A 24 V DC control circuit is a low-voltage power path that feeds devices such as PLC inputs, photoelectric sensors, relay coils, indicator lamps, and solenoid valves. The voltage is low enough that a small absolute loss becomes a large percentage loss. A 2 V drop may be ignored on a 240 V branch circuit, but the same 2 V loss is 8.3 percent of a 24 V control supply. That is why a panel can test perfectly at the power supply while the farthest field valve chatters, the relay pulls in slowly, or the sensor output disappears when several loads energize at once.

TL;DR

Use round-trip length for two-wire 24 V DC voltage drop.
Keep most control loads near 3 to 5 percent drop.
PLC inputs are light loads; solenoids and relays usually control wire size.
Check NEC 725 installation rules and IEC 60204-1 machine-control expectations.
For long multi-load runs, a local 24 V power supply may beat larger cable.

This guide is for electricians landing control cables, engineers reviewing panel drawings, maintenance teams troubleshooting nuisance faults, and DIY automation users building small pump, greenhouse, gate, or workshop controls. Use the DC voltage drop calculator for the numeric check, then use the wire resistance calculator and percentage voltage drop calculator to document why the selected conductor or local power supply is justified.

A programmable logic controller is an industrial computer used to read inputs and command outputs in machines and process systems. A solenoid is an electromagnetic coil that moves a plunger or valve when energized. Class 2 wiring is a limited-power circuit category used by the National Electrical Code to reduce fire and shock risk. None of those definitions removes the need to calculate voltage at the actual load terminals. International control panels usually point back to the same engineering discipline through the International Electrotechnical Commission and standards such as IEC 60204-1 for machine electrical equipment and IEC 61131-2 for PLC input and output characteristics.

“On 24 V controls, I do not start with breaker size. I start with the coil current and the round-trip copper length. A 0.7 A valve 150 ft from the panel can lose more voltage than ten PLC inputs on the same route.”
— Hommer Zhao, Technical Director

Why 24 V DC Voltage Drop Causes Real Control Failures

Most control designers are used to thinking in wide supply tolerances. A nominal 24 V DC supply may be adjusted to 24.5 V or 25 V at the panel. Many sensors list an operating range such as 10 V to 30 V DC. A PLC digital input may recognize an ON state above roughly 15 V, depending on the module. That makes voltage drop feel forgiving until the load is a coil, the cable run is long, and several outputs switch together.

The field problem usually appears during a state change. A solenoid coil may need a higher pickup voltage than holding voltage. A relay can pull in at 18 V to 19 V under ideal conditions but chatter when the cable, terminal blocks, and supply sag leave only 17 V during pickup. A three-wire sensor may keep its internal electronics alive while its output voltage falls below the PLC input threshold. When a control fault appears only at the farthest device or only when several devices operate together, voltage drop belongs near the top of the test list.

In one panel review, a greenhouse irrigation skid used a 24 V, 5 A supply to feed eight remote valve coils. The farthest manifold was 175 ft from the enclosure through 18 AWG cable, and two 0.55 A valves could open at the same time. The supply measured 24.2 V at the DIN rail, but the farthest coil saw about 21.1 V during pickup after cable and terminal losses. Upsizing the common feed pair to 14 AWG and splitting the manifold loads brought the measured pickup voltage above 22.8 V, which stopped the intermittent chatter without changing the PLC program.

The Calculation: Round-Trip Length, Load Current, and Wire Resistance

For a two-wire DC circuit, the current leaves the positive terminal, travels to the load, and returns on the 0 V conductor. That means the electrical length is the round-trip length. If a valve island is 150 ft away, the copper path is 300 ft. If the common return conductor carries several energized loads at once, calculate the shared segment at the combined current, not only the current of one device.

Voltage drop = load current x total circuit resistance

Total circuit resistance is conductor resistance per foot multiplied by the positive and return path length. For common AWG copper values, 18 AWG is about 6.385 ohms per 1000 ft, 16 AWG is about 4.016 ohms per 1000 ft, and 14 AWG is about 2.525 ohms per 1000 ft at 20 C.

Example: a 24 V DC solenoid draws 0.7 A and is located 150 ft from the power supply. With 18 AWG copper, the round-trip resistance is about 1.92 ohms, so the drop is 0.7 A x 1.92 ohms = 1.34 V. The load sees about 22.66 V, or 5.6 percent drop. If two identical solenoids share a common 18 AWG feed segment, the shared part carries 1.4 A and the voltage loss can double in that segment.

The same run feeding a 6 mA PLC input is completely different. At 0.006 A, the drop through 300 ft of 18 AWG is only about 0.012 V. That difference explains why the troubleshooting answer is not “all 24 V wiring needs large conductors.” The correct answer is that current, length, pickup voltage, and shared-return loading decide which runs need attention.

