How to Use the Wire Gauge Calculator
The calculator operates in two modes. In Amperage → AWG mode, enter the load amperage, pick copper or aluminum, select your temperature rating, and the tool returns the smallest NEC-compliant conductor size. In AWG → Ampacity mode, select a conductor size and material to see the full ampacity at all three temperature ratings.
The 80% continuous load rule is critical: NEC 210.19(A) and 215.2(A) require that conductors for continuous loads (3+ hours) be sized at 125% of the load current. The calculator flags when your input triggers this requirement and shows the adjusted minimum AWG.
For circuits with more than three current-carrying conductors in a conduit, NEC Table 310.15(B)(3)(a) requires derating. The calculator shows a note when this applies; multiply the base ampacity by the applicable correction factor (e.g., 0.80 for 4–6 conductors).
Worked example: 32 A EV charger circuit
An electrician is wiring a Level 2 EV charger that draws 32 A for hours at a time. The run uses copper THWN conductors landing on 75°C terminals.
- Mode: Amps to AWG, copper, 75°C, load 32 A
- Continuous load ticked: 32 A × 1.25 = 40.0 A required ampacity
- Result: 8 AWG copper, ampacity 50 A at 75°C
With the continuous box left unticked, the same inputs return 10 AWG, rated 35 A at 75°C. That is below the 40 A the 125% rule requires, which is why the checkbox matters for EV chargers and any other load that runs for 3 hours or more. Before you install, also check conduit fill, ambient temperature and voltage drop on the run.
NEC 310.15 Ampacity Tables Explained
NEC Table 310.15(B)(16) — formerly Table 310.16 — lists the allowable ampacities for insulated conductors in conduit, cable or earth at an ambient temperature of 30°C (86°F). There are three temperature columns because different insulation types have different maximum rated temperatures:
- 60°C (140°F) — used for older wiring and some specific applications. Typical insulation: TW, UF.
- 75°C (167°F) — the most common rating for commercial and industrial work. Typical insulation: THW, THWN, XHHW.
- 90°C (194°F) — higher capacity, but the termination temperature rating limits actual use to 75°C in most cases. Typical insulation: THHN, THWN-2, XHHW-2.
Aluminum conductors are two AWG sizes larger than equivalent copper for the same ampacity. Always check that terminals are rated for aluminum if using AL conductors.
Wire Sizing Best Practices for Electricians
Correct wire sizing protects people and property. Undersized conductors overheat, degrading insulation and potentially causing fires. Here are the key rules to remember:
- Continuous loads: Size at 125% (divide load by 0.8) before looking up AWG.
- Conduit fill: Derate for more than 3 current-carrying conductors per NEC Table 310.15(B)(3)(a).
- Ambient temperature: Derate per NEC Table 310.15(B)(2)(a) if ambient exceeds 30°C.
- Aluminum terminations: Use anti-oxidant compound and AL-rated lugs.
- Minimum sizes: 14 AWG copper minimum for 15 A branch circuits; 12 AWG for 20 A.
- Voltage drop: NEC recommends keeping total drop under 5% (3% on branch, 2% on feeder). Use the Voltage Drop Calculator to verify long runs.
Always consult the current NEC edition and local amendments. Some jurisdictions adopt amendments that change minimum sizes or add requirements.
Frequently asked questions
What AWG wire do I need for a 20-amp circuit?
For a standard 20-amp branch circuit, NEC requires a minimum of 12 AWG copper (or 10 AWG aluminum) at 60°C. If the load is continuous (running 3+ hours), size the conductor at 125% of the load — so a 20 A continuous load needs 25 A ampacity, which is 10 AWG copper at 60°C or 12 AWG at 75°C.
Why does the calculator show a larger AWG than I expected?
The most common reason is the 80% continuous load rule. If your load runs continuously, NEC requires the conductor to carry 125% of the load current. There may also be a conduit fill derate or high ambient temperature adjustment applied.
Can I use 90°C-rated wire at its full 90°C ampacity?
Usually no. NEC 110.14(C) limits the conductor to the lowest temperature rating of any connected termination. Most residential and commercial terminals are rated 60°C or 75°C. You can use 90°C wire but must apply the 75°C column ampacity when connected to 75°C-rated terminals.
Is aluminum wire safe for residential use?
Aluminum is safe when installed correctly with AL-rated terminals, proper torque, anti-oxidant compound and the correct conductor size (typically 2 AWG sizes larger than equivalent copper). It is widely used for service entrances, feeders and large appliance circuits. Avoid 15 A and 20 A branch circuit aluminum wiring without special AL-rated devices.
What does "derate for conduit fill" mean?
When more than 3 current-carrying conductors share a conduit, they generate more heat collectively than the table assumes. NEC Table 310.15(B)(3)(a) requires multiplying the base ampacity by a correction factor: 0.80 for 4–6 conductors, 0.70 for 7–9, 0.50 for 10–20, etc.