When sizing conductors for a branch circuit or feeder, the direct answer lies in the National Electrical Code (NEC). For standard residential and commercial copper wiring, the definitive electrical cable size chart amps reference is NEC Table 310.16. As a baseline rule of thumb: a 15A circuit requires 14 AWG, a 20A circuit requires 12 AWG, a 30A circuit requires 10 AWG, and a 50A circuit requires 6 AWG copper wire. However, pulling the right wire requires understanding which temperature column applies to your specific terminations and how conduit bundling alters those base numbers.

The NEC 310.16 Electrical Cable Size Chart Amps Reference

The table below provides the allowable ampacities for insulated copper conductors rated up to 2000 volts, assuming an ambient temperature of 30°C (86°F). This data is sourced directly from the NFPA 70 (National Electrical Code) Table 310.16.

How to Read This Table: The table is divided into three temperature columns: 60°C, 75°C, and 90°C. These correspond to the insulation rating of the wire (e.g., TW is 60°C, THWN is 75°C, THHN is 90°C). Crucially, you cannot simply use the highest number. The column you are legally allowed to use is dictated by the lowest temperature rating of any connected termination, device, or splice in the circuit (per NEC 110.14(C)). Most modern breakers and receptacles are rated for 75°C, while older devices or specific small-gauge terminals are limited to 60°C. The 90°C column is almost exclusively used as the starting point for calculating derating factors before applying the termination limit.
AWG / kcmil 60°C (140°F)
TW, UF		 75°C (167°F)
THHW, THWN, XHHW		 90°C (194°F)
THHN, THWN-2, XHHW-2
14 AWG (15A/20A circuits)152025
12 AWG (20A circuits)202530
10 AWG (30A circuits)303540
8 AWG405055
6 AWG (50A/60A circuits)556575
4 AWG708595
3 AWG85100115
2 AWG95115130
1 AWG110130145
1/0 AWG125150170
2/0 AWG145175195
3/0 AWG165200225
4/0 AWG195230260

Bookmark the quick-jump rows above (14, 12, 10, and 6 AWG) as they cover 95% of standard residential branch circuit and subpanel feeder queries.

Applying Derating Factors to Your Base Ampacity

The numbers in the chart above assume ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together in a raceway or cable. When real-world conditions deviate from this, you must apply derating factors. This is where the 90°C column earns its keep.

Ambient Temperature Correction: If your conduit runs through a hot attic where temperatures reach 110°F (43°C), you must multiply the base ampacity by a correction factor. For 90°C wire at 110°F, the factor is 0.87.

Conduit Bundling (More than 3 Conductors): When you pull multiple circuits through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to reduce the allowable ampacity. For 4 to 6 current-carrying conductors, the adjustment factor is 80%. For 7 to 9 conductors, it drops to 70%.

Worked Derating Example: You are pulling four 12 AWG THHN (90°C) wires through a single EMT conduit for two 20A circuits.
  • Base ampacity from the 90°C column: 30A.
  • Bundling factor for 4 conductors: 80%.
  • Adjusted ampacity: 30A × 0.80 = 24A.
Because 24A is still greater than the 20A breaker protecting the circuit, 12 AWG is perfectly legal and safe here, even though the 75°C column only lists 25A. You are permitted to use the 90°C column for derating calculations, provided the final derated ampacity is sufficient for the overcurrent device.

What This Electrical Cable Size Chart Amps Cannot Tell You

While NEC Table 310.16 is the bible for thermal limits (preventing the wire insulation from melting), it is not a complete design tool. Relying solely on this chart will lead to three common field failures:

  1. Voltage Drop: The NEC ampacity tables do not account for voltage drop over long distances. A 10 AWG wire might safely carry 30A thermally, but if you run it 250 feet to a detached garage, the voltage drop will exceed the recommended 3% limit, causing motors to overheat and lights to dim. For long runs, use a dedicated voltage drop calculator and expect to upsize your wire by one or two AWG sizes.
  2. The 125% Continuous Load Rule: If a load will run for 3 hours or more (like a commercial HVAC unit, an EV charger, or continuous lighting), the NEC requires you to size the wire and breaker at 125% of the continuous load. A 40A continuous EV charger requires a 50A breaker and wire sized for 50A (which means 6 AWG copper at 75°C), not 8 AWG.
  3. Terminal Temperature Limits (The Weakest Link): As mentioned earlier, NEC 110.14(C) dictates that you cannot use a wire's ampacity if the lug it lands on cannot handle the heat. If you pull 90°C THHN wire into a main panel where the lugs are only rated for 75°C, your maximum allowable ampacity is capped at the 75°C column, regardless of the wire's superior insulation.

Frequently Asked Questions

What wire size for a 50 amp breaker?

For a standard 50-amp breaker, you need 6 AWG copper wire (assuming 75°C terminations, which yields 65A). If you are using aluminum wire (like SER cable for a subpanel), you must step up to 4 AWG aluminum, as aluminum has a lower ampacity per cross-sectional area. Always verify that the breaker and panel lugs are rated for 75°C; if they are older 60°C rated terminals, you would need 4 AWG copper.

Can I use the 90°C column for breaker terminations?

Almost never. Unless the breaker, lug, or device is explicitly stamped or documented by the manufacturer as rated for 90°C (which is exceedingly rare for standard residential/commercial gear), you are legally bound by NEC 110.14(C) to use the 60°C or 75°C column for your final ampacity limit. The 90°C column is practically reserved as a mathematical starting point for conduit derating and ambient temperature corrections.

Does the neutral wire count for conduit derating?

It depends on the circuit type. In a standard single-phase, 120/240V multi-wire branch circuit (MWBC) or a standard 2-wire circuit, the neutral only carries the unbalanced load and does not count as a current-carrying conductor for derating purposes. However, in a 3-phase, 4-wire wye circuit where the major portion of the load consists of nonlinear loads (like LED drivers, computers, or VFDs that generate triplen harmonics), the neutral carries additive harmonic currents. In that specific scenario, the neutral must be counted as a current-carrying conductor when applying bundling derating factors.