Every field in stage 01 "Circuit & load" — circuit type, voltage, voltage-drop criterion, protective device, load, power factor, parallel runs, and length.
Stage 01 Circuit & load is the first of the two input stages. Filling these in does not calculate anything by itself — press Calculate (or Load a worked example) once both stages are set. See Getting started for the full calculate/recalculate flow.
A segmented control: DC / 1-ph AC / 3-ph AC. Changing it also syncs the circuit arrangement used in stage 02 (cable selection).
Nominal voltage (V) — the system voltage used to compute design current from a kW/kVA load and to express the voltage-drop percentage. Must be greater than zero, and cannot exceed the BS 7671 low-voltage scope this tool's Appendix 4 tables are drawn from: 1000 V for AC circuits, 1500 V for DC. A value above that ceiling is blocked, not just flagged — there is no applicable table data past it.
A select (Voltage-drop limit) that sets the allowable voltage-drop percentage:
| Option | Allowable VD |
|---|---|
| BS 7671 Table 4Ab — lighting (3 %) | 3% |
| BS 7671 Table 4Ab — other uses (5 %) | 5% |
| Custom / project specification | you type the value |
Choosing either BS-referenced option writes the matching percentage into the Custom limit field described below (it stays read-only until you switch to Custom). Selecting Custom / project specification reveals Custom limit as an editable field.
The panel displays this scope note next to the field:
Private-supply whole-installation 6%/8% limits and optional >100 m uplift are not applied.
That is, the two BS-referenced options only implement the BS 7671 Table 4Ab public LV supply / final circuit row (3% lighting, 5% other) — the same row the tool's formula-audit trail cites by name ("Public/final-circuit lighting" / "Public/final-circuit other"). The private-supply whole-installation limits and the optional uplift for runs over 100 m are not available — if your circuit falls under those provisions, select Custom / project specification and enter the applicable percentage yourself.
Only shown when Voltage-drop limit is set to Custom / project specification. The percentage voltage-drop limit used in the pass/fail check. Must be between 0.1% and 30%.
A select with five BS-referenced device types:
The adjacent current field's label changes with the selection:
| Device type | Field label |
|---|---|
| MCCB - BS EN 60947-2 | Long-time trip setting (symbol Ir) |
| all others | Protective device rating (symbol In) |
The field offers a list of standard ratings — 6, 10, 13, 16, 20, 25, 32, 40, 45, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600 A — as suggestions, but you can type any other positive value.
For MCCB, the value you enter is a user-supplied long-time trip setting (Ir) that the tool treats directly as the device's In for the coordination and capacity checks. The tool does not check trip curves, I2, frame/setting range, breaking capacity, discrimination/ selectivity, earth-fault loop impedance (Zs), or disconnection time — verify the entered value against the installed trip unit yourself.
For Semi-enclosed fuse - BS 3036, the tool applies a protective-device factor Cf = 0.725 to the required cable capacity (Cf = 1.00 for all other device types). See Correction factors for how Cf combines with the other correction factors.
A single Load field with an inline unit selector — kW, kVA, or A — next to it. The number you enter is read in whichever unit the selector is set to. Must resolve to a design current greater than zero.
For an AC circuit with kW or kVA selected, the design current is derived using the
Power factor field below. For a DC circuit with kW selected, design current is
P / Vn and power factor is not part of the formula (see
Method and formulas).
Always shown and required, for every circuit type including DC — the field does not hide when you pick DC. The tool supports lagging power factor only. Valid range: greater than 0 and no more than 1. DC's own design-current and voltage-drop formulas do not use the value, but the field must still hold a valid entry before the result panel commits.
A positive integer — the number of cable runs in parallel carrying the load between them. Parallel runs assume identical cables, routes, lengths, and terminations, with equal current sharing between runs. Must be a positive integer (≥ 1).
Run length — the single-direction cable run length, must be greater than zero.
Length unit — m or ft, selects the unit the length value is entered in.
| Rule | Applies to |
|---|---|
| Must be greater than zero | Nominal voltage, design current (derived from load/kW+PF/kVA), run length, protective-device rating |
| Must not exceed the BS 7671 LV scope | Nominal voltage — up to 1000 V for AC circuits, up to 1500 V for DC |
| Must be a positive integer | Parallel runs |
| Range 0.1%-30% | Custom limit (only enforced when Voltage-drop limit is set to Custom) |
| Range 0 < pf ≤ 1 | Power factor (required for every circuit type) |
Any failed rule shows an inline validation message on the offending field instead of a result. Not a blocking rule: a protective-device rating below the design current does not stop the commit — it is reported as a failed Device coordination check inside the result instead, alongside the other two checks. See Reading the result.