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.

Chute de tension en commande 24 V DC : entrees PLC, capteurs, relais et solenoides

Un circuit de commande 24 V DC alimente entrees PLC, capteurs, bobines de relais et electrovannes. Une perte de 2 V represente 8,3% et peut suffire a creer une panne intermittente.

En bref

24 V DC: 2 V = 8.3% voltage loss.
Pour une boucle DC deux fils, calculez aller et retour. Un appareil a 150 ft du bloc d alimentation represente environ 300 ft de conducteur.
Use 3% to 5% as a practical design target.
Check NEC 725, IEC 60204-1, and IEC 61131-2 beside the math.

Chute de tension en commande 24 V DC : entrees PLC, capteurs, relais et solenoides

Pour une boucle DC deux fils, calculez aller et retour. Un appareil a 150 ft du bloc d alimentation represente environ 300 ft de conducteur.

Une electrovanne de 0,7 A a 150 ft en 18 AWG cuivre perd environ 1,34 V; la tension utile tombe vers 22,66 V.

NEC 725 traite les circuits Classe 1, 2 et 3. IEC 60204-1 et IEC 61131-2 cadrent machines et E/S PLC.

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.

Conservez tension, distance, section, courant simultane et tension minimale aux bornes.

DC Calculator Wire Resistance Contact

Chute de tension en commande 24 V DC : entrees PLC, capteurs, relais et solenoides

En bref

Chute de tension en commande 24 V DC : entrees PLC, capteurs, relais et solenoides

24 V DC load comparison

Practical checks

Start Calculating

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