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.

24 V DC control voltage drop: PLC inputs, sensors, relays, at solenoids

Ang 24 V DC control circuit ay nagpapakain sa PLC inputs, sensors, relay coils, at solenoids. Ang 2 V loss ay 8.3%, kaya pwedeng pumalya ang malayong device.

Buod

24 V DC: 2 V = 8.3% voltage loss.
Sa two-wire DC, gamitin ang round-trip length. Kung 150 ft ang layo ng load, mga 300 ft ang dinadaanan ng current.
Use 3% to 5% as a practical design target.
Check NEC 725, IEC 60204-1, and IEC 61131-2 beside the math.

24 V DC control voltage drop: PLC inputs, sensors, relays, at solenoids

Sa two-wire DC, gamitin ang round-trip length. Kung 150 ft ang layo ng load, mga 300 ft ang dinadaanan ng current.

Ang 0.7 A solenoid sa 150 ft gamit ang 18 AWG copper ay may humigit-kumulang 1.34 V drop; nasa 22.66 V ang load.

NEC 725 covers Class 1/2/3 circuits; IEC 60204-1 at IEC 61131-2 ay mahalaga sa machine controls at PLC I/O.

NEC and IEC references define the installation context; delivered voltage is still a circuit-performance calculation.

24 V DC load comparison

Device	Current	150 ft, 18 AWG	Decision
PLC input	3-10 mA	<0.02 V	Noise and threshold matter most
Sensor	20-60 mA	0.04-0.12 V	Check output loading
Relay coil	80-200 mA	0.15-0.38 V	Check pickup voltage
Solenoid	300-700 mA	0.57-1.34 V	Often controls wire size
Valve group	1-3 A	1.9-5.7 V	Consider larger feed or local supply

Practical checks

Calculate the worst simultaneous current, include shared 0 V conductors, compare 18 AWG, 16 AWG, and 14 AWG, then decide whether a local 24 V supply is cleaner than a larger cable. A 24 V circuit should be checked at the device terminals, not only at the power supply.

I-record ang voltage, distance, wire size, simultaneous current, at minimum load voltage.

DC Calculator Wire Resistance Contact

24 V DC control voltage drop: PLC inputs, sensors, relays, at solenoids

Buod

24 V DC control voltage drop: PLC inputs, sensors, relays, at solenoids

24 V DC load comparison

Practical checks

Start Calculating

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