Power Supply Cable Sizing for Long DC Runs: Budget the Complete Circuit
Size a long DC power cable against two separate requirements: the receiver must still see an acceptable voltage, and the cable assembly must safely carry the intended current under its installation conditions. For the voltage calculation, include the complete outgoing and return path, plus connections. A power supply that delivers the correct voltage at its own terminals does not guarantee the same voltage at a distant load.
This is a purchasing and layout decision, not only a troubleshooting step. Establish the run length, load behavior and receiver limits before ordering cable, connectors and distribution hardware. Otherwise, a nominally suitable power supply can be paired with a feeder that consumes too much of the available voltage margin.
Define the complete circuit before selecting a conductor
Draw the source, outgoing conductor, receiver and return conductor. Include the terminal blocks, connectors, protective devices and joints that are actually in that current path. Do not calculate only the distance from the cabinet to the machine and forget the return route.
For a simple two-wire branch carrying a known current, the resistive drop is ΔV = I × Rloop. The loop resistance includes both conductors and the relevant connections. The referenced wiring theory guide explains that conductor length, cross-section and connection resistance all influence the drop. Use resistance data for the selected cable and applicable conditions rather than a generic wire-size slogan.
Record the receiver’s minimum operating voltage, its maximum permitted voltage and its steady and transient current requirements. Also establish the source’s minimum and maximum output under the intended conditions. The remaining margin, not merely the nameplate voltage, is what the feeder can consume.
A worked example with explicit assumptions
Assume a 12.0 V source, a receiver drawing 2.0 A, and a total circuit resistance of 0.20 Ω, including both conductors and connections. These are illustrative values, not measurements from the pictured equipment and not a recommendation for a particular cable size.
| Calculation | Result | What it tells the buyer |
|---|---|---|
| Voltage drop: 2.0 A × 0.20 Ω | 0.40 V | The complete feeder consumes 0.40 V at this assumed current |
| Receiver voltage: 12.0 V − 0.40 V | 11.60 V | Compare with the receiver’s actual permitted range |
| Circuit dissipation: (2.0 A)² × 0.20 Ω | 0.80 W | Resistive loss is distributed through the real current path |
| At an assumed 3.0 A with the same resistance | 0.60 V drop; 11.40 V at the receiver | A higher-current condition may change the voltage decision |
A receiver requiring at least 11.5 V would have little margin in the 2.0 A example and would fall below that assumed threshold at 3.0 A. A real assessment must also include source tolerance and the receiver’s behavior; a constant-power load, for example, cannot always be represented by an unchanged current. Do not treat this table as proof that any installed cable is thermally or electrically acceptable.
Keep the SD-50B-12 input and output feeders distinct
The WEHO SD series provides a DC-DC example. The referenced SD-50B-12 label identifies a 24 V DC nominal input with a 19–36 V range and a 12 V, 4 A output. It is not an AC-input supply. Its five terminals provide DC input positive and negative, frame ground, and DC output negative and positive.
The available family sheet lists 4.2 A for the 12 V variant, whereas this unit’s photographed label states 4 A. Do not silently upgrade the pictured unit to the higher value. Confirm the delivered revision and rating with the supplier. The worked example deliberately uses lower, hypothetical currents and does not resolve that documentation difference.
If the input feeder is also long, assess the voltage at the converter’s input separately. Do not combine input and output conductors into one simple current calculation: a DC-DC converter has distinct input and output conditions. The source evidence does not identify remote-sense terminals, so this guide does not propose remote-sense compensation.
Check current capacity and termination independently
A small calculated voltage drop does not establish the conductor’s permitted current in a hot enclosure, a bundle or a particular cable-management system. Obtain the applicable cable rating and installation requirements. Protective-device selection and fault behavior require their own design review.
Check the power supply terminal’s accepted conductor range and preparation. A cable selected for a long run may be too large for a particular terminal; do not trim away strands to force it to fit. Specify a suitable, documented transition or distribution arrangement and include its connection resistance in the budget. Strain relief and routing should prevent service movement from loading the electrical connection.
Resolve an excessive drop before the order is released
Possible design changes include a shorter route, a conductor with lower resistance, fewer unnecessary joints or a different distribution location. Review each change against the receiver, protection and installation requirements. Moving conversion closer to the load may be an architectural option, but it requires a fresh input, output and environmental assessment.
Do not simply raise the source voltage to compensate for one distant load without checking every receiver and operating state on that branch. A lightly loaded or shorter branch may then see a different voltage condition. Ask the equipment designer to define the acceptable solution and its verification method.
Frequently asked questions
Does a ten-metre run mean ten metres of conductor?
Not for a simple two-wire circuit with a ten-metre outward and ten-metre return route. Both conductors contribute resistance. Use the actual route lengths and include connections.
Does a larger power supply remove cable drop?
No. More available output current does not remove the resistance between source and receiver. Calculate the feeder against the load and permitted receiver voltage.
Can one voltage-drop result approve the cable?
No. Voltage margin, current capacity, termination, protection and installation conditions are separate checks.
Send a feeder schedule with the power supply enquiry
Provide the input source range, output load profile, receiver voltage limits, outward and return lengths, proposed cable data and connection list. Contact WEHO for the converter’s exact rating and terminal information, and have the equipment designer confirm the cable and protection arrangement before procurement.
