Short answer
Calculate the loop, not one wire: V_drop = I × R_loop, where R_loop includes the outbound conductor, return conductor, terminations, fuses, and connectors. For a uniform cable, R_loop = 2 × length × resistance per unit length. Use expected operating current and repeat the calculation with measured peak or startup current.
Quick reference: 24 V DC Industrial Cable Resistance & Maximum Run Distance Matrix (5% Drop Limit / 1.2V Max)
| Conductor Cross-Section / AWG | Loop Resistance per 100 ft (30 m) | Loop Resistance per 100 m (328 ft) | Max Distance @ 0.5 A Load | Max Distance @ 2.0 A Load |
|---|---|---|---|---|
| 22 AWG (0.34 mm²) | 3.23 Ω | 10.60 Ω | 74 ft (23 m) | 18 ft (5 m) |
| 20 AWG (0.50 mm²) | 2.03 Ω | 6.66 Ω | 118 ft (36 m) | 30 ft (9 m) |
| 18 AWG (0.75 mm²) | 1.28 Ω | 4.20 Ω | 188 ft (57 m) | 47 ft (14 m) |
| 16 AWG (1.50 mm²) | 0.80 Ω | 2.62 Ω | 300 ft (91 m) | 75 ft (23 m) |
| 14 AWG (2.50 mm²) | 0.50 Ω | 1.64 Ω | 480 ft (146 m) | 120 ft (37 m) |
| 12 AWG (4.00 mm²) | 0.32 Ω | 1.05 Ω | 750 ft (228 m) | 188 ft (57 m) |
What to look for
Connector and termination loss deserves its own measurement. A corroded pin, loose ferrule, undersized splice, or tired contact may add resistance that a cable table omits. Under a safely controlled loaded condition, measure supply positive-to-device positive and device return-to-supply return; then measure directly across the device's designated supply terminals.
Do not infer device voltage from an indicator at the panel supply. Trend readings while the real load transitions, because startup inrush and simultaneous outputs can expose a marginal circuit. Compare device-terminal voltage and symptoms with the equipment manual's operating limits; exact polarity, allowable range, and measurement points must be checked against that manual.
When the symptom appears in the plant
A remote pneumatic valve manifold occasionally failed to shift its valves, triggering timeout alarms in the PLC. The manifold was located 300 feet from the main control panel. The technician measured 24V at the panel, but only 19.5V at the manifold when multiple solenoids energized simultaneously. The long run of 18 AWG wire had too much resistance for the required current, causing a voltage drop below the manifold's minimum operating threshold. Upsizing the power wires to 14 AWG resolved the ghost faults.
The Inrush Current Voltage Sag Trap (Ghost Reset)
A digital DC voltmeter measured at a quiescent 24 V DC terminal block may show a solid 23.8 V DC. However, when inductive loads (such as pneumatic valve solenoid banks, DC contactors, or laser measurement heads) turn ON, their instantaneous inrush current can spike to 3× to 5× their steady-state operating current for 20 ms to 80 ms. Across a high-resistance long cable run, this momentary inrush current causes an acute voltage sag below 18.0 V DC. Standard digital multimeters update their display only 2 to 4 times per second (averaging out fast transients) and will completely miss this dip. The symptom is a microprocessor-based field sensor that randomly reboots or drops communication only when another actuator triggers. Use an oscilloscope or a multimeter equipped with a high-speed 1 ms Peak-Detect / Fast-Min function to catch transient inrush sags.
Five checks before replacement
- Measure the 24V DC voltage at the field device terminals under maximum load (e.g., when all valves or outputs are active).
- Calculate the expected voltage drop using the formula: V_drop = 2 * (Cable Length) * (Wire Resistance per foot) * (Load Current).
- Check the minimum operating voltage specification of the field device (many require at least 20.4V to operate reliably).
- Inspect terminal blocks and splices for loose connections, which can act as high-resistance points and cause localized voltage drops.
- Verify the 24V power supply output voltage; some can be adjusted slightly higher (e.g., to 24.5V) to compensate for long cable runs.
Always measure voltage drop under load; measuring an unloaded circuit will almost always show a healthy 24V.
When running power to remote islands or manifolds, calculate the current draw carefully and upsize the wire accordingly.
Separate sensor power from actuator power if possible to prevent inductive voltage dips from affecting delicate electronics.
Bench checklist
- Measure DC voltage at the furthest field device under simultaneous maximum inrush load (all solenoids engaged).
- Calculate total round-trip conductor resistance: R = 2 * length * resistance per meter for the specific wire gauge (AWG/mm²).
- Verify power supply terminal voltage: adjust the 24V DC trimmer pot to 24.5V if long feeder runs cause minor drop.
- Inspect terminal block connections for loose screw clamping or high-resistance oxidised wire strands.
- Segregate field actuator power (unregulated/solenoids) from sensitive instrument electronic power supplies.
Reference families
- NEC Voltage Drop Guidelines
- DC Power System Design Best Practices
Always verify the current edition and the exact device manual for the installation.
Official references
These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.
- IEC 60364-5-52: Wiring systems IEC
Official low-voltage installation standard landing page covering permitted voltage drop.
- SINUMERIK 808D: External 24 VDC power supply Siemens
Official commissioning documentation with a copper-cable voltage-drop equation.
- Continuous power Phoenix Contact
Official 24 V power-system documentation addressing long-cable voltage drop.