Comparison Table: Typical 24 V Control Loads

Device	Typical current	150 ft one-way on 18 AWG	Design note
PLC digital input	3-10 mA	Less than 0.02 V	Threshold and noise matter more than drop
Photoelectric sensor	20-60 mA	0.04-0.12 V	Check output load and cable shield practice
Interposing relay coil	80-200 mA	0.15-0.38 V	Pickup voltage is the important rating
Small solenoid valve	300-700 mA	0.57-1.34 V	Often controls wire size on long runs
Valve island or actuator group	1-3 A shared feed	1.9-5.7 V	Use larger feed or local supply

“For PLC inputs, a 200 ft run may be electrically harmless. For a valve island at 2 A, the same route can burn half the 24 V tolerance. Always separate signal current from actuator current before choosing cable.”
— Hommer Zhao, Technical Director

NEC and IEC Checks Beside the Math

NEC Article 725 is the first US code stop for many remote-control, signaling, and power-limited circuits. It helps determine whether the wiring is Class 1, Class 2, or Class 3, how it may be routed, and when separation from power conductors is required. NEC 300.3(C) and raceway-fill or cable-listing rules can also matter when control conductors share enclosures or pathways with other wiring. For machine panels, NFPA 79 and IEC 60204-1 are often the practical shop-floor references, while IEC 61131-2 helps define PLC I/O expectations.

These standards do not give a universal “24 V drop shall be X percent” rule for every control circuit. They instead require a design that is suitable for the equipment and environment. That is why good drawings show the supply rating, conductor size, terminal count, cable length, fuse or electronic circuit protection, and field device current. The voltage-drop calculation becomes the engineering evidence that the installation will work after the code installation rules are satisfied.

Temperature deserves a separate check. Copper resistance rises as conductor temperature rises. A cable tray above a hot machine, a rooftop control run, or a packed enclosure can add enough resistance to erase a small margin. If the calculation barely passes at 20 C, it may not pass in an enclosure that runs near 40 C. For sensitive controls, design with a margin instead of using the exact pass/fail edge.

Practical Design Workflow

List every 24 V load on the route, including startup or pickup current.
Mark the one-way distance to each device and double it for simple two-wire DC loops.
Calculate shared positive and 0 V segments at the worst simultaneous current.
Set a target, commonly 3 percent for tight controls or 5 percent for general devices.
Confirm the load terminal voltage stays above the device pickup or input threshold.
Check NEC 725, cable listing, separation, overcurrent protection, and machine-standard rules.
Document the final wire size or move the power supply closer to the loads.

The fastest way to compare options is to run the same load through the batch voltage drop calculator. Compare 18 AWG, 16 AWG, 14 AWG, and a local power supply option. For long routes, the lowest installed cost is often not the smallest cable; it is the design that prevents return trips, relay chatter, nuisance PLC alarms, and unexplained device replacements.

“A good 24 V drawing shows the field current budget. If the shared 0 V conductor can carry 2 A, I calculate that shared segment at 2 A, not at the 80 mA current of whichever relay I happened to notice first.”
— Hommer Zhao, Technical Director

FAQ: 24 V DC Control Voltage Drop

How much voltage drop is acceptable on a 24 V DC control circuit?

A 5 percent limit is often a practical maximum for 24 V DC control wiring because it leaves about 22.8 V at the load. Many PLC inputs and sensors tolerate 20.4 V to 28.8 V, but solenoids and relays should be checked against their pickup voltage.

Do I calculate 24 V DC voltage drop with one-way or round-trip length?

Use the round-trip conductor path for two-wire DC circuits. If the device is 180 ft from the power supply, the current travels about 360 ft through the positive and 0 V conductors, even though the physical route is 180 ft one way.

Does NEC Article 725 replace voltage-drop calculation for Class 2 wiring?

No. NEC Article 725 covers Class 1, Class 2, and Class 3 circuit installation requirements, but it does not guarantee delivered voltage. A Class 2 24 V circuit still needs conductor resistance checked when the load is far from the supply.

What wire size should I use for a 24 V DC solenoid 150 ft away?

For a 0.7 A solenoid at 150 ft one way, 18 AWG copper can drop about 1.9 V, or 7.9 percent. Moving to 16 AWG reduces the drop to about 1.2 V, or 5.0 percent, and 14 AWG gives more margin.

Why do PLC inputs work while solenoids fail on the same 24 V supply?

PLC inputs may draw only 3 mA to 10 mA, so voltage drop is tiny. A solenoid or relay coil may draw 0.3 A to 1.0 A, making the same 200 ft route a real voltage-drop problem.

When is a local 24 V power supply better than larger control cable?

A local power supply often wins when several loads share a route longer than 200 ft or total current exceeds about 2 A. Feed the local supply with an AC branch circuit, then keep the 24 V DC device wiring short.

Bottom Line for Field Work

A 24 V control circuit should be designed as a circuit, not as an afterthought attached to a PLC drawing. Calculate the round-trip resistance, use the real load current, account for shared conductors, and check the terminal voltage against the device rating. When the cable gets long or several devices switch together, compare larger wire against a local supply before the installation is built.

Ask for help reviewing a voltage-drop case

24 V DC Control Circuit Voltage Drop: PLC Inputs, Sensors, Relays, and Solenoids