# Limits and what is not checked

Source: https://meptools.com/tools/voltage-drop-cable-checker/docs/limits-and-assumptions/

The tool implements three specific checks against BS 7671 Appendix 4, for copper conductors.
Everything below states that boundary plainly, so a pass is never read as more than it is.

## What is checked

1. **Protective-device coordination** — Ib ≤ In ≤ Iz.
2. **Corrected current-carrying capacity** — the tabulated ampacity of the selected cable,
   derated by the installation-condition correction factors (Ca, Cs, Cd, Cg, Ci, Cf, Cc,
   Ccores), against the protective-device rating.
3. **Voltage drop** — the calculated drop along the run against the selected voltage-drop
   limit (3% lighting / 5% other / custom).
4. **Smallest-passing-size search** — within the selected cable family, installation method,
   and circuit arrangement, the tool identifies the smallest size that passes both the
   current-capacity and voltage-drop checks (disabled when a manufacturer coefficient
   override is active — see below).

See [Method and formulas](/tools/voltage-drop-cable-checker/docs/method) for the exact
equations behind each of these.

## What is not checked

- **Short-circuit withstand / adiabatic (I²t) rating.** The cable's ability to survive
  fault-current energy for the disconnection time is not evaluated.
- **Earth-fault loop impedance (Zs) and disconnection times.** Not calculated or checked
  against BS 7671 maximum disconnection-time tables.
- **Discrimination / selectivity** between protective devices.
- **Protective-device trip curves, I2, breaking capacity, and frame/setting range.** This
  matters most for **MCCB - BS EN 60947-2**: the long-time trip setting (Ir) you enter is
  taken directly as the device's In for the coordination and capacity checks, but the tool
  does not verify it against the installed trip unit's curve, breaking capacity, or
  frame/setting range.
- **Harmonics and neutral loading.**
- **Diversity.** The design current you enter (or derive from kW/kVA) is used as-is; the tool
  does not apply diversity factors.
- **Aluminium conductors.** The tool covers copper only — there is no aluminium data path.
- **Private-supply whole-installation voltage-drop limits (6%/8%) and the optional uplift for
  runs over 100 m.** Only the BS 7671 Table 4Ab public LV supply / final circuit row (3%
  lighting, 5% other) is implemented as a preset; if your installation falls under the
  private-supply provisions, set **Voltage-drop limit** to **Custom / project specification**
  and enter the applicable percentage yourself.
- **Medium- and high-voltage circuits.** Nominal voltage is capped at the BS 7671 low-voltage
  scope its Appendix 4 tables cover — 1000 V for AC circuits, 1500 V for DC. A value above that
  ceiling is rejected rather than calculated against tables that do not apply.

## Assumptions

- **BS 7671 Appendix 4 tabulated values** are used for ampacity and, unless overridden, for
  the voltage-drop coefficient.
- **Lagging power factor only.**
- **Balanced three-phase** loading.
- **Parallel runs assume identical cables, routes, lengths, and terminations**, with equal
  current sharing between runs — the tool does not model unequal impedance or unbalanced
  sharing across runs.
- **Manufacturer override values are user-entered and must be verified** against the project
  cable data — see
  [Manufacturer override](/tools/voltage-drop-cable-checker/docs/manufacturer-override). Using
  an override also disables the size-comparison table, because a manufacturer coefficient
  applies only to the specific cable it was measured for.
- **Correction-factor inputs are snapped to the nearest supported BS 7671 table value** before
  the factor is looked up — see
  [Correction factors](/tools/voltage-drop-cable-checker/docs/correction-factors) for the
  requested-vs-effective values shown in the panel.

## Disclaimer

The tool displays this notice, verbatim, alongside every result:

> **For engineering guidance and checking only.** Final cable selection must be verified by
> a qualified professional using the applicable code edition, local amendments, manufacturer
> data, and authority requirements.

## Next

- [Method and formulas](/tools/voltage-drop-cable-checker/docs/method) — the equations behind
  every checked quantity.
- [Manufacturer override](/tools/voltage-drop-cable-checker/docs/manufacturer-override) —
  using manufacturer R/X or a direct mV/A/m value, and what it disables.