A wire ampacity calculator tells you the maximum current a conductor can safely carry based on NEC Table 310.16 — the industry-standard reference for residential and commercial wiring. Selecting the correct wire gauge prevents overheating, insulation damage, and fire hazards. This tool applies temperature correction and conduit fill derating factors automatically so you get a safe, code-compliant result.
Wire & Installation Parameters
NEC 110.14(C): terminations limit effective ampacity to 60°C or 75°C even if wire is 90°C rated.
Standard reference is 30°C (86°F). Higher temps require derating.
Equipment grounding conductors (EGC) are not counted.
Ampacity Result
Derating Breakdown
NEC Table 310.16 Reference — Copper
| Wire Size | 60°C (A) | 75°C (A) | 90°C (A) |
|---|
Source: NEC Table 310.16, rated 0–2000V for conductors in raceway, cable, or earth. Not for free air installations.
How to Use the Wire Ampacity Calculator
Correctly sizing a wire for its load is one of the most fundamental electrical safety requirements. The National Electrical Code (NEC) Table 310.16 lists the maximum allowable current for copper and aluminum conductors under standard conditions — but real installations often require applying correction factors for high ambient temperatures and crowded conduits. This wire ampacity calculator handles all of that math in seconds.
Step 1: Select Wire Material and Size
Choose copper or aluminum from the material dropdown. Copper is standard for branch circuits and most residential wiring. Aluminum is common for service entrances, subpanel feeders, and large loads like electric dryers and ranges. Then select the wire gauge — AWG numbers go down as wire gets thicker (14 AWG is thinner than 2 AWG), while larger conductors use kcmil (thousand circular mils) designations starting at 250 kcmil.
Step 2: Choose the Insulation Temperature Rating
Most wire sold today is 90°C rated (THHN/THWN-2), but NEC 110.14(C) limits the actual working ampacity to the rating of the terminations — which are almost always rated at 75°C for circuits over 100A, or 60°C for smaller circuits. Select the temperature column that matches your termination rating. When in doubt, use 75°C for most residential and commercial applications.
Step 3: Enter the Ambient Temperature
NEC ampacity tables assume a 30°C (86°F) ambient temperature. If wiring runs through a hot attic, near a furnace, or in a climate with sustained high temperatures, the wire's ability to shed heat is reduced. The calculator applies the NEC Table 310.15(B)(1) correction factor automatically. For most conditioned spaces, 30°C is the standard default.
Step 4: Set the Number of Conductors in Conduit
When four or more current-carrying conductors share a conduit, raceway, or cable, they generate mutual heat that raises the conductor temperature above what the table assumes. NEC Table 310.15(C)(1) requires derating: 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20, and so on. Equipment grounding conductors (the bare or green wire) are not counted. A standard 3-wire (hot, neutral, ground) circuit uses only two current-carrying conductors — the hot and neutral — so no fill derating applies.
Step 5: Review the Results
The calculator shows the base ampacity from NEC Table 310.16, the fully derated ampacity after applying both correction factors, and the maximum recommended breaker size per NEC 240.4. The derating breakdown table shows each factor applied. A full NEC 310.16 reference table for the selected material is also displayed so you can compare adjacent wire sizes at a glance.
Understanding Breaker Size vs. Wire Ampacity
The breaker protects the wire, not the appliance. NEC 240.4 requires that the overcurrent device rating not exceed the wire's allowable ampacity. The calculator recommends the largest standard breaker size that stays at or below the derated ampacity. Note that some specific loads (motors, HVAC equipment) have their own sizing rules that may differ from the general table — always verify with the equipment nameplate and NEC Article 430 for motors.
FAQ
Is this wire ampacity calculator free?
Yes, this wire ampacity calculator is completely free with no account or signup required. All calculations run locally in your browser — your data is never sent to any server.
What NEC table does this calculator use?
This calculator uses NEC Table 310.16 (Allowable Ampacities of Insulated Conductors), which is the standard reference for copper and aluminum conductors rated 0–2000 volts in conduit, raceway, or cable. It is the most commonly applied ampacity table in residential and commercial wiring.
What is the difference between 60°C, 75°C, and 90°C temperature ratings?
These are the insulation temperature ratings for the conductor. Most residential wiring uses 60°C or 75°C rated wire. The 90°C column shows higher base ampacities, but NEC 110.14(C) limits terminations at panels and devices to 60°C or 75°C, so the 90°C column is mainly used to calculate derating — not as the final allowed current at terminations.
What is ampacity derating and when is it required?
Ampacity derating reduces the allowed current below the base table value when conditions increase heat buildup in the wire. Two main factors require derating: (1) elevated ambient temperature — when the surrounding air is hotter than 30°C (86°F), the wire cannot shed heat as effectively; and (2) conduit fill — when four or more current-carrying conductors share a conduit, they generate mutual heat and each conductor's ampacity must be reduced per NEC Table 310.15(C)(1).
What breaker size should I use?
Per NEC 240.4, the overcurrent protection (breaker or fuse) must not exceed the derated ampacity of the conductor. The calculator recommends the nearest standard breaker size at or below the derated ampacity. Standard sizes include 15A, 20A, 30A, 40A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A.
Can aluminum wire be used for residential wiring?
Yes, aluminum conductors are widely used for residential service entrances, subpanel feeds, and large appliance circuits (dryers, ranges, AC units). Aluminum has lower ampacity than copper for the same gauge, and connections require anti-oxidant compound and aluminum-rated terminals. For branch circuits supplying outlets, copper is generally preferred due to expansion/contraction and connection reliability concerns.
What does '4 or more conductors in conduit' mean?
NEC counts current-carrying conductors in a conduit to determine if a derating factor applies. If four or more current-carrying conductors share a single conduit, raceway, or cable (not counting equipment grounding conductors), the ampacity of each must be derated. The derating is 80% for 4–6 conductors, 70% for 7–9, and 50% for 10–20. This applies per NEC Table 310.15(C)(1).
Is this calculator a substitute for a licensed electrician?
No. This calculator is an educational reference tool based on NEC Table 310.16 values. Electrical work must comply with your local jurisdiction's adopted code version and may require permits and inspections. Always consult a licensed electrician for design, installation, and code compliance questions.