Cable Ampacity Calculator — Free Online Calculator
Find the maximum current rating (ampacity) for any wire gauge at different temperature ratings per NEC Table 310.16.
How to Use This Calculator
Select wire gauge, material, insulation temperature rating, and ambient temperature. The calculator returns the base ampacity from NEC Table 310.16 and applies temperature correction factors.
The Formula Explained
Ampacity values come from NEC Table 310.16, which lists maximum current for each conductor size based on insulation temperature rating and conductor material. When ambient temperature exceeds 30°C, correction factors from NEC Table 310.15(B)(1) reduce the allowable ampacity.
Understanding Ampacity vs. Current Rating
Ampacity is the maximum current a conductor can carry continuously without its insulation exceeding its rated temperature. It's not a fuse rating — a wire doesn't magically fail at 20.1 amps after being rated for 20. What happens is the insulation slowly degrades at elevated temperature, reducing its lifespan from 30 years to maybe 10, and increasing fire risk if any fault occurs. Ampacity is a lifetime-engineering number, not a fault-current number.
This distinction matters because different parts of the electrical system have different constraints. Breakers protect against faults — they trip when current exceeds their rating by a lot (5-10x for a few cycles, magnetic trip) or by a little for a long time (thermal trip, 125% for minutes to hours). Wire ampacity protects against sustained overload causing insulation breakdown. The two systems work together: the breaker limits maximum current, the wire is sized so the breaker's maximum current is below the ampacity.
Worked Example: Attic Feeder in Phoenix Summer
You're running a 60-amp subpanel feeder through a Phoenix attic. Ambient temperature in that attic can hit 60°C (140°F) in July. Starting point: 60A feeder, so baseline needs 6 AWG copper at 75°C (which gives 65A per Table 310.16). But that 65A assumes 30°C ambient.
Correction factor from NEC Table 310.15(B)(1)(1) for 60°C ambient with 75°C wire: 0.41 (drastic). Adjusted ampacity: 65 × 0.41 = 26.6A. The "60-amp" wire is actually a 26-amp wire in that attic. Not even close.
Solution: use 90°C wire (THHN or XHHW-2) and calculate at the 90°C column. Calibre 4 AWG THHN is 95A at 90°C, with correction factor 0.58 at 60°C ambient = 55A. Still not 60A. Bump to 3 AWG: 110A × 0.58 = 63.8A. That works. Alternative: reroute the conduit out of the attic through a wall cavity where ambient stays below 40°C. Engineering trade-off between wire size and install difficulty.
Worked Example: Direct Burial 100-Amp Feeder
A 100-amp feeder to a detached shop, direct buried 125 feet at 24-inch depth. Direct burial requires USE-2 or RHW-2 rated cable, both 90°C wet rated. Earth temperature at 24-inch depth averages about 20°C year-round in most of the continental US — actually cooler than the 30°C Table 310.16 baseline.
Starting point: 1 AWG copper at 90°C is 145A per Table 310.16. Ambient correction for 20°C ambient with 90°C insulation: 1.04 (slight increase). Adjusted ampacity: 145 × 1.04 = 150.8A. Way more than we need for 100A. The bottleneck isn't ampacity — it's voltage drop over 125 feet, which forces us to 2 AWG or 1 AWG for the drop calc. This is typical for long underground feeders: the earth's stable temperature gives you ampacity headroom, and voltage drop becomes the design driver.
Five Ampacity Calculation Errors
1. Using 90°C ampacity with 75°C terminations. Covered above but worth repeating because it's the single most common mistake. Your breaker lugs are 75°C rated, so the weakest link is 75°C, so you use the 75°C column.
2. Forgetting ambient temperature correction. Code tables assume 30°C (86°F) ambient. Attic, boiler room, outdoor in direct sun, industrial space with heat sources — all push ambient well above 30°C and require the correction factors in Table 310.15(B).
