Cable Ampacity Calculator

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

Inputs

Conductor Size (AWG)

AWG

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

Insulation Type

(1=THHN, 2=XHHW)

NEC 310.4: THHN ampacity based on 75°C column for most terminations (NEC 110.14(C))

Ambient Temperature

°F

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

Conductors in Raceway

conductors

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

Continuous Load?

(0=no, 1=yes)

NEC 210.19: continuous load conductor = 125% of load; breaker = 125% of continuous load

Tab to move between fields

Results

Derated Ampacity

40 A

Required Breaker Size

40 A

Base Ampacity40 A

Maximum Allowable Load32 A

How to Use This Calculator

Select the Conductor Size in AWG — smaller AWG numbers indicate larger wires (e.g., AWG 10 is larger than AWG 14).
Select the Insulation Type: THHN is the most common general-purpose wire; XHHW is used in wet locations.
Enter the Ambient Temperature in °C around the conductors — NEC base is 30°C; higher temperatures require derating.
Enter the number of Current-Carrying Conductors in the raceway — three or fewer requires no fill derating.
Indicate whether this is a Continuous Load (3+ hours) — NEC requires conductors to be derated to 80% for continuous loads.
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 Raceway	Derated Ampacity	Required Breaker Size
4.9	32 A	35 A
15	20 A	20 A
26	18 A	20 A
36	18 A	20 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?: Select 1 if 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

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

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