When you pull wire for a new circuit, guessing the gauge isn't just a code violation—it's a fire hazard. The definitive electrical wire sizing chart is published by the NFPA in the National Electrical Code (NEC) under Table 310.16. This table dictates the allowable ampacity for insulated conductors. The values below assume copper conductors, an ambient temperature of 30°C (86°F), and no more than three current-carrying conductors bundled in a single raceway or cable.
The Master Electrical Wire Sizing Chart (NEC Table 310.16)
Before using this chart, you need to understand how to read the temperature columns. Wire insulation is rated for specific maximum temperatures. Older UF-B cable is typically rated for 60°C. Standard THWN and older THW are rated for 75°C. Modern THHN/THWN-2 wire, which is the standard for conduit pulls today, is rated for 90°C. However, as we will cover below, the 90°C column is almost never used for final breaker sizing in residential work; it is primarily reserved as a baseline for calculating derating factors.
| AWG / kcmil | 60°C (140°F) | 75°C (167°F) | 90°C (194°F) | Common Residential Application |
|---|---|---|---|---|
| 14 | 15A | 20A | 25A | 15A Lighting / General Receptacles |
| 12 | 20A | 25A | 30A | 20A Kitchen / Bath Receptacles |
| 10 | 30A | 35A | 40A | 30A Dryer / Water Heater |
| 8 | 40A | 50A | 55A | 40A Range / Heat Pump |
| 6 | 55A | 65A | 75A | 50A-60A Subpanel Feeders |
| 4 | 70A | 85A | 95A | 70A-85A Heavy Subpanel Feeders |
| 3 | 85A | 100A | 110A | 100A Subpanel Feeders |
| 2 | 95A | 115A | 130A | 100A-125A Service Entrance |
| 1 | 110A | 130A | 145A | 125A Service Entrance |
| 1/0 | 125A | 150A | 170A | 150A Service Entrance |
Which Column Applies to Your Installation?
The most common mistake DIYers and junior apprentices make is looking at a spool of 90°C THHN wire, looking at the 90°C column, and assuming 12 AWG is good for 30A. It isn't. To determine which column governs your final breaker size, you must apply the "Weakest Link" rule defined in NFPA 70 National Electrical Code Article 110.14(C).
NEC 110.14(C) states that the temperature rating of the wire cannot exceed the temperature rating of the termination (the lug on the breaker or the terminal on the receptacle). Almost all residential breakers under 100A (like standard Square D QO or Eaton BR lines) and standard receptacles are rated for 75°C terminations. Therefore, even if your wire is rated for 90°C, your ampacity is capped by the 75°C column.
Furthermore, NEC 240.4(D) places hard limits on small conductors regardless of the termination rating. For 14, 12, and 10 AWG copper, the maximum overcurrent protection is strictly capped at 15A, 20A, and 30A respectively, effectively forcing you to use the 60°C column values for these sizes in standard branch circuits.
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For the most queried residential breaker sizes, here is your exact wire requirement based on the governing NEC columns:
- 15 Amp Breaker: Use 14 AWG. (Governed by 60°C column / NEC 240.4(D)).
- 20 Amp Breaker: Use 12 AWG. (Governed by 60°C column / NEC 240.4(D)).
- 30 Amp Breaker: Use 10 AWG. (Governed by 60°C column / NEC 240.4(D)).
- 40 Amp Breaker: Use 8 AWG. (Governed by 75°C column, allowing 50A, but 40A is the standard breaker match).
- 50 Amp Breaker: Use 6 AWG. (Governed by 75°C column, which allows 65A. 6 AWG is the standard, safe choice for 50A EV chargers and ranges).
- 100 Amp Subpanel: Use 3 AWG. (Governed by 75°C column, hitting exactly 100A. If using aluminum SER, you must step up to 1/0 AWG).
How Derating Modifies Your Base Ampacity
The electrical wire sizing chart above represents ideal conditions: a comfortable 30°C (86°F) ambient temperature and a maximum of three current-carrying conductors in a conduit. When real-world conditions deviate from this baseline, you must apply derating factors to the 90°C column to find your new adjusted ampacity, and then verify that adjusted ampacity still supports your breaker size.
Ambient Temperature Correction
If you run conduit through a hot attic that reaches 45°C (113°F) in the summer, the wire cannot dissipate heat as efficiently. According to NEC Table 310.15(B)(1), you must multiply the 90°C ampacity by 0.87. If you pull 10 AWG THHN (90°C rating = 40A) through that attic, your new ampacity is 34.8A. This is still safely above the 30A maximum for 10 AWG, so you can proceed. However, if the attic hits 55°C (131°F), the multiplier drops to 0.71. Your 10 AWG wire is now derated to 28.4A, meaning it can no longer legally be protected by a 30A breaker. You must upsize to 8 AWG.
Conductor Bundling (More Than 3 Wires)
When you pull more than three current-carrying conductors through a single raceway, they heat each other up. If you pull four current-carrying wires (for example, two separate 120V circuits where the neutrals are not shared in a multi-wire branch circuit configuration), you must apply an 80% derating factor.
Worked Example: You are pulling four 12 AWG THHN wires in a conduit. The 90°C column lists 12 AWG at 30A. Multiply 30A by 0.80 = 24A. Because 24A is greater than your 20A breaker size, the 12 AWG wire is still legally permitted. If you were pulling eight conductors (70% derating), 30A x 0.70 = 21A. You are now dangerously close to the 20A limit, and upsizing to 10 AWG is the professional move to prevent nuisance tripping and thermal degradation.
What the Table Cannot Tell You: Voltage Drop & Continuous Loads
NEC Table 310.16 is strictly a thermal safety chart. It tells you the size at which the wire will not melt or catch fire. It does not guarantee the voltage will actually reach the end of the run. For long runs, you must calculate voltage drop. A 120V circuit pulling 15A over 100 feet of 14 AWG wire will experience a voltage drop of roughly 7.7 volts (over 6%). While 14 AWG won't burn down the house on a 15A breaker, your power tools or appliances at the end of the line will run hot, slow, and inefficiently. For a 100-foot run, upsizing to 10 AWG or 8 AWG is required to keep the drop under the recommended 3% threshold. Tools like the Southwire Voltage Drop Calculator are essential for these long feeder calculations.
Finally, the table assumes non-continuous loads. If your load will run for three hours or more continuously (like a hardwired EV charger, baseboard heaters, or commercial lighting), NEC 210.20(A) requires you to multiply the load by 125%. A 20A continuous load requires wire and an overcurrent device rated for at least 25A. Since 25A is not a standard residential breaker size for small panels, you must step up to a 30A breaker and use 10 AWG wire, even though 12 AWG would technically handle the raw 20A thermal load. Always size for the continuous reality, not just the nameplate number.
