Ampacity Calculator
The purpose of this calculator is to determine the ampacity of conductors in conduit, cable, or directly buried. All calculations are based on the National Electrical Code®.
Information about the ampacity calculation methodology and code requirements is included below the calculator.
WIRE / CABLE TYPE:
While high-performance conductors such as THHN/THWN-2, XHHW-2, and USE-2/RHH/RHW-2 feature a 90°C insulation rating—allowing for temperature corrections and ampacity adjustments from that baseline—the final allowable ampacity is often dictated by the connected equipment.
Per NEC 110.14(C)(1), thermal limitations at terminal points (like circuit breakers or panelboards) are critical. Unless otherwise specified, conductors sized 14 AWG through 1 AWG are assumed to be connected to 60°C terminals, while conductors 1/0 AWG and larger default to a 75°C rating. Because many 600-volt panelboards are capped at 75°C, it is essential to verify actual equipment ratings to ensure the entire system operates within safe thermal bounds.
Certain cable types may also have a specified maximum temperature limit. For example, Type NM-B is limited to the ampacity values in the 60℃ column, even though the insulation on the conductors is required to be rated at 90℃.
Note: The calculator will permit choosing NM-B and UF-B up to 2 AWG. NM-B is permitted to be manufactured with conductors as large as 2 AWG, but multiconductor UF-B is limited to conductors no larger than 6 AWG by the UL standard.
AMBIENT TEMPERATURE:
The environment surrounding a wiring method can affect the ampacity of the conductors. NEC® Table 310.15(B)(16) ampacities are based on an ambient temperature of 86℉. If the temperature is greater than 86℉, the ampacity from the table must be corrected based on the values found in NEC Table 310.15(B)(2)(a). If the temperature is lower than 78℉, the ampacity is permitted to be corrected in accordance with the values in that table.
NUMBER OF CURRENT-CARRYING CONDUCTORS:
There are times when it is necessary or desired to run two or more conductors in parallel to carry the required current for a circuit. Paralleled conductors are permitted if sized at 1/0 AWG or larger. The number of parallel runs is the number of separate conductors installed within each phase, polarity, etc. There are several restrictions for the installation of paralleled conductors that can be found in NEC® 310.10(H).
OUTPUT PAGE:
The calculated ampacity value is the minimum of: the corrected and adjusted ampacity; the equipment temperature limitation default values as described in NEC® 110.14(C)(1); or the cable type temperature limitation. The initial input values are also shown for verification, and they will be emailed along with the calculated ampacity if the email option is chosen. The insulation temperature rating is the rated temperature of the insulation surrounding the conductor metal, and the termination temperature rating is based on the default values shown in NEC 110.14(C)(1). If a cable wiring method was chosen, the temperature limit is 60℃ for NM-B and UF-B, and it is 75℃ for SER, SEU and MC cable.
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