DC Voltage Drop and Cable Sizing: Formulas, Examples, and Safety Checks

NetOps Tools · Practical electrical design guide

DC cable sizing, step by step.

Estimate cable voltage drop, calculate a preliminary conductor cross-section and understand why electrical protection and installation conditions matter just as much as the math.

ΔV = (2 × L × I × ρ) / A

24 V DCPower supply

20 m · one-way routeTwo conductor paths

10 AConstant-current example load

A cable that carries enough current can still deliver insufficient voltage. That is particularly important in 12 V, 24 V and 48 V DC systems, where even a small voltage loss can be a significant percentage of the source voltage.

24 VSource voltage

10 AAssumed steady load

20 mOne-way cable distance

1. Why DC cable size matters

Every conductor has electrical resistance. A longer cable increases resistance; a larger copper cross-sectional area reduces it. Under a fixed-current approximation, this resistance creates a voltage difference between the supply and the equipment.

The same loss in volts has very different consequences depending on the source voltage. Losing 1 V from 12 V is 8.33%, while losing 1 V from 48 V is approximately 2.08%.

A remote access-control device, LED driver, sensor, industrial control module or battery-fed load may operate normally with one cable and reset with another. The correct design depends on the device’s minimum input voltage, starting current, operating characteristics and protection requirements—not on one universal percentage.

Related reading: Our separate guide to the 3% and 5% voltage-drop recommendations explains the origin and limits of those common design figures. Here, we focus on the conductor-sizing calculation itself.

2. Calculate DC voltage drop correctly

For a simplified two-conductor DC circuit with an outgoing conductor and a return conductor of equal material, area and length:

ΔV = 2 × L × I × ρ ÷ A

ΔV = conductor voltage drop in volts · L = one-way length (m) · I = current (A) · ρ = resistivity (Ω·mm²/m) · A = conductor area (mm²)

For the worked examples below, we use ρ = 0.0172 Ω·mm²/m as an approximate resistivity of copper at 20 °C. Real cable resistance varies with conductor construction, temperature, material and the manufacturer’s specification.

Measure the physical one-way run. Follow the real cable route, including detours—not the straight-line distance between devices.
Use a defensible current assumption. Consider normal draw, expected peak load and whether the device behaves approximately as a fixed-current load.
Choose the conductor area. Metric cable is usually marked in mm²; AWG uses a different size scale.
Account for the return path. The factor of two belongs in this simple two-wire model because both positive and negative wires add resistance. Do not double the length yourself when using the NetOps calculator.
Calculate voltage at the device. Use Vload = Vsource − ΔV and drop % = 100 × ΔV / Vsource.

Different wiring layout? If return and supply wires differ in area or length, calculate each path’s resistance separately and add them. Shared returns, multiple loads, connectors, terminals and distribution points can also change the total drop.

3. Worked example: 24 V DC, 10 A, 20 m

Consider a 24 V power supply connected to an illustrative 10 A fixed-current load, using copper conductors over a 20 m one-way route. First, estimate the result with 4 mm² copper:

ΔV = 2 × 20 × 10 × 0.0172 ÷ 4

ΔV = 1.72 V

Voltage drop ≈ 7.17% · Device voltage ≈ 22.28 V · Conductor loss ≈ 17.2 W at 10 A.

The modeled 4 mm² cable loses more than 7% of the source voltage. Whether the device can operate at approximately 22.28 V depends on its specifications. For a 3% illustrative planning target, it would not meet that target.

Now compare several metric conductor sizes under identical assumptions:

Copper area	Drop	Drop (%)	Device voltage	At 3% target?
2.5 mm²	2.752 V	11.47%	21.248 V	No
4 mm²	1.720 V	7.17%	22.280 V	No
6 mm²	1.147 V	4.78%	22.853 V	No
10 mm²	0.688 V	2.87%	23.312 V	Yes, at 20 °C
16 mm²	0.430 V	1.79%	23.570 V	Yes, at 20 °C

Illustrative 20 °C copper-resistance estimates only, not approved sizes or verified ampacities. Results rounded for display.

