A cable can have sufficient ampacity for a load and still be a poor choice for a long underground run. The reason is voltage drop. Every conductor has resistance. As current travels through the conductor, some voltage is lost along the way. The farther the electrical equipment is from the source, the more important that loss becomes.
Direct-burial and underground installations frequently involve long distances across commercial properties, industrial plants, campuses, utilities, outdoor equipment installations and infrastructure sites. That can turn voltage drop into a major design consideration even when the conductor would otherwise have adequate current-carrying capacity.
What Is Voltage Drop?
Voltage drop is the reduction in voltage between the source of a circuit and the equipment receiving power. Conductor resistance, current and circuit length all affect the amount of drop. Increasing the distance increases resistance. Increasing the current increases the voltage lost across that resistance.
A load at the far end of a long cable run may therefore receive less voltage than the source is supplying. Whether that difference matters depends on the equipment.
Some loads tolerate modest voltage variation without noticeable effects. Motors, controls, lighting, electronic equipment and other devices can be more sensitive, particularly when the voltage at the equipment falls outside the range expected by its manufacturer.
Is There a Maximum Acceptable Voltage Drop?
There is no single percentage that answers every installation. A commonly referenced design guideline is to limit voltage drop to about 3 percent on a branch circuit and approximately 5 percent across the combined feeder and branch circuit to the farthest load. These percentages are often used as targets for reasonable operating efficiency rather than as a universal rule that determines whether every circuit passes or fails electrical code requirements.
Project specifications, equipment requirements and applicable electrical codes can require different treatment. That is why voltage drop should be evaluated as part of the electrical design rather than treated as one fixed number that applies to every underground cable run.
Why Underground Runs Bring the Issue to the Surface
Distance is frequently the deciding factor. Inside a building, a branch circuit may travel a relatively modest distance from a panel to the equipment. Underground power distribution can extend hundreds of feet across a property.
A conductor selected only from an ampacity table may be capable of carrying the current safely but still lose more voltage over that distance than the project allows. This is one reason underground cable selection should consider run length before the cable is purchased and installed.
Once a direct-burial installation has been trenched, placed and covered, correcting an undersized conductor can become far more expensive than addressing voltage drop during design.
Conductor Size Can Reduce Voltage Drop
Increasing conductor size reduces conductor resistance. For a long run, that can mean using a larger conductor than the minimum size that would otherwise be selected based on ampacity.
That does not mean every long cable run needs to be oversized. The required size depends on the source voltage, current, circuit length, conductor material, load characteristics and acceptable voltage at the equipment. A voltage-drop calculation allows those factors to be considered together.
Copper and Aluminum Need to Be Evaluated Separately
Conductor material matters too. Copper has lower electrical resistance than aluminum for an equivalent conductor size. Aluminum is widely used for feeders, distribution circuits and underground installations, but an aluminum conductor may need a larger cross-sectional area than copper when the design is targeting similar electrical performance.
Cost, conductor weight, installation conditions, termination requirements and available cable constructions may all influence the final decision. The important point is that a cable should not be sized by simply transferring the same AWG number from one conductor material to another.
When considering aluminum cable for a long underground run, the voltage-drop calculation should use the properties of the conductor being specified.
Load Matters as Much as Distance
A 300-foot cable run carrying a light load is a different electrical problem from the same 300-foot run operating near its full design current. As current increases, so does voltage drop.
Designers therefore need a realistic understanding of the expected load rather than simply the maximum voltage rating printed on the cable. The type of equipment should also be considered. Motors and other equipment may have operating characteristics that make adequate voltage at the load particularly important.
The Voltage Rating of the Cable Does Not Solve Voltage Drop
A common misunderstanding is to confuse cable voltage rating with the amount of voltage that will reach the load. A cable rated for 600 volts, for example, is designed for circuits within its applicable voltage rating. That designation does not mean voltage will remain constant across a long run.
Voltage drop is determined by the electrical characteristics of the circuit, including resistance, current and distance. The insulation rating and voltage-drop calculation answer two different questions.
Underground Conditions Still Matter After the Calculation
Solving the voltage-drop problem does not automatically mean the cable is appropriate for burial. The cable must still be suitable for the installation environment.
A below-grade installation can expose wiring to moisture, soil conditions, physical stress and other hazards. Some cables are specifically rated for direct burial, while others may be appropriate underground only in an approved raceway. If the installation requires a direct-burial construction, increasing the conductor size does not replace that requirement. Both electrical performance and environmental suitability need to be addressed.
Calculate Before the Trench Is Closed
Voltage drop is much easier to solve during planning than after equipment is operating poorly. Before cable is ordered for a long underground run, determine the load, circuit voltage, one-way distance, conductor material and acceptable voltage drop. Then use those requirements to determine whether the conductor size selected for ampacity is also sufficient for the distance. If it is not, moving to a larger conductor may improve voltage at the load.
The best underground cable specification considers the complete installation: ampacity, voltage drop, conductor material, insulation and jacket construction, installation method, environmental exposure and the equipment being served.
Frequently Asked Questions
No. Burial depth does not directly reduce the electrical resistance that causes voltage drop.
The important electrical factors are conductor material, conductor size, current and circuit length. Burial depth can affect other installation considerations, including mechanical protection and thermal conditions, but placing the same conductor deeper in the ground does not make its voltage drop disappear. Required burial depth should be determined from the applicable installation rules, not as a method of controlling voltage drop.
No. Conduit provides a pathway and physical protection, but it does not reduce the resistance of the conductor.
A cable run inside underground conduit can experience essentially the same voltage-drop concern as another run of the same length, conductor material and size. Conduit can also contain moisture, so the conductors still need to be suitable for the environment in which they are installed.
Yes. Ampacity and voltage drop measure different aspects of the circuit.
Ampacity addresses how much current a conductor can carry under specified conditions without exceeding allowable temperature limits. Voltage drop addresses how much voltage is lost while current travels through the circuit. On a long run, a conductor can have adequate ampacity yet deliver less voltage to the load than the system designer or equipment manufacturer considers acceptable.
They can. Motors can draw substantially more current during starting than during normal operation.
The temporary increase in current can cause a larger momentary voltage drop than the steady-state calculation suggests. Whether this is significant depends on motor size, starting method, circuit impedance and the equipment connected to the system. Motor circuits should therefore be evaluated using the requirements applicable to that specific equipment rather than relying only on a generic voltage-drop percentage.
The same number of volts lost represents a smaller percentage of a higher system voltage, but the complete circuit still needs to be evaluated.
For example, a given voltage loss represents a larger percentage of a 120-volt supply than of a 480-volt supply. Current levels, conductor size, load characteristics and circuit design also change between systems, so system voltage by itself does not determine whether voltage drop will be acceptable.
