To prevent excessive voltage drop in a long DC output cable, calculate the complete outgoing-and-returning circuit length, select a conductor with enough cross-sectional area, and check the voltage at the load while it is operating. Also account for current, conductor material, operating temperature, connectors, and the equipment’s acceptable input-voltage range. In machinery applications, we recommend treating cable selection as part of the power-system design—not simply choosing a cable based on its current rating.
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This guide is for machinery manufacturers, system integrators, electrical designers, and purchasing teams specifying DC power connections over long runs. It is relevant to equipment such as remote control panels, sensors, actuators, lighting, and other DC loads supplied from a central power source. The same principles apply whether a cable is supplied with a DC output adapter, power supply, or machine assembly.
Our aim is to help you identify the main causes of voltage drop and compare practical ways to reduce it. The calculations below are examples, not a substitute for checking applicable electrical codes, cable ratings, installation conditions, and the load manufacturer’s requirements.
A cable has electrical resistance, and current flowing through that resistance produces a voltage loss. For a two-wire DC circuit, current travels to the load through one conductor and returns through the other, so the relevant cable length is the full circuit loop—not only the one-way distance. A longer loop, higher current, or smaller conductor generally means a larger voltage drop.
For a first estimate using copper conductors at approximately 20°C, use this formula:
Voltage drop (V) = 2 × one-way length (m) × current (A) × resistivity (Ω·m) ÷ conductor area (m²)
Copper resistivity is approximately 1.724 × 10-8 Ω·m at 20°C. This calculation estimates conductor loss; it does not include extra resistance from connectors, terminals, or other components. Actual conductor resistance also increases as the conductor gets hotter, so a calculation made at room temperature may understate the drop in a warm operating environment.
Start with the cable route from the DC source to the load, then include the return path. Use the load’s expected operating current and consider startup or peak current if the equipment draws more during certain operating conditions. If several loads share a supply, assess the current in each cable segment rather than assuming that every section carries the same current.
For example, consider a 24 V DC load drawing 10 A through a 20 m one-way run using 4 mm² copper conductors. The circuit loop is 40 m, and the estimated cable drop at 20°C is about 1.72 V, or 7.2% of the source voltage. The load would receive approximately 22.28 V before allowing for connector losses or temperature effects.
A larger conductor has lower resistance, so increasing cross-sectional area is one of the most direct ways to reduce voltage drop. In the same illustrative 24 V, 10 A, 20 m example, using 6 mm² copper reduces the estimated cable drop to about 1.15 V, or 4.8%, before connector losses and temperature effects. These figures are calculation examples, not a recommendation that a particular drop percentage is acceptable for every machine.
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Choose a design limit based on the power supply’s output tolerance and the load’s minimum input voltage. A motor, control device, or electronic module may respond differently to low voltage, so verify its documentation rather than applying one percentage as a universal rule. Also check the cable’s permitted current for its actual installation conditions; a voltage-drop calculation alone does not establish a safe current rating.
Reducing cable length lowers the resistance of the circuit and can reduce both voltage drop and conductor heating. Review the machine layout for a shorter routing path, or consider placing a suitable power supply nearer to a remote load. Before changing the arrangement, confirm that the new location meets environmental, access, ventilation, and electrical requirements.
For a given power level, distributing power at a higher voltage can reduce the current required, which may lower cable voltage drop. This can be useful in a system designed for an appropriate higher DC bus voltage with conversion close to the load. Do not raise the source voltage to compensate for cable loss unless every connected component is rated for the resulting voltage and operating conditions.
Loose, contaminated, undersized, or poorly terminated connections add resistance beyond the cable itself. Confirm that connector and terminal ratings suit the current, conductor size, and installation environment, and follow the specified termination method. A poor return connection can cause the same practical symptom as an undersized outgoing conductor.
When diagnosing an existing installation, measure voltage at the source and at the load while the equipment is operating under a representative load. A measurement taken with little or no current may not reveal the drop that occurs during normal operation. Follow safe electrical work procedures and use instruments appropriate to the circuit.
Conductor resistance rises as temperature rises, increasing voltage drop for the same current. Cables bundled together, installed in warm spaces, or routed near heat sources may operate at a higher temperature than a free cable in room conditions. Use cable data applicable to the conductor and installation, and include a suitable engineering margin where actual conditions are uncertain.
Keep the assumptions with the calculation, including cable length, current, conductor area, material, and temperature basis. This makes it easier for engineering and purchasing teams to compare supplier proposals on the same terms. If the actual load or route changes, review the sizing rather than reusing the original result without checking it.
At Keerda, we work with machinery buyers and engineering teams evaluating power products and related connection requirements. Share the DC output voltage, load current, one-way cable length, conductor preference, connector details, and operating environment when requesting a review. With those inputs, we can discuss the relevant product or sourcing requirements and help clarify what information is needed to assess a suitable cable solution.
The most reliable way to prevent excessive voltage drop is to calculate the full DC loop, size the conductor for both voltage performance and current rating, and verify voltage at the load under real operating conditions. For long runs, consider a larger conductor, a shorter route, or a properly engineered power-distribution change; check connector losses and temperature as part of the same review. There is no single cable size or voltage-drop target that fits every machine.
Before requesting a quotation or finalizing a design, prepare the output voltage, continuous and peak current, route length, acceptable load-voltage range, installation conditions, and termination requirements. Contact Keerda with these details to discuss your machinery power connection needs and evaluate the next steps for your application.
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