24 VDC Control Voltage Drop Calculator

A 24VDC Control Voltage Drop Calculator estimates the voltage lost in the outgoing and return conductors of a control circuit, then shows the voltage remaining at the connected device. The key field result is Estimated load voltage—the voltage available at the control load after conductor loss.

This calculation is used when reviewing 24 VDC PLC outputs, field sensors, solenoid valves, relays, contactors, control panels, remote I/O, annunciators, and similar DC control loads. It helps determine whether the selected cable length and conductor resistance can support the device’s required operating voltage.

The calculator compares the calculated load voltage against the entered Minimum load voltage and reports a Minimum-voltage margin. A positive margin indicates that the estimated load voltage is at or above the selected screening threshold.

Control-Loop Inputs

The calculation uses the following interface fields.

Input	Electrical meaning
Control load current	The current, in amperes, carried by the 24 VDC control loop
One-way cable length	The distance in feet from the DC source to the load
Cable resistance	The resistance basis for one conductor, expressed in ohm/1000 ft
Supply voltage	The available DC source voltage in VDC
Minimum load voltage	The minimum voltage used for the load-voltage comparison, based on the connected device requirement or project criterion

The entered One-way cable length is not the total circuit length. Current must travel from the source to the load and return to the source, so the calculator doubles this value to establish the conductor path used for voltage-drop calculation.

The Cable resistance value must represent one conductor. It should be based on the actual conductor material, AWG or kcmil size, and applicable operating conditions used for the design review. A smaller conductor generally has higher resistance per 1,000 ft and produces more voltage drop at the same current and length.

Voltage-Drop Calculation

The calculator uses a round-trip resistance method for a two-conductor DC control loop.

\(\displaystyle L_{RT}=2 \times L_{one-way}\)

\(\displaystyle V_{drop}=I \times L_{RT} \times \frac{R}{1000}\)

\(\displaystyle V_{load}=V_{supply}-V_{drop}\)

\(\displaystyle \text{Drop percent}=\frac{V_{drop}}{V_{supply}} \times 100\%\)

\(\displaystyle \text{Minimum-voltage margin}=V_{load}-V_{minimum}\)

Where:

• L_{RT} = Round-trip length in feet
• I = Control load current in amperes
• R = Cable resistance in ohm/1000 ft for one conductor
• V_{drop} = Voltage drop
• V_{load} = Estimated load voltage
• V_{minimum} = Minimum load voltage

The resulting Voltage drop is expressed both in volts and as Voltage drop percentage. The percentage shows the loss relative to the entered Supply voltage, while the voltage value shows the actual reduction available to the load.

Calculation Example

A 24 VDC control load draws 1 A and is installed 100 ft from the supply. The selected conductor has a resistance of 1 ohm/1000 ft per conductor. The supply is 24 VDC, and the design comparison uses a Minimum load voltage of 21.6 VDC.

Field	Value
Control load current	1 A
One-way cable length	100 ft
Cable resistance	1 ohm/1000 ft
Supply voltage	24 VDC
Minimum load voltage	21.6 VDC

First, calculate the complete outgoing-and-return conductor path:

\(\displaystyle L_{RT}=2 \times 100=200\text{ ft}\)

Then calculate the conductor voltage drop:

\(\displaystyle V_{drop}=1 \times 200 \times \frac{1}{1000}=0.2\text{ V}\)

The estimated voltage at the control load is:

\(\displaystyle V_{load}=24-0.2=23.8\text{ VDC}\)

The resulting values are:

Result	Value
Round-trip length	200 ft
Voltage drop	0.2 V
Estimated load voltage	23.8 V
Voltage drop	0.8333%
Minimum-voltage margin	2.2 V
Minimum-voltage status	At or above entered minimum voltage

A 21.6 VDC minimum is 10% below a 24 VDC supply. That value can be a useful project screening point, but the actual acceptable minimum voltage must come from the control-device data sheet and the operating requirements of the circuit.

Interpreting Load Voltage and Margin

The Estimated load voltage determines whether conductor loss is likely to interfere with a control device’s intended operation. A device can receive nominal 24 VDC at the panel while receiving materially less voltage at a remote field location.

The Minimum-voltage margin provides a direct comparison:

• A positive margin means the calculated load voltage is above the entered Minimum load voltage
• A zero margin means the calculated load voltage equals the entered minimum
• A negative margin means the calculated load voltage falls below the entered minimum

A result near the minimum threshold may warrant a revised conductor selection, a shorter routing path, a local power supply arrangement, or a review of actual load current and source voltage under operating conditions. The required solution depends on the equipment design, circuit arrangement, and manufacturer instructions.

Field Verification

This calculation evaluates conductor resistance and DC voltage drop only. Confirm the actual cable resistance, conductor temperature, terminal and connection behavior, DC power-supply capacity, protective-device arrangement, control-device minimum operating voltage, installation method, and manufacturer instructions before finalizing the circuit.

The calculation does not establish conductor ampacity, overcurrent protection, terminal rating, insulation temperature rating, raceway fill, voltage-drop design criteria, or code compliance. Those decisions require a separate review of the installed system, applicable electrical requirements, project specifications, and AHJ requirements.

For control circuits with multiple loads, intermittent loads, inrush conditions, long raceway routes, shared returns, or electronic devices with strict undervoltage limits, use the current and conductor path that represent the actual circuit condition being evaluated.