3. Counting only hot conductors for derating. Neutrals in single-phase circuits that carry only unbalanced current don't count. But neutrals in three-phase wye circuits with significant harmonic content (LED drivers, VFDs, switching power supplies) DO count — they can carry more current than the phase conductors due to triplen harmonics.
4. Ignoring bundled conductor derating. More than 3 current-carrying conductors bundled for more than 24 inches requires derating per Table 310.15(C)(1). 80% for 4-6, 70% for 7-9, 50% for 10-20, then it gets worse.
5. Stacking corrections and thinking they don't compound. If you have hot ambient AND bundled conductors, you multiply both factors. Example: 40°C ambient (0.88) × 5 conductors bundled (0.80) = 0.70 combined. That's a 30% ampacity reduction from the baseline.
Ampacity Best Practices
Always route conduit through cool spaces when possible. Wall cavities, basements, and below-slab runs stay close to baseline 30°C. Attics, above-ceiling spaces near roofs, and direct-sun exterior runs can hit 50-60°C. The ampacity penalty for hot routing can force you up a wire gauge or two.
Use 90°C wire even when limited to 75°C terminations. You pay almost nothing extra for THHN/THWN-2 vs plain THWN, and you gain margin when you apply correction factors. A 75°C wire derated to 0.82 for ambient is at 61.5% of its theoretical maximum. A 90°C wire derated the same way still has more absolute ampacity because its starting point was higher.
Design for worst-case ambient. If your attic hits 55°C on the hottest day of the year, the wire was at 55°C ambient for those few hours. Insulation damage is cumulative — minutes at high temperature add up over decades.
When in doubt, measure. Infrared thermometers are cheap. If you've got wire carrying unknown load at unknown ambient, point an IR gun at the conduit and see what temperature it actually reaches at maximum load.
NEC Ampacity Code References
Table 310.16 — raceway/cable ampacity for 60°C, 75°C, 90°C at 30°C ambient baseline. Table 310.17 — free air ampacity (higher values because of better heat dissipation). Table 310.15(B)(1)(1) — ambient temperature correction factors from 10°C to 80°C. Table 310.15(C)(1) — adjustment factors for more than 3 current-carrying conductors bundled.
110.14(C) — the critical rule that limits you to the temperature rating of the weakest termination in the circuit (usually 75°C for breakers and lugs, even when wire is 90°C rated). 240.4(D) — small conductor rule that overrides ampacity for 14/12/10 AWG. 310.12 — reduced ampacity allowance for single-phase residential services (allows using 75°C values for 120/240V single-phase dwelling services with specific size mapping).
Cable ampacity: from a single AWG number to a real-world current rating
Ampacity is the maximum continuous current a conductor can carry before its insulation overheats. The NEC publishes base ampacities in Table 310.16, but those numbers assume specific conditions: 30 C ambient air, not more than three current-carrying conductors in a raceway, and 75 C or 90 C insulation. Step outside those conditions and you have to derate.
This calculator starts from the table value and applies the correction factors that actually apply to your install: ambient temperature, number of bundled current-carrying conductors, and conduit on a sunlit roof (NEC 310.15(B)(2)).
The formula and what it does
The base ampacity in NEC 310.16 is multiplied by the ambient correction factor from 310.15(B)(1) and the adjustment factor from 310.15(C)(1). On a roof with conduit in sun, you also add a temperature adder from 310.15(B)(2) Table. The product is the ampacity for your specific install, which must equal or exceed the load current (with the 125 percent rule for continuous loads).
Worked example
Scenario: 6 AWG copper THHN, 5 current-carrying conductors in conduit, 40 C ambient (hot mechanical room).
Base 75 C ampacity (NEC 310.16): 65 A. Ambient correction for 40 C at 75 C insulation: 0.88. Adjustment for 5 conductors: 0.80. Final ampacity: 65 x 0.88 x 0.80 = 45.8 A. The conductor now safely carries 45 A, not the 65 A its bare gauge suggested. A 50 A breaker on this conductor is a code violation.
