A 24 VDC control circuit can lose enough voltage in its cable conductors to leave a relay, sensor, solenoid, PLC input, actuator, or other load below its required operating voltage. As cable length increases, loop resistance increases, producing a higher voltage-drop result and a lower estimated voltage at the device.
The 24 VDC Control Voltage Drop Calculator estimates:
\(\displaystyle \text{Load Voltage} = \text{Supply Voltage} - \text{Voltage Drop}\)
The load-voltage result is the number used to determine whether the selected conductor size and route length can deliver adequate voltage to the connected control device.
A circuit can have conductors with adequate ampacity yet still perform poorly because of voltage drop. This is common with small-AWG control wiring, extended runs across industrial equipment, remote field devices, and DC solenoids with higher pull-in current.
DC Voltage-Drop Calculation
The calculation uses the full circuit path, not only the one-way distance from the power supply to the device. Current travels out on one conductor and returns on another conductor, so the calculator doubles the one-way cable length to determine loop length.
\(\displaystyle \text{Loop Length} = 2 \times \text{One-Way Cable Length}\)
Voltage drop is then calculated from current and conductor resistance:
\(\displaystyle \text{Voltage Drop} = \frac{\text{Loop Length} \times \text{Load Current} \times \text{Wire Resistance per 1,000 ft}}{1,000}\)
The calculator subtracts that result from the supply voltage:
\(\displaystyle \text{Estimated Load Voltage} = 24\text{ V} - \text{Voltage Drop}\)
For example, doubling a 250 ft one-way run produces a 500 ft loop length. If the same wire size and load current are retained, a 500 ft one-way run has twice the loop resistance and approximately twice the voltage drop.
Cable Length and Loop Resistance
Conductor resistance rises in direct proportion to cable length when AWG size, conductor material, and temperature remain unchanged.
| One-Way Cable Length | Loop Length Used in Calculation | Relative Voltage Drop |
|---|---|---|
| Short run | Two times the one-way length | Lower |
| Medium run | Two times the one-way length | Proportional to length |
| Long run | Two times the one-way length | Higher |
A longer route does not automatically require a larger conductor based on ampacity. It may require a larger conductor because the voltage available at the far-end device becomes too low.
For a fixed current draw:
- Doubling cable length doubles resistance and voltage drop.
- Doubling load current doubles voltage drop.
- Selecting a conductor with lower resistance per 1,000 ft reduces voltage drop.
- Raising the actual supply voltage, where permitted by the equipment design, increases the voltage margin at the load.
The calculator’s wire-resistance input is especially useful when evaluating different AWG conductor sizes. A lower AWG number generally has lower resistance than a higher AWG number, but the resistance value entered into the calculation controls the result.
Calculation Example
Use the same 24 VDC supply, device current, and conductor resistance value for short, medium, and long cable runs. Enter each one-way cable length separately and compare the calculated voltage drop and estimated load voltage.
| Comparison Run | One-Way Length Input | Calculation Effect | Result to Review |
|---|---|---|---|
| Short | Short distance to device | Low loop resistance | Estimated load voltage |
| Medium | Increased distance | Higher loop resistance | Estimated load voltage |
| Long | Extended route | Highest loop resistance | Estimated load voltage and device minimum |
The field decision is not based on voltage drop alone. Compare the calculator’s estimated load voltage with the device manufacturer’s minimum operating voltage at the actual condition of use.
For a solenoid, contactor coil, or actuator, check the voltage requirement for pull-in or startup—not merely the voltage needed to remain energized after operation. A device may hold at a lower voltage than it needs to start.
Conductor Sizing Workflow
Voltage-drop review fits alongside conductor ampacity, raceway layout, termination, and equipment requirements.
A practical control-circuit workflow is:
- Identify the actual DC supply voltage and the expected device current.
- Measure or estimate the one-way cable route, including vertical risers, panel routing, machine wireway travel, and raceway offsets.
- Select the applicable conductor resistance per 1,000 ft for the proposed AWG size.
- Calculate loop length by doubling the one-way length.
- Calculate voltage drop and estimated voltage at the device.
- Compare estimated load voltage with the manufacturer’s minimum voltage requirement.
- If the load voltage is inadequate, evaluate a shorter route, lower-resistance conductor, revised power-distribution location, or another equipment design solution.
Where multiple loads share a common 24 VDC feeder, evaluate the current and voltage conditions at the relevant point in the circuit. The farthest device may not always have the lowest voltage if loads, taps, conductor sizes, and supply locations differ.
Device Load Review
The calculator uses the entered current and wire resistance to estimate steady DC voltage drop. Actual control-circuit performance can also be affected by:
- Device startup or inrush current
- Coil pull-in current
- Solenoid or actuator transients
- Terminal and connector resistance
- Fuse, circuit-breaker, or protection-device voltage loss
- Power-supply regulation and output behavior
- Shared conductors and simultaneous load operation
- Manufacturer voltage limits and wiring requirements
These conditions require separate equipment and field review. A calculated load voltage that appears acceptable under steady current may not confirm successful operation during startup or under simultaneous loading.
Use the Calculator
Use the following Elecatrix tools to calculate the control-circuit voltage drop, confirm the DC formula inputs, and identify the resistance value for the selected AWG conductor: