Ampacity Calculator

Ampacity Calculator Information & Instructions:

This app uses the calculation requirements found in the 2017 National Electrical Code® to determine the ampacity of insulated conductors and certain cable-type wiring methods. This calculator also complies with the 2020 NEC requirements. Certain installations (such as cable trays and medium-voltage conductors) are not included in this app. Insulated single conductors are assumed to be installed in conduit or direct buried. All calculations assume a voltage of no more than 2000 volts and that the appropriate NEC® requirements are followed by the installer, engineer or other responsible party. For more information on ampacity calculations, please consult our technical resources.

The primary two metals used for electrical conductors are copper and aluminum. Each has a different ampacity for a specified gauge size, and these ampacities are found in NEC® Table 310.15(B)(16) for the calculations in this app. For example, 3/0 AWG copper has an ampacity of 200 amps at 75℃, while 3/0 AWG aluminum has an ampacity of 155 amps at 75℃.

WIRE / CABLE TYPE:
Insulated wire types like THHN/THWN-2, XHHW-2 and USE-2/RHH/RHW-2 have an insulation temperature rating of 90℃, and any temperature correction factors or ampacity adjustment factors can be applied to the insulation temperature rating. The resulting value is the ampacity of the wire installed throughout the building or other installation.

However, electrical equipment (such as panelboards or circuit breakers) typically has an additional limiting temperature rating that must be considered when determining the ampacity of the wire that connects to the equipment. As an example, 600-volt panelboards have a maximum temperature limit of 75℃. Based on NEC 110.14(C)(1), conductors sized 14 AWG to 1 AWG are assumed to be connected to equipment limited to 60℃, and conductors 1/0 AWG and larger are assumed to be connected to equipment limited to 75℃. The installed equipment may have different temperature limitations, so be sure to check the actual ratings.

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℃.

This app assumes installation in conduit or direct burial when individual insulated wire types are chosen. The app limits the final ampacity of the wire to the most conservative 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.

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:
The number of current-carrying conductors in a wiring method can affect the ampacity of the conductors. NEC® Table 310.15(B)(16) ampacities are based on no more than three current-carrying conductors. If there are more than three current- carrying conductors, the ampacity from the table must be adjusted based on the values found in NEC Table 310.15(B)(3)(a). Adjustment factors range from 80% for 4 to 6 conductors, all the way to 35% for 41 or more conductors.

Do not include equipment grounding conductors, and do not include neutral conductors that meet the exception conditions in NEC 310.15(B)(5). Do include “spare” conductors that are intended to be energized at some point in the future. If there are multiple sets of conductors in a wiring method that supply electrical components that cannot be simultaneously energized, only the number of conductors that will be energized at any one time must be counted. Use the most conservative conditions anticipated to avoid overheating the conductors and creating a hazardous condition. 

NUMBER OF PARALLEL RUNS:
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.