Code references and standards
NEC 310.16 is the base ampacity table for insulated conductors in raceway, cable, or earth. Three temperature columns (60, 75, 90 C). Termination temperature (110.14(C)) almost always forces you to 75 C.
NEC 310.15(B)(1) ambient temperature correction. The table maps ambient temperature to a multiplier; hot environments cut ampacity, cold environments raise it.
NEC 310.15(C)(1) bundling adjustment. 4-6 CCC: x0.80. 7-9 CCC: x0.70. 10-20: x0.50. The neutral counts as a current-carrying conductor on shared-neutral multiwire circuits.
NEC 310.15(B)(2) rooftop conduit. Adds 17-33 C to ambient depending on the height above roof, then you derate the new total. This catches a lot of solar PV installs where the DC conduit lies right on a hot membrane.
NEC 310.16 ampacity, 75 C copper column
| AWG | Ampacity (A) | Typical use |
|---|---|---|
| 14 | 15 | Lighting branch |
| 12 | 20 | General outlets |
| 10 | 30 | Dryer, water heater |
| 8 | 50 | 40 A EV charger, range |
| 6 | 65 | 60 A subpanel |
| 4 | 85 | 100 A feeder |
| 2 | 115 | 125 A service |
| 2/0 | 175 | 200 A copper service |
| 4/0 | 230 | 200 A aluminum SE |
Common mistakes to avoid
Ignoring derating entirely. The table value is the starting point, not the answer. Long subpanel feeders sharing a conduit, hot attics, and rooftop solar all trigger derating.
Confusing ampacity with breaker size. Ampacity is the conductor capacity. The breaker rating is the protection threshold. They are usually the same number but they answer different questions.
Frequently asked questions
How does conduit on a hot roof affect ampacity?
NEC 310.15(B)(2) adds 17-33 C to the ambient depending on height of conduit above roof. So 35 C ambient + 33 C adder = 68 C effective ambient, which slashes the 75 C-rated ampacity to roughly 35 percent of nameplate. Solar installers learned this the hard way.
Does a neutral count as a current-carrying conductor?
On a standard multiwire branch circuit (shared neutral, balanced loads) the neutral carries only imbalance and does not count. On a circuit with significant non-linear loads (LEDs, electronics, VFD outputs) the neutral can carry triplen harmonics and does count, per NEC 310.15(E).
What if my install runs cooler than 30 C?
You can take a positive correction factor. NEC 310.15(B)(1) gives multipliers above 1.0 for ambient below 30 C. In a freezing outdoor install (-10 C), 10 AWG copper ampacity bumps from 30 A to about 33 A.
Does the calculator apply the 90 C column then derate down?
You can do that math (NEC 310.15(B) lets you start from 90 C ampacity for derating then verify against the 75 C terminal limit at the load and source), but the calculator defaults to the conservative path of starting from the 75 C column. The conservative path is what most inspectors expect.
What is the difference between ampacity and continuous current rating?
Ampacity is the conductor capacity. Continuous current rating is the load expectation. NEC 210.19(A)(1) and 215.2(A)(1) require continuous loads (3+ hours) to be sized at 125 percent of load current. So a 32 A continuous load needs 40 A ampacity, even though the load is only 32 A.
Does cable type (NM-B vs THHN vs MC) change ampacity?
Yes. NM-B (Romex) is capped at the 60 C column per NEC 334.80, even though the conductor itself is rated 90 C. THHN/THWN-2 uses the 75 C column. MC cable follows the conductor insulation rating. The calculator usually defaults to THHN in conduit; pick NM-B if your install uses that cable type.
How precise are the table values?
They are conservative steady-state ratings based on conductor temperature reaching but not exceeding the insulation rating. Real-world ampacity at lower duty cycles is higher, but the table is what is code-compliant.