Calculating voltage at a remote device

What you should be able to do

Evaluate the voltage available at a remote device for stated operating conditions and compare it with that device's input limits.

Sources

Define the complete voltage path

For a simple dedicated two-wire branch: Device voltage = source voltage - wire-loop drop - other series drops. Wire-loop drop = current x total outgoing-and-return wire resistance.

Speco's paired-wire guidance counts both the feed and return. If loop resistance is already supplied, do not double it again. Keep connection, protection and switching drops separate only when they are not already included in the chosen resistance or measured drop.

Worked through

Device permitted input: 21.0 to 28.0 V DC. Total wire-loop resistance at the evaluated condition: 2.00 ohms. Other series voltage drop: assumed 0.20 V in each scenario. Normal source: 24.0 V with 0.50 A load. Reduced source: 22.0 V with 0.80 A load.

The current values are assumed operating-point data. The 0.20 V allowance is a given for this exercise, not a typical relay or fuse drop. Real component drops can change with current and temperature. The two scenarios are separate conditions, not simultaneous loads.

Sources

Sources

Check the upper end too

Evaluate high-source and low-load conditions against the maximum device input. Raising source voltage to fix a distant device can expose another device to too much voltage. Changes require checking the whole affected design and manufacturer limits.

Do not describe a result exactly at a limit as comfortable margin. Account for relevant tolerances and design requirements. No universal percentage drop allowance is established by this lesson.

Understand the model's limits

A constant-power device may draw more current as its input voltage falls. A startup pulse can be brief but significant. Shared wiring carries the combined downstream current on common segments. Use actual load behavior and segment currents rather than forcing every system into this simple model.

Document the calculation inputs and their evidence. Field validation, when authorized and performed by suitably trained personnel, should evaluate the relevant operating condition at the device terminals. This lesson does not instruct an apprentice to probe energized enclosures or interrupt active services.

Practice

1. With 22.0 V source, 0.80 A and 0.20 V other drop, what device voltage results if loop resistance is 1.00 ohm?
2. Is that result above the fictional minimum?
3. Is a single normal-condition measurement enough to prove startup operation?
4. Should the 0.20 V example drop be copied into every design?

Answers

Calculation record

Device ID, exact model and input range: Source ID and evaluated voltage: Operating scenario and current: Loop resistance, temperature basis and source: Other drops and inclusion boundaries: Calculated device-terminal voltage: Comparison with both input limits: Tolerance/startup/standby checks outstanding: Reviewer, revision and disposition:

Sources