What the voltage-drop hero figure, the three checks, the metrics, the cable-size comparison table, and the formula audit each mean.
The result panel is empty until you press Calculate (or Load a worked example, which runs immediately) — it does not recompute as you type. Once a result is showing, editing any input flips the action rail from Calculated to Recalculate and shows a strip: "Inputs changed. You're looking at the previous run." — everything below still reflects that previous run until you press Recalculate. See Getting started for the full calculate/recalculate cycle.
The hero figure is Voltage drop, the calculated drop as a percentage of nominal voltage, shown to two decimal places. Beneath it a badge reads PASSES or FAILS against your selected voltage-drop limit (the Allowable VD % set in stage 01). Below that:
<VD> V drop · receiving end <Vr> V — the drop restated in volts, and the resulting
receiving-end voltage.Below the headline, three checks are shown side by side:
| Check | What it reports |
|---|---|
| Device coordination | PASS or FAILS — whether the design current, device rating, and cable capacity satisfy Ib ≤ In ≤ Iz. |
| Current capacity | PASS or FAILS, with the corrected Iz per run in amps — the cable's tabulated ampacity after installation correction factors. |
| Voltage drop | PASS or FAILS, with the voltage-drop percentage again. |
The overall result (the colour of the whole readout) passes only if all three checks pass. A single failing check fails the overall result even if the other two pass.
A row of four metric tiles beneath the checks:
| Metric | Unit | Meaning |
|---|---|---|
| Design current / run | A | The design current per parallel run (Ib), after dividing by the number of parallel runs. |
| Corrected Iz / run | A | The tabulated current-carrying capacity after installation correction factors, per parallel run. |
| Coefficient | mV/A/m | The voltage-drop coefficient used for the calculation — from the BS 7671 table, or your manufacturer override if active. |
| Max length | m | The longest one-way run length that would still pass the voltage-drop check at these currents and this coefficient. |
The Cable size comparison table lists every compatible cable size in the selected family/installation-method/circuit combination, with one row per size:
| Column | Meaning |
|---|---|
| Size | Cable size, mm². |
| Table Iz | The tabulated (uncorrected) ampacity for that size. |
| Corrected Iz | Table Iz after installation correction factors. |
| mV/A/m | The voltage-drop coefficient for that size. |
| VD (V) | Calculated voltage drop in volts at your current and length. |
| VD % | Calculated voltage drop as a percentage of nominal voltage. |
| L max | The longest run length that size could carry within the allowable VD %. |
| Status | See below. |
Status values:
| Status | Meaning |
|---|---|
| AMPACITY FAIL | Corrected Iz is below the device rating for that size. |
| VD FAIL | Ampacity passes but the voltage-drop percentage exceeds your allowable VD %. |
| SELECTED · SMALLEST OK | This is both the size you selected and the smallest size that passes both checks. |
| SMALLEST OK | The smallest size that passes both checks, but not the one currently selected. |
| SELECTED | The size you selected, passing both checks, but not the smallest that does. |
| PASS | Passes both checks; neither selected nor the smallest passing size. |
The tool always identifies the smallest compatible size where both the current-capacity and voltage-drop checks pass, regardless of which size you have selected. To keep the table manageable, only a window of rows around the selected and smallest-passing sizes is shown initially; a Show all N compatible sizes button reveals the full list.
The comparison is only available when the coefficient comes from the BS table — if a manufacturer coefficient override is active, the comparison is hidden because that coefficient applies only to the single selected cable. See Cable and installation selection for the manufacturer override.
The Calculation steps panel (open by default) is a full audit trail: for every step — design current, device coordination (Ib ≤ In ≤ Iz), each correction factor applied, required/ selected/corrected capacity, the voltage-drop coefficient, the voltage-drop limit, the calculated VD and VD%, the receiving voltage, and the maximum length — it shows the symbolic formula, the substituted values, and the numeric result, with a source-table reference where one applies.
When the comparison table is available, a final Minimum-size evaluation step is appended, stating how many compatible sizes were evaluated and which is the smallest suitable size (or that none passes).
See Method and formulas for the equation behind each audit step.
The collapsible Assumptions and warnings panel lists:
These two notes are always shown, independent of your inputs.