) | NM-B (Romex) | 10 AWG Copper | 30A Double-Pole |
| ≤ 32A (40A max circuit) | THHN in Conduit | 8 AWG Copper (75°C col) | 40A Double-Pole |
| ≤ 40A (50A max circuit) | THHN in Conduit | 6 AWG Copper (75°C col) | 50A Double-Pole |
| ≤ 48A (60A max circuit) | THHN in Conduit | 6 AWG Copper (75°C col = 65A) | 60A Double-Pole |
Note on the 60A pick: NEC 240.4(B) allows the 'next size up' rule. Since 6 AWG in the 75°C column is rated for 65A, and 65A breakers do not exist as standard sizes, you are legally permitted to protect it with a 60A breaker. If using NM-B for a 60A circuit, you must step up to 4 AWG (rated 70A at 60°C).
Derating and Adjustments: When the Base Chart Fails
The ampacity chart above assumes ideal conditions: 30°C ambient air and a maximum of three current-carrying conductors (CCCs) bundled together. When you exceed these parameters, the wire cannot dissipate heat efficiently, and you must apply derating factors to the 90°C column (even if your terminations are 75°C). For detailed bundling rules, refer to the Copper Development Association's building wire guidelines.
Worked Numeric Example (Bundling):
You are pulling six current-carrying conductors (two 3-phase circuits, no neutral) through a single EMT conduit to a workshop subpanel. You want to use 10 AWG THHN copper for a 30A load.
- Base Ampacity: 10 AWG in the 90°C column is 40A.
- Adjustment Factor: NEC Table 310.15(C)(1) states that 4 to 6 CCCs require an 80% derating factor.
- Derated Ampacity: 40A × 0.80 = 32A.
- Termination Check: The breaker lugs are rated 75°C. The 75°C column for 10 AWG is 35A.
- Final Verdict: The derated value (32A) is lower than the termination value (35A). Therefore, the wire's true allowable ampacity is 32A. You cannot put this on a 35A or 40A breaker. You must protect it with a 30A breaker. If your calculated load is exactly 30A, 10 AWG passes. If the load is 32A, you must step up to 8 AWG.
What This Table Cannot Tell You
While the NEC ampacity chart dictates the thermal limits of the wire insulation, it completely ignores voltage drop. A 14 AWG wire might be thermally safe carrying 15A over a 200-foot run, but the resistance of that much copper will cause a massive voltage drop, starving your appliances and causing motors to overheat.
As a rule of thumb, limit voltage drop to 3% for branch circuits and 5% for the total feeder + branch combined. If you are running a 120V, 15A circuit to a detached shed 150 feet away, the ampacity chart says 14 AWG is fine. However, voltage drop calculators will show a 6.4% drop. To fix this, you must upsize to 10 AWG or even 8 AWG purely for voltage regulation, despite the breaker remaining at 15A or 20A.
Furthermore, this table does not account for physical lug constraints. A 4/0 AWG copper wire carries 230A at 75°C, but it will not physically fit into the lugs of a standard 200A residential main breaker without using a mechanical lug reducer or a specialized terminal block. Always check the manufacturer's datasheet for the specific panelboard or breaker to verify maximum wire sizing.
The Default Recommendation: When in doubt for standard 120V/240V residential receptacles and lighting, default to 12 AWG copper NM-B on a 20A breaker. The marginal material cost increase over 14 AWG is roughly $15 to $20 per 250-foot roll, but it inherently mitigates voltage drop on long runs, provides a safer thermal buffer for continuous loads, and allows future circuit upgrades without pulling new wire.
