Check 24 VDC voltage margin before blaming the PLC or field device.
This calculator estimates DC cable voltage drop, endpoint voltage and supply headroom using values taken from the actual circuit. It is intended for remote I/O, transmitters, relays, solenoids and other control-panel loads.
Cable voltage drop and endpoint margin
Two-conductor DC estimate: loop resistance = 2 × one-way length × conductor resistance. Extra series resistance can represent known fuses, barriers, contacts or other measured/documented series elements.
Power-supply continuous-load budget
The utilization percentage is a project planning input, not a universal standard. Verify manufacturer derating, ambient temperature, redundancy philosophy and startup/inrush requirements separately.
Field checks that prevent false power-supply diagnoses
Measure under load
Open-circuit voltage can look normal while the endpoint collapses when the device energizes. Record both source and endpoint voltage at the real operating current.
Use the actual conductor resistance
Cable resistance changes with conductor size, material and temperature. Use the cable datasheet or a measured value instead of assuming a generic wire size.
Include series devices
Fuses, isolators, barriers, contacts, connectors and damaged terminals can add voltage loss that a cable-only calculation will miss.
Separate continuous load from inrush
A supply can be acceptable for steady load yet dip during relay, solenoid or capacitive input startup. Capture transient behavior when symptoms are intermittent.
Check shared commons and returns
Unexpected return-path resistance can create channel-to-channel symptoms. Measure voltage with reference to the correct supply return point.
Record minimum device voltage
The useful result is not only percentage drop. Compare the calculated and measured endpoint voltage with the device's documented operating requirement.
Suggested commissioning record
| Check | Record | Why it matters |
|---|---|---|
| Supply output | Measured V DC at source, unloaded and loaded where appropriate | Separates source regulation from downstream loss. |
| Load | Continuous current and known startup/inrush behavior | Defines the real electrical demand. |
| Cable | One-way length and conductor resistance | Supports a reproducible voltage-drop calculation. |
| Series path | Fuses, barriers, isolators, contacts and terminals | Captures non-cable resistance. |
| Endpoint | Measured voltage at the device under operating load | Confirms whether voltage margin exists in the real circuit. |
| Supply budget | Rated current, planning limit, existing load, added load | Shows remaining continuous-load headroom. |
Reusable worksheet
Download 24 VDC control-power CSVRelated reference
Case 04 — PLC Power Supply Failure Troubleshooting. Use that case for rack-level isolation after basic source and voltage-margin checks are complete.
Maintainer
Maintained by PLCProTech Ltd. as an open industrial automation engineering reference.