When sizing conductors for a branch circuit or feeder, the amp chart for wire is your foundational reference. Based on NEC Table 310.16 (NFPA 70), this chart dictates the maximum continuous current a wire can safely carry before its insulation degrades or the conductor overheats. For standard residential 15A and 20A circuits, you will typically use 14 AWG and 12 AWG copper wire, referencing the 60°C column. For larger 240V appliances like ranges or dryers, you will move up to 10 AWG, 8 AWG, or 6 AWG, often utilizing the 75°C column.
How to Read This Amp Chart for Wire
The most common mistake DIYers make when looking up wire ampacity is choosing the wrong temperature column. The amp chart for wire is divided into three temperature ratings: 60°C (140°F), 75°C (167°F), and 90°C (194°F). These columns represent the maximum temperature the wire's insulation can handle at the termination points (breakers, lugs, and receptacles), not just the wire itself.
- 60°C Column: Use this column for Non-Metallic Sheathed Cable (NM-B, commonly known as Romex) and any circuits rated 100 amps or less where the termination temperature is unknown. Even though modern NM-B wire has 90°C insulation, NEC Article 334.80 strictly limits its ampacity to the 60°C column.
- 75°C Column: Use this column for most modern breakers, SER (Service Entrance) cable, and THWN wire in wet locations. If you are wiring a 50A EV charger using individual THWN wires in conduit, and the breaker is rated for 75°C, this is your governing column.
- 90°C Column: Use this column for THHN/THWN-2 wire in dry locations. Crucial caveat: You almost never use the 90°C column to size your final breaker. It is used exclusively as the starting baseline for derating calculations (adjusting for heat buildup in bundled wires) before applying the termination limits of the 60°C or 75°C columns.
Complete NEC 310.16 Ampacity Table (Copper & Aluminum)
The following data is sourced directly from NFPA 70 (National Electrical Code) 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. For quick reference, the most commonly queried residential wire gauges are highlighted with bookmark-friendly IDs.
| Wire Size (AWG/kcmil) | Copper 60°C (NM-B) | Copper 75°C (THWN) | Copper 90°C (THHN) | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|
| 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 | 110A | 75A | 85A |
| 2 AWG | 95A | 115A | 130A | 90A | 100A |
| 1 AWG | 110A | 130A | 145A | 100A | 115A |
| 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 |
Note: Aluminum wire smaller than 8 AWG is generally not permitted for residential branch circuits. Always verify local codes regarding aluminum feeder usage.
Derating, Bundling, and What This Chart Cannot Tell You
The base values in the amp chart for wire assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When real-world conditions deviate from this baseline, you must apply derating factors found in NEC 310.15.
How Derating Modifies the Base Value
When you pull more than three current-carrying conductors through a single conduit, the heat generated by the wires cannot dissipate efficiently. You must multiply the 90°C column ampacity by a specific percentage. For example, if you pull four to six current-carrying conductors in a conduit, you multiply the 90°C ampacity by 80%.
Worked Example: You are running a multi-wire branch circuit (MWBC) with four current-carrying 12 AWG THHN wires in a single EMT conduit.
1. Base 90°C ampacity for 12 AWG = 30A.
2. Derating factor for 4-6 wires = 80%.
3. 30A × 0.80 = 24A.
4. Because 24A is greater than your 20A breaker requirement, 12 AWG is still legally permissible. However, if you added two more wires (totaling six), you would still be at 80%. But if you pulled 7-9 wires, the factor drops to 70% (30A × 0.70 = 21A), forcing you to upsize to 10 AWG to safely protect a 20A circuit.
What the Amp Chart Cannot Tell You
Relying solely on the amp chart for wire will leave critical gaps in your design. The table does not account for:
- Voltage Drop: A 4 AWG copper wire has an ampacity of 85A at 75°C. However, if you run it 200 feet to a 60A subpanel, the resistance of the wire will cause a voltage drop exceeding the recommended 3% threshold. You must calculate voltage drop separately using resistance values from NEC Chapter 9, Table 8, often requiring you to upsize to 2 AWG or 1 AWG despite the ampacity chart saying 4 AWG is sufficient for the heat limit.
- Conduit Fill: The chart tells you how much current the wire can handle, but NEC Chapter 9, Table 1 dictates how many wires physically fit inside a specific trade size of conduit (e.g., 40% fill limit for three or more wires). You cannot legally stuff twenty 12 AWG wires into a 1/2-inch EMT conduit, regardless of derating math.
- Physical Termination Limits: A 50A breaker might accept up to 4 AWG wire, but if you are using 8 AWG, you must ensure the specific lug on your equipment is rated to accept the smaller gauge securely without creating a high-resistance hot spot.
Frequently Asked Questions About Wire Ampacity
Can I use the 90°C column to size my breaker for NM-B Romex?
No. This is a frequent code violation. While modern NM-B (Romex) cable is manufactured with 90°C rated insulation, NEC Article 334.80 explicitly states that the ampacity of NM-B cable must be determined using the 60°C column. Therefore, 14 AWG NM-B is strictly limited to 15A, 12 AWG to 20A, and 10 AWG to 30A, regardless of the 90°C values printed on the cable jacket. The 90°C rating on Romex is only useful if you need to apply derating factors for bundling before hitting a junction box, but the final overcurrent protection must match the 60°C limits.
How does bundling wires in a conduit change the amp chart for wire values?
Bundling wires traps heat, which degrades insulation over time and increases fire risk. When you have more than three current-carrying conductors in a single raceway or cable, you must apply the adjustment factors from NEC Table 310.15(C)(1). You take the 90°C ampacity of the wire, multiply it by the derating percentage (e.g., 80% for 4-6 wires, 70% for 7-9 wires), and then ensure that resulting number is still higher than your breaker size and the termination temperature limits of your 60°C or 75°C equipment.
Why is my 50-amp hot tub wired with 6 AWG copper instead of 8 AWG?
According to the 75°C column, 8 AWG copper is rated for exactly 50A. Technically, 8 AWG meets the minimum ampacity requirement. However, most hot tub manufacturers explicitly require 6 AWG copper in their installation manuals to account for voltage drop over the distance from the panel to the outdoor spa, and to ensure a robust connection at the outdoor disconnect switch. Furthermore, many local AHJs have adopted blanket rules requiring 6 AWG for all 50A outdoor spa feeds to eliminate nuisance tripping and heating at the lugs. Always follow the manufacturer's minimum wire gauge specification, as it overrides the base NEC table under NEC 110.3(B).
Does the equipment grounding conductor count for derating?
No. When counting the number of current-carrying conductors in a conduit to determine your derating factor, the equipment grounding conductor (EGC) or bare copper ground wire is explicitly excluded. It only carries current during a fault condition, not during normal operation, so it does not contribute to the ambient heat buildup inside the conduit under normal load. For a standard 240V circuit with two hots and a ground, you only count two current-carrying conductors, meaning no derating is required.
