DC Cable Voltage Drop: Specify Voltage at the Equipment

Agree where the voltage requirement applies

When a buyer requests a power adapter with a longer DC cable, the quote should identify more than cable length and connector size. Ask the equipment engineer to specify the acceptable voltage at the equipment input and the load conditions under which that requirement applies.

A reading at the power supply output and a reading at the far end of a loaded cable answer different questions. Put the agreed measurement point in the RFQ so a supplier cannot inadvertently quote against a different acceptance point. This is especially useful when comparing standard cables, custom lengths and distributor replacement options.

Use a simple calculation to frame the question

TDK-Lambda explains that resistance in the output wiring produces a voltage drop as load current flows. For a steady DC illustration, voltage drop is current multiplied by total circuit-path resistance.

Illustrative calculation, not a product specification: suppose the combined outgoing and return path resistance is 0.20 ohm and current is 2.0 A. The estimated drop is 2.0 x 0.20 = 0.40 V. If the source terminal voltage at that load is 12.0 V, the estimated equipment-side voltage is 11.60 V. At 0.5 A through the same assumed resistance, the estimated drop is 0.10 V.

The resistance in this example represents the complete path being evaluated; do not use the resistance of only one conductor and forget the return path. These assumed values are not recommended limits. The calculation is a first comparison, not a substitute for testing the actual supply, connectors and cable assembly.

Ask suppliers to quote the complete assembly

Equipment requirement

State the acceptable input voltage range at the agreed measurement point. Have engineering identify the operating states that need evaluation.

Load

Provide relevant running currents and startup or peak-load information. Label measured data and estimates separately.

Cable assembly

Identify length, conductor specification, connector, polarity and any adapters or extension joints included in the application.

Supplier evidence

Request the assembly specification and any available resistance or loaded-voltage results, including measurement conditions.

Acceptance record

Define the sample configuration, measurement point, test conditions and pass criteria before approving the order.

Where information is missing, ask the supplier to identify the assumption in its quotation. A statement that the cable is suitable should be tied to your requirement rather than treated as a complete technical answer.

Compare remedies without silently changing the voltage

Ask the supplier and equipment engineer to compare cable options against the same equipment-side requirement. A shorter cable or a different conductor specification may be a candidate for evaluation. Compare the resulting assembly, usability, price and supporting evidence together.

Remote sensing is another design-specific topic. TDK-Lambda explains that supplies with this feature can regulate using the voltage at the load, within a model-specific compensation range. Do not assume a conventional two-wire adapter provides remote sensing. Ask for explicit confirmation and the relevant instructions if the feature is part of a proposed solution.

Do not simply order a higher nominal output voltage to conceal an unexplained cable drop. Refer that change to the equipment engineer, including the lower-load condition and every intended cable option. Purchasing needs approval of the complete configuration, not just a reading that looks correct at one operating point.

Keep cable variants in the sample approval record

Imagine a distributor ordering one adapter family with short and long cable options. This is a planning example, not a customer case. Evaluating the short-cable sample does not resolve the long-cable request. Give each approved assembly an identifiable specification and record which sample results support it.

During approval, ask for readings at the source and equipment input under the agreed load conditions, with the actual connector arrangement in place. Have engineering define the equipment operating states and acceptance criteria; this guide does not prescribe a universal allowable percentage drop.

For repeat orders, retain cable and connector revisions alongside the adapter model. Review substitutions before shipment so a change of cable supplier or assembly specification is not hidden behind an unchanged adapter part number.

For a Voltivo Power inquiry, share the required equipment-side voltage range, load, cable length, connector, destination markets and quantity. The proposed cable assembly and available verification evidence need confirmation for the specific product configuration.