← Back to Tools Directory 24V SELV/PELV Guide →

24V DC Control Cable & Voltage Drop Sizer

Calculate two-wire DC loop resistance, conductor voltage drop, and terminal operating voltage for industrial sensors, PLCs, safety scanners, and solenoid valves. Verifies that field device voltage stays strictly above the mandatory 20.4 Vdc (-15%) operating threshold.

Supply & loadPower supply output voltageNominal output voltage at power supply terminals.Target field device thresholdMinimum required voltage for reliable device operation.Total branch load current (Ib)Amperes. Combined current of all sensors, relays, or slice modules on this branch.One-way cable run length (L)Metres. The calculator automatically doubles this for the complete two-wire circuit loop (2 × L).

Conductor & environmentConductor cross-sectional area (CSA)Copper conductor cross-section.Ambient operating temperatureCopper resistivity increases by +0.393% per °C rise above 20°C.

Results

Terminal voltage at load -

Loop voltage drop -

Voltage drop percentage -

Total loop resistance -

Cable power dissipation -

Min required conductor CSA -

Why 24V DC loop resistance causes false zero logic faults. Standard PLC digital input cards (compliant with BS EN 61131-2 Type 1/2/3) require a minimum of 20.4 Vdc to guarantee that an input bit turns ON (logic '1'). When long field sensor cables experience voltage drop below 20.4 V, the sensor LED may illuminate on site, but the PLC input card fails to register the signal, causing intermittent and hard-to-diagnose automation stoppages.

Worked example

24.0 V supply, 35 m one-way run (70 m loop), 2.0 A sensor load, 0.75 mm² cable at 40°C:

Resistivity at 40°C = 0.0175 × [1 + 0.00393 × (40 - 20)] = 0.01888 Ω·mm²/m

Loop resistance = (70 m × 0.01888) / 0.75 mm² = 1.76 Ω

Voltage drop = 2.0 A × 1.76 Ω = 3.52 V (14.7%)

Terminal voltage = 24.0 V - 3.52 V = 20.48 V (Passes 20.4 V limit with 0.08 V margin)

⚠️ Educational design aid. For safety-critical control circuits (E-Stops, light curtains), always allow at least 1.0 V operating headroom above the manufacturer's low-voltage cut-off to accommodate supply ripple and power supply aging.

Regulation reference: BS EN 61131-2 specifies DC power supply tolerance zones for industrial automation. BS EN 60204-1 §6.4 and §9.4 govern control circuit voltages and safe disconnection under earth fault conditions.

Frequently Asked Questions: 24V DC Control Loops

Why is voltage drop critical on 24V DC industrial control circuits?

Unlike 230V mains where a 5% drop is 11.5V, a 5% drop on 24V DC is only 1.2V. Relays, contactor coils, and PLC digital inputs commonly drop out below 20.4V DC (-15%), causing intermittent control failures and machine downtime on long cable runs.

How do you calculate voltage drop on 24V DC circuits?

Use loop length (double the one-way route distance): V_drop = (2 * Length * rho * Current) / CSA, where rho is copper resistivity (0.0175 Ω·mm²/m at 20°C, temperature-corrected for operating conditions).