# Frequently asked questions

Source: https://meptools.com/tools/voltage-drop-cable-checker/docs/faq/

## Which standard does this tool use?

**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](/tools/voltage-drop-cable-checker/docs/method) for the exact equations.

## Does it check short-circuit withstand or earth-fault loop impedance (Zs)?

No. Adiabatic (I²t) short-circuit withstand, Zs, and disconnection times are not calculated.
See [Limits and what is not checked](/tools/voltage-drop-cable-checker/docs/limits-and-assumptions)
for the full list of what falls outside the tool's scope.

## Can I use manufacturer cable data instead of the BS tables?

Yes. The [Manufacturer override](/tools/voltage-drop-cable-checker/docs/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.

## Why did the cable-size comparison table disappear?

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.

## What voltage-drop limit does it apply?

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](/tools/voltage-drop-cable-checker/docs/method#voltage-drop-limit-basis).

## Do I need an account to use it?

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](/tools/voltage-drop-cable-checker/docs/getting-started).

## Does it size aluminium conductors?

No. The tool covers copper conductors only — there is no aluminium data path anywhere in the
calculation.

## What does the MCCB "Ir" field mean?

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](/tools/voltage-drop-cable-checker/docs/limits-and-assumptions).

## Why does a result say "PASS" but the tool still shows warnings?

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.