In this article, we show a guide for selecting the appropriate electrical conductor size according to the NEC (National Electrical Code), based on the load’s amperage.
Determine the Load’s Amperage
Identify the current (in amperes) required by your load. This can be found in the equipment specifications from the manufacturer or calculated based on the power and voltage.
Here is the basis for selecting the proper conductor: avoid choosing one with a lower amperage rating, as this could lead to overheating, and avoid one that’s too large, as it will significantly increase costs.
Check the NEC Table for electrical conductor size
The NEC provides tables that match amperage with conductor sizes. Table 310.15 in the NEC is commonly used for copper or aluminum conductors, and it takes temperature and insulation type into account.
Look up your required amperage in the table, under the correct temperature and insulation category.
You multiply the load’s amperage by 1.25, then look up the result in the NEC ampacity table.
You should select the conductor immediately above the value obtained. If the installation is expected to expand in the future, it’s recommended to choose the next larger conductor to avoid issues with future expansion.
Select the Conductor’s Insulation Type
Choose the insulation based on the maximum temperature the conductor will be exposed to. Common insulation types include:
THHN: Rated up to 90 °C, often used in dry conduit applications.
XHHW: Rated up to 90 °C, suitable for wet environments.
Make sure to select the appropriate insulation for the location and environmental conditions.
To use a temperature rating above 75°C, you must be sure that the terminals of the devices being used can withstand that temperature, as most are designed to operate at 75°C.
Adjust for Cable Length (Voltage Drop)
For long cable runs, consider voltage drop. The NEC recommends keeping voltage drop below 3% for lighting circuits and 5% for other circuits.
Apply Correction Factors for electrical conductor size
Apply temperature and conductor bundling correction factors from NEC Tables 310.15(B)(2)(a) and 310.15(B)(3)(a). For conductors in higher temperatures or grouped with other conductors, the current-carrying capacity decreases.
Multiply these factors by the rated ampacity to get the corrected ampacity.
Example Selection
Suppose you have a load that requires 50 amperes, and you plan to use a copper conductor with THHN insulation in an ambient temperature of 30 °C.
I = 50 * 1.25 = 62.5 A
Refer to the NEC conductor table, where you’ll see that for 62.5 amperes, a #6 AWG copper conductor with THHN insulation is sufficient, 70 °C
Apply any correction factors as needed based on temperature or bundling.
Following these steps will help you make an initial selection of the appropriate conductor based on NEC requirements.
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