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.

נפילת מתח 24 V DC בבקרה: PLC, חיישנים, ממסרים וסולנואידים

מעגל בקרה 24 V DC מזין כניסות PLC, חיישנים, סלילי ממסר וסולנואידים. אובדן 2 V הוא 8.3% ועלול לגרום לתקלות בקצה הרחוק.

תקציר

24 V DC: 2 V = 8.3% voltage loss.
במעגל DC דו-גידי מחשבים הלוך וחזור. עומס במרחק 150 ft יוצר מסלול חשמלי של כ-300 ft.
Use 3% to 5% as a practical design target.
Check NEC 725, IEC 60204-1, and IEC 61131-2 beside the math.