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.

4–20 mA current loop diagramA 24 volt supply powers a transmitter and returns a 4–20 mA signal to a PLC analog input.TWO-WIRE CURRENT LOOP24 V DCSUPPLYTRANSMITTER4–20 mAPLC AIINPUT+ loopsignalreturn / commonREAD HERE Original diagram · Signal Field Guide / field note power & wiring

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.

FIELD CASE / REASONING PATH

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.

DIAGNOSTIC SEQUENCE

Five checks before replacement

  1. Measure the 24V DC voltage at the field device terminals under maximum load (e.g., when all valves or outputs are active).
  2. Calculate the expected voltage drop using the formula: V_drop = 2 * (Cable Length) * (Wire Resistance per foot) * (Load Current).
  3. Check the minimum operating voltage specification of the field device (many require at least 20.4V to operate reliably).
  4. Inspect terminal blocks and splices for loose connections, which can act as high-resistance points and cause localized voltage drops.
  5. Verify the 24V power supply output voltage; some can be adjusted slightly higher (e.g., to 24.5V) to compensate for long cable runs.
PRACTICAL NOTES

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.

BEFORE YOU REPLACE THE PART

Bench checklist

  • Confirm the supply at the device terminals.
  • Confirm the meter is in the correct mode and position.
  • Confirm the input range matches the signal type.
  • Confirm the reference/common is intentional.
  • Record the reading before changing the wiring.

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.

READ THE PRIMARY MATERIAL

Official references

These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.

This note is an educational reference, not a substitute for the equipment manual, site procedures, or qualified electrical work.