About Conductor Ampacity Calculator (NEC 310.16)
The conductor ampacity calculator sizes building wire per the NEC. It embeds the 2023 Table 310.16 allowable ampacities for copper and aluminum conductors from 14 AWG to 500 kcmil in the 60 °C (TW), 75 °C (THWN), and 90 °C (THHN/XHHW-2) insulation columns, then applies the ambient temperature correction of 310.15(B)(1) and the adjustment factors of 310.15(C)(1) for more than three current-carrying conductors in a raceway or cable.
In sizing mode it walks the table upward to the smallest conductor whose fully derated ampacity covers the required load, including the 125% multiplier for continuous loads and the 110.14(C) rule that terminations rated 60 or 75 °C cap the usable ampacity regardless of the insulation column. In check mode it reports the corrected, adjusted, and termination-limited ampacity of a conductor you already have, with a utilization percentage against the load.
How It Works
- Enter the load current and mark it continuous if it runs for three hours or more; the required ampacity becomes 125% of the load per NEC 210.19(A) and 215.2(A).
- Select the conductor material, insulation temperature rating, ambient temperature, number of current-carrying conductors, and the termination rating of the connected equipment (60 °C for most gear up to 100 A, 75 °C above, per 110.14(C)).
- The calculator applies the ambient correction factor sqrt((Tc - Ta)/(Tc - 30)) and the bundling adjustment (0.8 for 4-6 conductors, 0.7 for 7-9, 0.5 for 10-20, down to 0.35 for 41+) to the base table ampacity.
- In sizing mode it selects the smallest size whose derated ampacity and termination-limited base ampacity both meet the requirement; in check mode it reports the allowable ampacity of the chosen size, the governing limit, and the utilization percentage.
Worked Example
A feeder carries a 40 A continuous load using copper THWN (75 °C) conductors in conduit with three current-carrying conductors at 30 °C ambient, landing on 75 °C terminations. The required ampacity is 1.25 x 40 = 50 A. At the table conditions the correction and adjustment factors are both 1.0, so the calculator walks Table 310.16: 14 AWG (20 A), 12 AWG (25 A), and 10 AWG (35 A) all fall short, while 8 AWG copper at 75 °C carries exactly 50 A, so 8 AWG is selected at 100% utilization. If the same circuit ran through a 40 °C attic with 90 °C THHN insulation, the correction factor would be sqrt((90 - 40)/60) = 0.913, giving 8 AWG THHN a derated ampacity of 0.913 x 55 = 50.2 A, still adequate but now also capped by the 75 °C termination value of 50 A.
Formulas
- Required ampacity (continuous load)
I_req = 1.25 * I_load (continuous); I_req = I_load (non-continuous)- Ambient temperature correction
F_temp = sqrt((Tc - Ta) / (Tc - 30))- Bundling adjustment (Table 310.15(C)(1))
F_adj = 1.0 (1-3), 0.8 (4-6), 0.7 (7-9), 0.5 (10-20), 0.45 (21-30), 0.4 (31-40), 0.35 (41+)- Derated and allowable ampacity
I_derated = I_table * F_temp * F_adj; I_allow = min(I_derated, I_termination)
Standards & References
- NEC 2023 Table 310.16 -- Allowable ampacities of insulated conductors
- NEC 310.15(B)(1) -- Ambient temperature correction factors
- NEC 310.15(C)(1) -- Adjustment for more than three conductors
- NEC 110.14(C) -- Temperature limitations at terminations
- NEC 210.19(A) / 215.2(A) -- Continuous load sizing
Frequently Asked Questions
Why can I not just use the 90 °C THHN ampacity column?
Because NEC 110.14(C) limits the usable ampacity to the temperature rating of the terminations, which are typically 60 °C for equipment rated 100 A or less and 75 °C above that. The 90 °C column may still be used as the starting point for derating, which is its real value: after ambient and bundling factors are applied, the result must simply not exceed the termination-column ampacity.
What is the 125% continuous load factor and when does it apply?
A continuous load is one expected to run at maximum current for three hours or more, such as lighting, EV charging, or water heating. NEC 210.19(A) and 215.2(A) require conductors (and standard breakers) to be sized at 125% of the continuous load plus 100% of the non-continuous load, so a 40 A continuous load needs a conductor good for 50 A.
How does ambient temperature change conductor ampacity?
Table 310.16 is based on 30 °C (86 °F) ambient. In hotter locations the conductor can dissipate less heat, so the ampacity is multiplied by sqrt((Tc - Ta)/(Tc - 30)), where Tc is the insulation rating. For 90 °C insulation at 40 °C ambient the factor is sqrt(50/60) = 0.913; at ambients below 30 °C the factor exceeds 1.0 and the ampacity increases.
When do bundling adjustment factors apply?
Whenever more than three current-carrying conductors share a raceway or cable, or are bundled longer than 24 inches, Table 310.15(C)(1) reduces the ampacity: 80% for 4-6 conductors, 70% for 7-9, 50% for 10-20, 45% for 21-30, 40% for 31-40, and 35% for 41 or more. Neutrals carrying only unbalanced current and equipment grounds generally do not count as current-carrying.
What is the difference between copper and aluminum ampacity?
Aluminum has higher resistivity, so an aluminum conductor of the same size carries roughly 20 to 25 percent less current: 1/0 aluminum at 75 °C is 120 A versus 150 A for copper. Aluminum is common for feeders and services because it is lighter and cheaper per amp, but it needs about two sizes larger than copper for the same load, and terminations must be rated AL/CU.
Does this calculator size the overcurrent device or account for voltage drop?
No. It sizes the conductor for ampacity only. The breaker or fuse must still be selected per NEC 240.4, including the small-conductor caps of 240.4(D) (15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG copper), and long runs should also be checked for voltage drop, which often governs before ampacity does. Use the voltage drop calculator alongside this tool.