AUWiring

Voltage sag calculator

Calculate voltage sag percentage from entered pre-event and event voltage records for Australian power-quality review.

  • Calculator
  • Power quality
  • Australia
Use a switchboard, feeder, analyser or event-log reference.
V
Enter the voltage immediately before the event on the same measurement basis.
V
Enter the lowest or selected event voltage from the same record.
ms
Enter the event duration from the analyser or project note.
%
Enter the event-change percentage used for this project review.
Name the project, equipment, report or engineering basis for the entered threshold.
Sag_% = (V_pre - V_event) / V_pre x 100; Remaining_% = V_event / V_pre x 100
  • V_pre and V_event must come from one measurement record.
  • The entered threshold is a project, equipment, report or engineering review value, not a universal hidden limit.
  • Event classification, instrument setup and equipment response remain outside this calculator.
Formula variables
VariableMeaningUnitUse
V_prePre-event voltageVVoltage immediately before the event on the same measurement basis.
V_eventEvent voltageVVoltage recorded during the event.
Sag_%Voltage sag%Voltage change divided by pre-event voltage.
Remaining_%Remaining voltage%Event voltage divided by pre-event voltage.
durationDurationmsEvent duration from the same record.
thresholdReview threshold%User-entered review value for this event record.
More

Voltage sag calculator technical guide

Calculate voltage sag percentage from entered pre-event and event voltage records for Australian power-quality review.

Use this calculator when the starting point is a voltage event record: a voltage before the event, a voltage during the event and a duration. It is intended for Australian power-quality notes, switchboard logs, analyser records and equipment discussions where the arithmetic needs to stay transparent.

The result is a measurement record. It does not classify every disturbance, set network requirements, diagnose equipment behaviour or replace formal logging. It shows the voltage change, sag percentage, remaining voltage and the threshold basis entered by the user.

Event Record Use Cases

Voltage sag use cases
Work settingReal questionUseful action from this page
PQ analyser eventHow large was the entered voltage reduction?Calculate sag percent and remaining voltage.
Plant-room complaintDoes the event value deserve record review?Compare with an entered project or equipment threshold.
Motor-start observationIs the event better handled by motor dip arithmetic?Use this record first, then move to motor voltage dip if the event is a start.
Tenancy report noteAre event values being kept on one basis?Keep pre-event and event voltage together.
Maintenance handoverWhat should be carried forward?Export a traceable event worksheet.

The useful output is the whole record, not only the percentage. A sag value without duration, location and threshold basis is weak evidence. A record that keeps those values visible is easier to review.

Measurement Boundary

What belongs in the record
ItemIncluded in the arithmeticBoundary to keep separate
Pre-event voltageEntered voltage before the event.Instrument setup and measurement window remain source records.
Event voltageEntered voltage during the event.Event classification remains outside this arithmetic.
DurationDisplayed and carried with the result.Flicker, interruption and detailed disturbance methods remain outside this page.
ThresholdUser-entered review percentage.The calculator does not create a universal event limit.

Australian low-voltage records commonly sit around 230/400 V and 50 Hz, but the entered record controls the arithmetic. Keep line-to-line and phase-to-neutral values separate.

Review Workflow

  1. Name the event record, analyser row or site note.
  2. Enter pre-event voltage and event voltage from the same measurement basis.
  3. Enter duration in milliseconds.
  4. Enter the review threshold and threshold basis.
  5. Read sag percentage and remaining voltage together.
  6. Use voltage unbalance, motor voltage dip or formal logging when the next question moves beyond this event arithmetic.

Worked Records

Voltage sag examples
SituationInputsResult patternInterpretation
230 V equipment event230 V before, 184 V during, 200 ms20% sag, 80% remainingReview the measurement basis and event context.
Small voltage-change note230 V before, 220 V during, 80 ms4.35% sagBelow the entered threshold for the worksheet record.
Longer event record400 V before, 320 V during, 1200 ms20% sag with longer durationCarry duration into a separate event review.

Stop Points

  • The pre-event and event voltages are from different measurement bases.
  • The duration is unknown but being treated as a logged event.
  • The threshold has no stated project, equipment, report or engineering basis.
  • The event is being used to adjust equipment or network settings without separate review.
  • The concern is actually steady voltage drop, voltage unbalance, motor starting or harmonic distortion.

230 V equipment event

A power-quality event log shows 230 V before the event and 184 V during the event for 200 ms.

Reference
PQ-SAG-01
Pre-event voltage
230 V
Event voltage
184 V
Duration
200 ms
  1. Voltage change46 V
  2. Sag20%
  3. Remaining voltage80%
Voltage sag20%

80% of the pre-event voltage remains during the event.

The event is above the entered review threshold and needs the measurement record checked before reuse.

  • The event voltage belongs to the same measurement record.
  • The review threshold is entered by the user.
  • Instrument settings remain external.

Small voltage-change note

A plant-room note records a short drop from 230 V to 220 V.

Reference
PLANT-SAG-LOW
Pre-event voltage
230 V
Event voltage
220 V
Duration
80 ms
  1. Voltage change10 V
  2. Sag4.35%
  3. Remaining voltage95.65%
Voltage sag4.35%

95.65% of the pre-event voltage remains during the event.

The arithmetic remains below the entered review threshold, while the event context still belongs in the record.

  • The event is short.
  • The threshold is not a universal limit.
  • Equipment response is not assessed here.

Longer event record

A site log captures a longer event with a large voltage reduction.

Reference
PQ-LONG-EVENT
Pre-event voltage
400 V
Event voltage
320 V
Duration
1200 ms
  1. Voltage change80 V
  2. Sag20%
  3. Remaining voltage80%
Voltage sag20%

80% of the pre-event voltage remains during the event.

The duration and change both point to review of the measurement basis and event classification outside the calculator.

  • Line voltage is used consistently.
  • Duration comes from the same event capture.
  • Formal disturbance classification is separate.

Questions

Is voltage sag the same as voltage drop?

No. Voltage drop is a steady circuit calculation. This page records an event voltage reduction from a measurement record.

Does the calculator classify the disturbance?

No. It calculates the entered voltage change and keeps event classification outside the worksheet.

Where should the threshold come from?

Use a project, equipment, report or engineering threshold that belongs to the record being reviewed.

Can I use line-to-line and phase-to-neutral values together?

No. Keep the pre-event and event voltages on the same measurement basis.