Voltage Sag Calculator

Electrical Purpose

A sudden load step—motor starting, welder firing, large contactor closing—draws a current spike against the source impedance of the supply. That current spike produces a momentary drop in terminal voltage, commonly called voltage sag or voltage dip. The Voltage Sag Calculator converts two field-measurable quantities, available fault current and a load-current step, into an estimated sag in volts and percent, using the source impedance implied by the fault current.

This is not a voltage-drop-over-distance calculation. Voltage drop from conductor resistance is a steady-state IR loss along a known conductor length; voltage sag is a transient dip driven by source impedance reacting to a current step. The two concepts use different inputs and answer different questions, but both are checked when a load causes flicker, nuisance tripping, or equipment reset at start-up.

Inputs

• System voltage — the voltage basis for the sag estimate, in volts. Enter line-to-line voltage for three-phase estimates (480 V in the example).
• Available fault current — the available fault current at the point of interest, in amperes, used as a source-strength screening proxy (10,000 A in the example). Higher fault current implies a stiffer source and lower impedance.
• Load-current step — the sudden load or motor-starting current step being screened, in amperes (800 A in the example).
• Phase count — Single-phase or Three-phase, selected to set the impedance and sag formula used for the display.

Outputs

• Estimated source impedance — the implied source impedance in ohms, derived from system voltage and available fault current.
• Estimated voltage sag — the voltage dip in volts produced by the load-current step across that source impedance.
• Estimated sag — the dip expressed as a percentage of system voltage.
• Remaining voltage — the voltage remaining during the step, in volts and as a percent of system voltage.

Formula and Calculation Logic

For a three-phase, line-to-line voltage basis, the calculator derives source impedance from the fault current, then applies the load step to that impedance:

Estimated source impedance: \(Z = \frac{V_{system}}{\sqrt{3} \times I_{fault}}\)

Estimated voltage sag: \(V_{sag} = \sqrt{3} \times I_{step} \times Z\)

Estimated sag (%): \(\text{Sag (\%)} = \frac{V_{sag}}{V_{system}} \times 100\)

Remaining voltage: \(V_{remaining} = V_{system} – V_{sag}\)

The single-phase form drops the √3 term and applies \(Z = V_{system} / I_{fault}\) and \(V_{sag} = I_{step} \times Z\) directly. In both cases, the calculator treats fault current as a proxy for source strength rather than measuring true Thevenin impedance, phase angle, or X/R ratio.

Calculation Example

With system voltage 480 V, available fault current 10,000 A, load-current step 800 A, and phase count Three-phase:

1. Estimated source impedance: \(480 / (\sqrt{3} \times 10{,}000) = 480 / 17{,}320.5 = 0.0277\ \Omega\)

2. Estimated voltage sag: \(\sqrt{3} \times 800 \times 0.0277 = 38.4\ \text{V}\)

3. Estimated sag: \(38.4 / 480 \times 100 = 8\%\)

4. Remaining voltage: \(480 – 38.4 = 441.6\ \text{V}\), or (92%) of system voltage

An 8% sag at motor start is within the range that typically causes visible flicker and can trip undervoltage-sensitive contactors or drives set to trip below roughly 90% of nominal, depending on manufacturer settings.

Where This Result Is Used

The sag percentage and remaining voltage feed directly into equipment-tolerance review: contactor and PLC undervoltage settings, VFD ride-through settings, and lighting flicker complaints tied to motor or compressor starting. A calculated 8% sag against a load with a documented 10% dropout threshold indicates the load will hold through the start; a load rated for only 5% dropout will not. The same source-impedance figure is a starting point for reviewing available fault current at a panel, transformer sizing adequacy, and whether a soft starter, VFD, or reduced-voltage starting method is justified to limit the current step rather than accept the sag.

Field and Application Limits

Available fault current entered here is a scalar screening value, not a full short-circuit study result with X/R ratio, motor contribution, or utility source impedance breakdown. System voltage must match the actual line-to-line basis at the point being evaluated; mixing a line-to-neutral value into a three-phase calculation misstates the impedance and sag by a factor of √3. The calculator does not account for transformer impedance separately from the fault-current-derived impedance, does not model harmonic content, and does not apply any NEC article or utility power-quality standard—those checks (equipment undervoltage ratings, utility flicker limits, IEEE 519/1159 references where applicable) are separate engineering and utility-coordination steps that must be verified against actual field measurements, a certified short-circuit study, or the serving utility’s requirements before treating an 8% or any other calculated sag figure as accepted for a given installation.