What changing conductor area does

Voltage-drop percentage under the same 24 V / 10 A / 20 m assumptions.

2.5 mm²

11.47%

4 mm²

7.17%

6 mm²

4.78%

10 mm²

2.87%

16 mm²

1.79%

Shorter bars mean less modeled voltage loss. Colors are visual aids, not an electrical safety or compliance certification.

4. Solve for minimum area by a chosen drop target

Instead of checking one conductor at a time, rearrange the resistance formula to calculate an initial copper cross-section:

A ≥ 2 × L × I × ρ ÷ ΔVmax

For a percentage target, ΔVmax = Source voltage × target percentage / 100.

If we choose a 3% design target for this example, the allowable conductor drop is 0.72 V (3% of 24 V):

A ≥ 6.88 ÷ 0.72

A ≥ 9.56 mm²

A common nominal copper size of 10 mm² is the next size up in this example and models approximately 2.87% drop at 20 °C.

Calculation result ≠ approved conductor. Ten square millimetres is only the preliminary result of this voltage-drop calculation. An actual installation still needs verified current-carrying capacity, wiring method, conductor temperature, terminals, short-circuit conditions, protective devices and local code requirements.

5. What changes when copper becomes hot?

Resistance rises as copper temperature rises. An approximate temperature adjustment over a moderate range is:

R(T) ≈ R(20°C) × [1 + α(T − 20)]

For this illustration, α ≈ 0.00393 per °C. Use cable-manufacturer resistance data at the design temperature whenever available.

For the 10 mm² example above, warming the conductor from 20 °C to an assumed 60 °C increases the calculated drop from approximately 0.688 V (2.87%) to 0.796 V (3.32%). Under those assumptions, it no longer meets the illustrative 3% target.

Conductor operating temperature is not necessarily ambient temperature. Insulation type, grouping, cable routing, load duration, termination ratings and installation method affect cable selection and permissible temperature. Treat the cold-resistance calculation as a starting point, not an installation design.

6. How far can you run a 24 V cable?

We can also rearrange the formula to estimate the one-way maximum length for a selected loss target:

Lmax = (ΔVmax × A) ÷ (2 × I × ρ)

Again, the formula assumes matched outgoing and return copper conductors at the stated temperature and fixed load current.

Illustrative maximum one-way length

24 V, 10 A, 3% drop target and copper at 20 °C — not ampacity checks.

2.5 mm²

5.2 m

4 mm²

8.4 m

6 mm²

12.6 m

10 mm²

20.9 m

16 mm²

33.5 m

Distance grows with copper area under these simplified fixed-current and 20 °C assumptions. Actual permissible lengths may be lower.

7. Practical example: a remote DC-powered device

Suppose a remote device has a nominal 12 V supply, a modeled constant current of 2 A and a 30 m one-way copper run. Consider the following illustrative values:

Copper area	Voltage drop	Drop (%)	Estimated at device
1.5 mm²	1.376 V	11.47%	10.624 V
4 mm²	0.516 V	4.30%	11.484 V
6 mm²	0.344 V	2.87%	11.656 V

These numbers do not establish that any cable will work for the actual device. The manufacturer might specify a narrower voltage range, a higher startup current, a different power requirement, specific wiring topology or a different recommended cable.

Important for security and network equipment: do not apply this generic two-wire model to certify Power over Ethernet (PoE). PoE uses standardized powering methods and twisted-pair cabling, requiring dedicated checks of pair resistance, power class, source capacity and powered-device requirements.

8. The cable selection checklist: voltage drop is only one test

A final cable selection must consider more than conductor resistance. The actual requirements depend on the applicable electrical code and the equipment manufacturer’s instructions.

✓Load: continuous current, startup current, duty cycle and possible constant-power behavior.

✓Ampacity: conductor current rating with ambient temperature, grouping and installation-method corrections.

✓Protection: overcurrent protection, available fault energy and appropriate device coordination.

