How to Use This Calculator
Wire size (gauge or AWG) is chosen based on the amperage the circuit carries and the ambient temperature where it runs. The NEC Table 310.16 (“Allowable Ampacities of Insulated Conductors…”) lists the maximum safe current for each wire size at standard conditions (60°C, 75°C, and 90°C insulation ratings, in free air or conduit). This calculator looks up those values and applies correction factors if your installation is hotter or more congested than the standard.
Enter your circuit current in amps, the ambient temperature of the installation area, and the number of conductors in the conduit or cable. The calculator adjusts the base ampacity for temperature (higher temps = lower ampacity) and for conduit fill (more wires crammed together = lower ampacity due to heat buildup). It then recommends the smallest wire size that safely handles your circuit.
Example: You’re running a 50-amp circuit 100 feet in a hot attic (95°F ambient). Base 50 amps at 75°C calls for 8 AWG copper (55 amps allowed). But at 95°F, the temperature correction factor is 0.91, so your effective ampacity is 55 × 0.91 = 50 amps—just barely adequate. If you add a third circuit in the same conduit, you derating for fill, and 8 AWG is no longer safe. You’d upsize to 6 AWG.
Formula
Adjusted Ampacity: Adjusted ampacity = table ampacity × temperature factor × fill factor
Where:
- Table ampacity is from NEC Table 310.16 at the conductor’s temperature rating (75°C or 90°C)
- Temperature factor is from NEC Table 310.15(B)(2)(a) — reduces at higher ambient temps
- Fill factor is from NEC Section 310.15(B)(3) — reduces when more than three current-carrying conductors are in the same conduit
Wire Selection: Choose the smallest wire size whose adjusted ampacity meets or exceeds your circuit current.
Standard ampacities at 75°C (in free air, 3 or fewer conductors):
- 14 AWG copper: 15 amps
- 12 AWG copper: 20 amps
- 10 AWG copper: 30 amps
- 8 AWG copper: 40 amps
- 6 AWG copper: 55 amps
- 4 AWG copper: 70 amps
- 2 AWG copper: 95 amps
When to Use This
Before every breaker installation and wire run. Undersizing causes overheating and fire. Oversizing wastes money and makes conduit crowded. The NEC enforces this strictly, and inspectors will check your work. A 20-amp circuit on 12 AWG is code-compliant and efficient. The same current on 14 AWG is a fire hazard and a violation.
Attics, sunlit exterior walls, and mechanical rooms run hot—you must derate. Four circuits bundled in one conduit must be derated. A 200-foot feeder from the main panel to a subpanel may require uprating to keep voltage drop reasonable. Get the wire size right and you protect equipment, satisfy code, and avoid callbacks.
Code References
- NEC Article 310.16: Allowable ampacities of insulated conductors rated 0-2000V
- NEC Table 310.15(B)(2)(a): Ambient temperature correction factors
- NEC Table 310.15(B)(3): Adjustment factors for conductor fill (more than 3 current-carrying conductors in conduit)
- NEC Article 110.6: Temperature rating of terminations must be matched to conductor and equipment
Frequently Asked Questions
Why is 12 AWG rated at 20 amps if a 20-amp breaker exists?
The breaker and wire both serve different purposes. A 20-amp breaker protects the circuit from overload. 12 AWG wire is rated to safely carry 20 amps continuously (at 75°C, in free air or under-filled conduit). If you run 12 AWG inside a hot conduit with five other wires, its effective ampacity drops below 20 amps due to derating, so the breaker setting may need to be reduced. Always check adjusted ampacity, not just the base rating.
Do I use 60°C, 75°C, or 90°C from the table?
Use the temperature rating marked on the conductor insulation. Most residential and commercial wire is 75°C rated (THHN, THW, XHHW). Some older wire is 60°C (Type T). Never size by the 90°C column alone—you’re allowed to use 90°C ampacities only if both the conductor and the termination (breaker, panel, equipment) are rated for 90°C. Most breakers and panels are 75°C, so use that column.
What if my attic gets to 110°F in summer?
Apply the temperature correction. At 110°F ambient, a 75°C-rated conductor loses about 20% of its ampacity. 8 AWG copper is normally 40 amps, but in a 110°F attic, it’s safely 40 × 0.80 = 32 amps. Size up to maintain safety. A attics in hot climates, 6 AWG becomes common for what would normally be 8 AWG circuits in cooler areas.
Can I put six circuits in one conduit if each is 14 AWG?
No. Putting 6 current-carrying conductors (the neutral doesn’t count as current-carrying for derating if used for a single phase) in one conduit triggers derating. Each conductor’s ampacity is reduced to 50% of its table value (per NEC Table 310.15(B)(3)(a)). A 14 AWG at 50% of 15 amps = 7.5 amps. That circuit could only handle 7.5 amps, making the 15-amp breaker unsafe. Don’t overcrowd conduit.
What about ground conductors—do they count toward fill?
No. The equipment ground (green) and any ground rods or grounds at the equipment don’t count as current-carrying conductors. Only conductors carrying current (hot, neutral in AC circuits, minus the neutral if it serves only one phase) trigger fill derating.