Common questions about scope, standards, inputs, and results for the Voltage Drop & Cable Size Checker.
BS 7671 Appendix 4, for copper conductors only. Ampacity, correction factors, and (unless you override it) the voltage-drop coefficient all come from Appendix 4 tables. See Method and formulas for the exact equations.
No. Adiabatic (I²t) short-circuit withstand, Zs, and disconnection times are not calculated. See Limits and what is not checked for the full list of what falls outside the tool's scope.
Yes. The Manufacturer override panel lets you enter AC resistance and reactance (R + X) per conductor, or a direct mV/A/m value, in place of the tabulated voltage-drop coefficient. The current-capacity check still runs against the BS 7671 tabulated ampacity regardless of which coefficient source you choose.
A manufacturer coefficient (R + X or direct mV/A/m) applies only to the specific cable it was measured for, so the tool cannot use it to evaluate other sizes in the family. The comparison table is disabled whenever a manufacturer override is active — switch the coefficient source back to BS table values to restore it.
Whichever you select under Voltage-drop criterion: 3% for public/final-circuit lighting, 5% for public/final-circuit other, or a value you type under Custom. The BS-referenced presets come from Table 4Ab, public LV supply / final circuit only — private-supply whole-installation limits (6%/8%) and the optional uplift for runs over 100 m are not applied. Use Custom if your circuit needs one of those. See Method and formulas.
No. Calculating is free and requires no sign-in. An account is only needed to save a project or export a PDF report. See Getting started.
No. The tool covers copper conductors only — there is no aluminium data path anywhere in the calculation.
For MCCB - BS EN 60947-2, the field is labelled Long-time trip setting (Ir), and whatever you enter is used directly as the device's In for the coordination and current-capacity checks. The tool does not check the trip curve, I2, breaking capacity, or frame/setting range against an actual trip unit — verify the entered value yourself. See Limits and what is not checked.
A pass means the three implemented checks (device coordination, corrected capacity, voltage drop) succeeded for your inputs — it is not a full BS 7671 compliance declaration. The warnings panel lists standing scope notes (such as the voltage-drop limit basis or a manufacturer-override reminder) and any condition-specific flags, and they apply whether the overall result is a pass or a fail.