✓Temperature: conductor resistance at its expected operating temperature, not just 20 °C.

✓Connections: termination suitability, connector resistance, polarity, and manufacturer instructions.

✓Equipment limits: minimum and maximum supply voltage, permissible ripple and source variations.

✓Real route: physical one-way distance, branches, shared return conductors and environmental exposure.

✓Local requirements: applicable national/local wiring code, installation classification and qualified review.

Schneider Electric’s Electrical Installation Guide explains how conductor sizing incorporates installation conditions and correction factors. A voltage-drop result alone does not calculate those factors.

9. Does a 3% voltage-drop rule apply to every DC cable?

No. Three percent can be a useful example target for comparing cable sizes, but it is not a universal legal limit for 12 V, 24 V or 48 V DC equipment. General US NEC informational recommendations often cited for branch and feeder circuit efficiency include approximately 3% at the farthest branch outlet and 5% across feeder and branch combined. Those informational recommendations must not be treated as blanket permission or mandatory rules for every DC installation.

Equipment specifications and applicable codes or contractual design criteria can require a different voltage at the load. Specialized installations may have separate binding provisions. See our dedicated explanation of the 3% and 5% guidance for more context.

10. Mistakes to avoid

Doubling distance twice: Enter one-way distance in our calculator; its DC method already accounts for the return path.
Confusing mm² and diameter: Cable cross-sectional area in square millimeters is not wire diameter in millimeters.
Assuming a cold conductor: The 20 °C resistance model understates the drop if the actual conductor operates hotter.
Using a fuse size as the operating current: The load current and circuit protection rating are not the same quantity.
Ignoring connection resistance: Corroded contacts, undersized terminals or poor connections may create extra losses and heating.
Using voltage-drop calculations as ampacity approval: A conductor may meet a percentage target and still be unsuitable or unsafe.

Frequently asked questions

Does the NetOps calculator require one-way or round-trip length?

Enter the physical one-way cable distance in metres. For its simplified DC mode, the calculator includes the outgoing and return paths. Do not multiply the distance by two before entering it.

Is a thicker wire always the right answer?

Greater conductor area generally reduces resistive voltage drop under equivalent conditions. But the selected cable still needs appropriate current-carrying capacity, termination compatibility, protection, installation suitability and equipment approval.

Why does a 12 V circuit lose a higher percentage than a 48 V circuit?

For the same absolute conductor drop, source voltage is the denominator. A 1 V loss is about 8.33% at 12 V, 4.17% at 24 V and 2.08% at 48 V. Actual current and system design may differ between the circuits.

Can the formula be used for aluminum cables?

The electrical approach can be adapted, but not with the copper resistivity value shown here. Use aluminum conductor data at the appropriate temperature, and check installation rules and approved terminals for the actual conductor type.

Do these calculations determine fuse or breaker size?

No. Overcurrent-protection selection is a separate safety task based on the installation, conductor ratings, connected equipment, fault conditions and applicable regulations.

Compare your own cable sizes

Open the free NetOps AC/DC Voltage Drop Calculator. Select DC, enter voltage, current, one-way cable distance and copper cross-sectional area. Compare the estimated voltage reaching the load.

Open AC/DC Voltage Drop Calculator ↗Browse Technical Guides ↗

Technical references

Georgia State University, HyperPhysics — Resistivity and Temperature Coefficient at 20 °C (illustrative copper-resistivity and temperature-coefficient data).
Schneider Electric — Electrical Installation Guide: General Method for Cable Sizing (ampacity, ambient and installation correction factors).
NFPA 70 — National Electrical Code (consult the edition adopted in the installation’s jurisdiction; informative voltage-drop notes are not general mandatory thresholds).
NetOps Tools — AC/DC Voltage Drop Calculator (public tool methodology and limitations).

Last reviewed: October 27, 2026 · Maintained by: NetOps Tools · Purpose: Educational calculation and preliminary technical planning. No installation is approved or certified by this article.