Sizing Electrical Wire for Underground Circuit Cable

Underground wire for a residential circuit usually is installed with underground feeder (UF) cable, which is rated for outdoor use and direct burial. Such an installation is typical when running a circuit to an outdoor location, such as a garage, shed or other outbuilding, or to a yard light or water feature. Sizing the wires, or conductors, for an underground cable is no different than sizing for other household circuits and is typically based on the total load, or electrical demand, of the devices on the circuit. However, if the cable run is long, as underground runs often are, you may need to increase the wire size to account for voltage drop—a loss of voltage in the circuit caused by the natural resistance in the wires. Voltage drop occurs in any wire run, but it is rarely a problem with indoor wiring, where the distance from the circuit breaker panel to the end of the circuit is usually relatively short. Outdoor circuits, however, are often quite long and the voltage drop can be significant.

Understanding Voltage Drop

All conductors of electricity, including wires, impose some resistance to the flow of electricity. One effect of this resistance, also called impedance, is a loss of voltage. This is known as voltage drop and is represented as a percentage of the total voltage supplied at the power source of the circuit. If you measure the voltage of a circuit at the service panel (breaker box), you should get a reading of around 120 volts (for a standard circuit). If you take another measurement of the circuit at the farthest device from the panel and get a reading of 114 volts—a difference of 6 volts—that circuit has a 5 percent voltage drop (5 percent of 120 = 6).

Excessive voltage drop means that motors, appliances, and other devices don't run as fast or efficiently as they are designed to, even with dedicated electrical circuits. This can lead to decreased performance, unnecessary wear, and even premature failure of electrical equipment. Voltage drop is also a waste of electricity because the energy is lost as heat rather than being available for use by the circuit devices.

Causes of Voltage Drop

Since voltage drop is caused by the resistance of conductors, the more conductors you have, the greater the voltage drop. When it comes to underground wire, the longer the wire, the greater the voltage drop. Wire size is another factor: Smaller-diameter wires have more resistance than larger-diameter wires. Copper wire has lower resistance than aluminum wire, but chances are good that you'll be using copper in any case. These days, the only aluminum used in most new residential projects is in the service entrance cables from the utility, although you may see aluminum show up on voltage drop tables.

How Load Affects Voltage Drop

Voltage drop increases as the load on a circuit increases, and overloading a circuit contributes to excessive voltage drop. In other words, if you put too many loads on one circuit and exceed the standard 80 percent safe capacity (1,440 watts for 15-amp circuits; 1,920 watts for 20-amp circuits), you'll add unnecessary voltage drop. The solution is simple: Keep the total load on the circuit to 80 percent or less of total capacity. This condition is assumed in many voltage drop calculations and tables.

Sizing the Conductors

The National Electrical Code (NEC) recommends a maximum voltage drop of 3 percent for individual household circuits (known as branch circuits). This is a good goal to shoot for when sizing the conductors for an underground cable. The following are the maximum lengths of cable you can use while still maintaining a 3 percent voltage drop for the given wire size (AWG) and circuit voltage. As an example, for a 120-volt circuit, you can run up to 50 feet of 14 AWG cable without exceeding 3 percent voltage drop.

For 120-volt circuits:

For 240-volt circuits: