Cable Knowledge

How to Check Cable Voltage Drop

Voltage drop can determine cable size on long feeders even when thermal ampacity is adequate. Excessive drop can cause poor motor starting, dim lighting, control malfunction and avoidable energy loss.

Inputs

A calculation requires system voltage, phase, current, conductor material, size, route length, power factor and operating temperature. Cable data should provide resistance and, where needed, reactance.

AC and DC Circuits

DC circuits depend mainly on loop resistance. Single-phase and three-phase AC formulas differ, and power factor changes the relative effect of resistance and reactance.

Motor Starting

Starting current may be several times running current. Check whether the terminal voltage remains sufficient for acceleration and contactor or control operation.

Reducing Drop

Increase conductor cross-section
Reduce route length
Raise distribution voltage where design permits
Use parallel conductors under applicable rules
Improve power factor where appropriate
Locate equipment or transformers closer to the load

Use the Operating Case That Matters

A feeder may satisfy running voltage drop but fail to start a distant motor, while a PV string may meet thermal limits yet lose too much energy over its life. State the operating scenario and acceptance criterion before calculating.

Use conductor resistance at relevant temperature.
Include power factor and reactance for AC where required.
Check both normal and transient conditions.

Information to Confirm

Voltage and circuit type
Running and starting current
Route length
Power factor
Cable resistance/reactance
Maximum permitted voltage drop

Engineering note: Final voltage-drop limits should follow the project specification, equipment requirements and local code.

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