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Cable Ampacity Calculator

Determine cable ampacity with NEC derating factors for temperature, conduit fill, and continuous loads.

About this calculator

A wire's ampacity rating on a datasheet assumes ideal conditions, and this calculator walks backward from that base figure to what the conductor can actually carry in your installation, following the derating framework NFPA 70 (the National Electrical Code) sets out for exactly this purpose. It starts from a lookup table of base ampacities by AWG size for THHN or XHHW insulation (values that approximate the 90°C column of NEC Table 310.16), then applies two independent derating multipliers. The first is a temperature derating factor stepped at 30°C (no derating), 40°C (0.91×), 45°C (0.87×), 50°C (0.82×), and above 50°C (0.76×), reflecting that hotter ambient air limits how much additional heat the conductor's insulation can tolerate from current flow. The second is a conduit-fill derating based on how many current-carrying conductors share the raceway — three or fewer see no penalty, 4-6 conductors are derated to 80%, 7-9 to 70%, 10-20 to 50%, and more than 20 to 45%, per NEC Table 310.15(C)(1), since bundled conductors trap each other's heat.

Multiplying the base ampacity by both factors gives the Derated Ampacity. If the load is continuous (running 3+ hours), NEC 210.19(A)(1) requires sizing the conductor for 125% of the load, which this calculator implements by capping the usable Maximum Allowable Load at 80% of the derated ampacity — the reciprocal of that 125% rule. Finally it picks the next-larger standard breaker size at or above the derated ampacity. A frequent mistake is stacking corrections twice — this tool's derating is meant to be compared to actual load, not further multiplied by outside safety margins.

Inputs

AWG

NEC 310.16: #14 AWG=15A; #12=20A; #10=30A; #8=50A; #6=65A (60°C THHN, 3 conductors)

°F

NEC base: 30°C; 40°C: derate to 0.87×; 50°C: 0.75×; 60°C: 0.58× of base ampacity

conductors

NEC 310.15(C)(1): ≤3 = 100%; 4–6 = 80%; 7–9 = 70%; 10–20 = 50%; 21–30 = 45%

Results

Derated Ampacity

40 A

Required Breaker Size

40 A

Base Ampacity40 A
Maximum Allowable Load32 A

Figures current as of 2026. Source: National Fire Protection Association, NFPA 70, National Electrical Code (NEC), 2026 Edition

How to Use This Calculator
  1. Select the Conductor Size in AWG — smaller AWG numbers indicate larger wires (e.g., AWG 10 is larger than AWG 14).
  2. Select the Insulation Type: THHN is the most common general-purpose wire; XHHW is used in wet locations.
  3. Enter the Ambient Temperature in °C around the conductors — NEC base is 30°C; higher temperatures require derating.
  4. Enter the number of Current-Carrying Conductors in the raceway — three or fewer requires no fill derating.
  5. Indicate whether this is a Continuous Load (3+ hours) — NEC requires conductors to be derated to 80% for continuous loads.
  6. Read the Derated Ampacity in amperes and the Required Breaker Size to confirm the circuit is properly protected.

How the result changes with Conductors in Raceway

Conductors in RacewayDerated AmpacityRequired Breaker Size
1.540 A40 A
2.2540 A40 A
4.532 A35 A
7.528 A30 A

What each input means

Conductor Size (AWG)
AWG wire size per NEC Table 310.12 (residential) or Table 310.16 (commercial/industrial). 14, 12, 10, 8, 6, 4, 3, 2, 1, 0 (for 1/0). Smaller number = larger wire.
Insulation Type
Conductor insulation type per NEC Article 310. THHN: 90°C dry/75°C wet, most common in conduit. XHHW: 90°C dry/75°C wet, better moisture resistance for wet locations.
Ambient Temperature
Ambient temperature around the conductors. NEC Table 310.16 base is 30°C (86°F). Apply correction factors from NEC Table 310.15(B)(1) for higher ambient temperatures.
Conductors in Raceway
Total current-carrying conductors in the raceway per NEC Table 310.15(C)(1). ≤3 conductors = no derating; 4–6 = 80%; 7–9 = 70%; 10–20 = 50%.
Continuous Load?
Turn on if the load operates for 3+ hours continuously. NEC 210.19(A)(1) and 215.2(A)(1) require conductors sized to 125% of continuous load (equivalent to 80% conductor loading).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Conductor Size (AWG) = 10, Insulation Type = 1, Ambient Temperature = 30, Conductors in Raceway = 3 = 5 input(s) provided
  2. Calculate Derated Ampacity
    Derated Ampacity
    40 = 40
  3. Calculate Required Breaker Size
    Required Breaker Size
    40 = 40
  4. Calculate Base Ampacity
    Base Ampacity
    40 = 40
  5. Calculate Maximum Allowable Load
    Maximum Allowable Load
    32 = 32

Figures and sources

Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does switching Insulation Type from THHN to XHHW not change the result?

In this calculator's lookup tables, thhnAmpacities and xhhwAmpacities hold identical values for every AWG size — the code notes this reflects practical equivalence at the 90°C rating used here. In field practice the two insulation types can differ slightly on conductor outer diameter (which matters for conduit fill), but for base ampacity the tool treats them the same.

Do the temperature derating and conduit-fill derating stack, or is only the worse one applied?

They stack: Derated Ampacity = baseAmpacity × tempDerating × fillDerating, so both penalties multiply together rather than the calculator taking whichever is more severe. A hot ambient temperature (say 0.87×) combined with a crowded raceway (say 0.7×) compounds to roughly 0.61× of the base rating, not just 0.7×.

Why does turning on 'Continuous Load' reduce the Maximum Allowable Load instead of asking for a bigger wire?

The calculator implements NEC 210.19(A)(1)'s 125%-of-continuous-load conductor sizing rule by working backward: continuousFactor is set to 0.8 when the load runs 3+ hours, and Maximum Allowable Load is deratedAmpacity × continuousFactor. Capping usable load at 80% of the derated ampacity is mathematically the same requirement as sizing the conductor for 125% of the actual load — the tool just expresses it as a load ceiling rather than a wire-size multiplier.

How does the calculator pick the Required Breaker Size?

It walks a fixed list of standard breaker sizes (15, 20, 25, 30 A, and so on up to 600 A) in ascending order and picks the first one at or above the Derated Ampacity. That means the breaker is sized to the derated (already temperature- and fill-corrected) ampacity, not the raw base ampacity from the lookup table.

Where do these ampacity and derating figures actually come from?

They're drawn from NFPA 70, the National Electrical Code — specifically Table 310.16 for base conductor ampacity, Table 310.15(B)(1) for the ambient-temperature correction factors, and Table 310.15(C)(1) for the adjustment applied when more than three current-carrying conductors share a raceway. The 125% continuous-load rule this calculator implements as an 80% loading cap comes from NEC 210.19(A)(1) and 210.20.

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