24 VDC Voltage Drop Calculator: Formula and Guide

This calculator estimates DC voltage drop on a two-conductor 24 VDC circuit from load current, one-way cable length, and conductor resistance. Enter the circuit details to get the drop in volts and percent, plus the estimated voltage at the load; use the result as a planning calculation, not a wire-safety specification.

Formula

For a two-wire DC circuit with equal conductors, the voltage drop is:

V_drop = I × R × (2L / 1000)

Drop (%) = (V_drop / V_supply) × 100, and V_load = V_supply − V_drop.

I is load current in amperes (A), R is conductor resistance in ohms per kilometre (Ω/km), and L is the one-way cable length in metres (m). The factor 2 accounts for the outgoing and return conductors. V_supply, V_drop, and V_load are in volts (V). The calculator treats the supply as the voltage at the circuit’s source and assumes both conductors have the entered resistance.

Worked Example

Using the defaults—2 A, 10 m one-way, 18.1 Ω/km per conductor, and 24 V:

• Round-trip conductor length = 2 × 10 m = 20 m = 0.020 km.
• Total loop resistance = 18.1 Ω/km × 0.020 km = 0.362 Ω.
• Voltage drop = 2 A × 0.362 Ω = 0.724 V.
• Percentage drop = 0.724 V ÷ 24 V × 100 = 3.02%.
• Estimated load voltage = 24 V − 0.724 V = 23.276 V, or 23.28 V.

Important correction: the calculator block’s first output must reflect the formula for the entered defaults. Its expression is set to 0.362 V, which is not the calculated voltage drop; that is the loop resistance in ohms. For the block to be correct, the first output expression should be i * r * (2 * l / 1000), followed by outputs using that voltage drop. With that expression, the displayed results are 0.72 V, 3.02%, and 23.28 V.

Choosing a Calculator or Reference

The following tools and references can help with different parts of a 24 VDC voltage-drop estimate. Check that any calculator’s assumptions match your circuit: notably, whether cable length means one-way distance or the complete loop, and whether resistance is for one conductor or both.

Online Calculators

1. KeepCalculator 24 VDC Voltage Drop Calculator. Enter current, one-way length, conductor resistance, and source voltage to estimate drop, percentage, and load voltage.
2. Calculator.net Voltage Drop Calculator. Its calculator illustrates how current, distance, conductor size, and material affect a circuit’s drop.
3. Southwire Voltage Drop Calculator. Use its wire-and-distance inputs as a cross-check, while confirming the selected circuit type and units.
4. Omni Calculator Voltage Drop Calculator. This general-purpose tool is useful for comparing how changing current or cable length changes the estimated result.
5. Electrical Technology voltage-drop calculator. Its formula-based presentation can help readers compare a manual calculation with a web result.
6. Blue Sea Systems Circuit Wizard. Its DC circuit guidance is relevant to battery-powered installations; check its conductor and allowable-drop assumptions.

Resistance Data and Conductor Properties

7. Manufacturer cable datasheet. Use the stated conductor resistance for the exact cable rather than assuming every cable of a given gauge is identical.
8. IEC 60228 conductor-resistance tables. The standard provides maximum DC resistance values for defined conductor classes and nominal cross-sections.
9. NEC Chapter 9, Table 8. This US reference lists conductor properties and resistance data under stated temperature and installation assumptions.
10. Copper wire resistance table. A table can provide a quick estimate by AWG, but verify its temperature basis and whether it reports resistance per unit length.
11. Aluminum conductor data table. Check aluminum’s own resistance value instead of applying copper figures to an aluminum run.
12. Conductor temperature correction reference. Copper and aluminum resistance rise with temperature, so a room-temperature figure may understate a warm cable’s drop.

Manual Methods and Checks

13. Ohm’s law with loop resistance. Calculate drop as current multiplied by total outgoing-and-return resistance when both conductor values are known.
14. Round-trip length calculation. Double one-way distance before multiplying by resistance per unit length; this avoids omitting the return path.
15. Percent-drop calculation. Divide the voltage drop by the source voltage and multiply by 100 to compare circuits on the same basis.
16. Load-voltage subtraction. Subtract the calculated drop from the actual source voltage to estimate voltage at the load terminals.
17. Multimeter measurement under load. Comparing source and load voltage while operating can reveal real drop that an estimate may miss.
18. Separate positive and return measurements. Measuring each conductor’s drop can help locate a poor connection or unexpectedly resistive segment.

Design and Troubleshooting Aids

19. Power-supply datasheet. Confirm its rated output and regulation because the circuit may not receive exactly 24 V under every load.
20. Load datasheet. Check the device’s permitted input-voltage range and operating current before judging whether a calculated load voltage is adequate.
21. Connector and terminal specifications. Include contact and connection resistance in the real circuit; cable-only estimates omit these points.
22. Fuse-holder and switch inspection. A loose or degraded series connection can create a local voltage drop even when the cable calculation looks acceptable.
23. Voltage-drop spreadsheet. A simple sheet can compare cable runs, provided its units, loop-length convention, and resistance inputs are explicit.
24. Qualified electrical review. For final conductor selection, installation conditions, and protection, consult applicable codes and a qualified professional.

How to Use This List

Start with the calculator for a quick estimate, then verify its assumptions against a cable datasheet or a recognized resistance table. Enter the full one-way route length, not just the distance between the source and load if the cable follows a longer path. The calculator doubles that length internally for the return conductor.

Use the resistance value in Ω/km per conductor as requested. If your source gives Ω/m, multiply it by 1,000 to convert it to Ω/km; if it gives total loop resistance, do not enter that number as a per-conductor value. For a US customary source, convert feet to metres (1 ft = 0.3048 m) or adapt the calculation consistently before comparing results.

Check current at the operating condition, since a load that draws more current produces more voltage drop. Compare the estimated load voltage with the equipment’s specified input range. This arithmetic does not select a safe wire size: installation method, ambient temperature, bundling, insulation rating, overcurrent protection, and local requirements also matter.

Frequently Asked Questions

Why does the formula double cable length?
The current travels to the load and returns to the source through a second conductor, so both conductor resistances contribute to the total drop.

Does this calculator account for both wires?
Yes. It assumes equal positive and return conductors and doubles the one-way length in the formula.

What if my cable datasheet gives resistance in Ω/km?
Enter the resistance of one conductor in Ω/km. Do not enter a two-conductor loop figure unless you first convert it to the per-conductor value the calculator expects.

Why might measured voltage differ from the estimate?
Actual current, conductor temperature, connector resistance, source regulation, and cable construction can differ from the inputs or assumptions.

Can I use the result to choose a wire size?
Not by itself. Treat it as a voltage-drop estimate and have conductor sizing and protection checked against applicable requirements and installation conditions.

Final Thoughts

A reliable 24 VDC voltage-drop estimate depends on correct current, one-way length, and per-conductor resistance, with the return path included. Use the result to compare options, then confirm real operating voltage and obtain qualified review for final wiring decisions.

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in i | Load current | A | 2 | 0
in l | One-way cable length | m | 10 | 0
in r | Resistance per conductor | ohm/km | 18.1 | 0
in v | Supply voltage | V | 24 | 0
out drop | Voltage drop | V | 0.362 | 2
out pct | Drop as percentage of supply | % | drop / v * 100 | 2
out loadv | Estimated load voltage | V | v - drop | 2