A Comprehensive Guide to Sizing Electrical Wire for Underground Circuit Cable

When it comes to installing underground wire for a residential circuit, using an underground feeder (UF) cable is the way to go. This type of cable is specifically designed for outdoor use and direct burial, making it ideal for running circuits to outdoor locations like garages, sheds, yard lights, or water features. Sizing the wires for an underground cable is crucial to ensure the safe and efficient operation of the circuit.

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Why Wire Sizing Matters

Sizing the wires, also known as conductors, for an underground circuit is similar to sizing for other household circuits. It is primarily based on the total load or electrical demand of the devices on the circuit. However, when it comes to underground runs, the length of the cable can play a significant role in determining the wire size needed. This is because of voltage drop, which is the loss of voltage in a circuit caused by the natural resistance in the wires.

Understanding Voltage Drop

Voltage drop is a critical factor to consider when sizing wires for an underground circuit. All conductors of electricity, including wires, have some resistance to the flow of electricity, which results in a loss of voltage. This loss is expressed as a percentage of the total voltage supplied at the power source of the circuit. Excessive voltage drop can lead to decreased performance, unnecessary wear, and premature failure of electrical equipment.

Causes of Voltage Drop

Voltage drop is primarily caused by the resistance of conductors. Factors that contribute to voltage drop include:

Length of the wire: The longer the wire, the greater the voltage drop.
Wire size: Smaller-diameter wires have more resistance than larger-diameter wires.
Material: Copper wire has lower resistance than aluminum wire.

While aluminum wire may show up on voltage drop tables, copper is typically the preferred choice for residential projects due to its lower resistance.

How Load Affects Voltage Drop

Voltage drop increases as the load on a circuit increases. Overloading a circuit can result in excessive voltage drop, affecting the efficiency of devices on the circuit. It is essential to keep the total load on a circuit to 80 percent or less of its total capacity to prevent unnecessary voltage drop.

Sizing the Conductors

The National Electrical Code (NEC) recommends a maximum voltage drop of 3 percent for individual household circuits, also known as branch circuits. When sizing the conductors for an underground cable, aiming for a 3 percent voltage drop is a good rule of thumb. The following are the maximum lengths of cable you can use while maintaining a 3 percent voltage drop for various wire sizes and circuit voltages:

For 120-volt circuits:
14 AWG: Up to 50 feet
For 240-volt circuits:
Recommendations vary based on wire size and specific installation circumstances.

Additional Resources

For more detailed information on sizing electrical wire for underground circuit cables, refer to the following resources:

United States Department of Energy: Provides guidelines for eliminating excessive voltage drops in in-plant distribution systems.
National Electric Code, 2020: Includes regulations for minimum ampacity and wire size for conductors.

By following these recommendations and guidelines, you can ensure the safe and efficient operation of your underground circuit cables. Whether you are running a circuit to an outdoor location or addressing voltage drop concerns, proper wire sizing is key to a successful electrical installation.