# Method and formulas

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

This page states the calculation exactly as implemented, so you can reproduce any reported
figure by hand. The tool covers **BS 7671 Appendix 4, copper conductors only** — there is no
other standard or conductor material behind any result it shows.

## Design current (Ib)

If you enter the load directly as a current, that value is used unchanged. Otherwise Ib is
derived from the load and circuit type:

```text
Entered directly:            Ib = Iuser

From kW:
  DC:                         Ib = P / Vn
  1-phase AC:                 Ib = P / (Vn · cosφ)
  3-phase AC:                 Ib = P / (√3 · VLL · cosφ)

From kVA:
  1-phase AC:                 Ib = S / Vn
  3-phase AC:                 Ib = S / (√3 · VLL)
```

`P` is active power in W, `S` is apparent power in VA, `Vn` is the nominal (phase) voltage,
`VLL` is the line-to-line voltage, and `cosφ` is the power factor.

## Corrected current-carrying capacity (Iz)

```text
Iz = It × Ctotal
```

`It` is the BS 7671 Appendix 4 tabulated ampacity for the selected cable family, installation
method, and size. `Ctotal` is the product of every applicable correction factor — see
[Correction factors](/tools/voltage-drop-cable-checker/docs/correction-factors) for what each
term (Ca, Cs, Cd, Cg, Ci, Cf, Cc, Ccores) corrects for:

```text
Ctotal = Ca × Cs × Cd × Cg × Ci × Cf × Cc × Ccores
```

The required tabulated capacity for the protective device to be covered is:

```text
It ≥ (In / runs) ÷ Ctotal
```

### Protective-device coordination

```text
Ib ≤ In ≤ Iz
```

For every device type except MCCB, `In` is the rated current you select. For **MCCB - BS EN
60947-2**, the value you enter is a user-supplied long-time trip setting (Ir), and the
calculation uses it directly as `In` — see
[Limits and what is not checked](/tools/voltage-drop-cable-checker/docs/limits-and-assumptions)
for what that does and does not verify.

## Voltage-drop coefficient (mV/A/m)

The coefficient can come from three sources, controlled by
[Manufacturer override](/tools/voltage-drop-cable-checker/docs/manufacturer-override):

**BS table value (default).** Where the Appendix 4 table gives a single scalar mV/A/m figure,
it is used directly. Where the table splits the value into resistive and reactive components:

```text
mV/A/m = r · cosφ + x · sinφ
```

**Manufacturer R + X**, entered in Ω/km:

```text
DC:            mV/A/m = 2R
1-phase AC:    mV/A/m = 2(R · cosφ + X · sinφ)
3-phase AC:    mV/A/m = √3(R · cosφ + X · sinφ)
```

R can optionally be entered at 20°C and corrected to the conductor's operating temperature:

```text
R(θ) = R20 × (1 + α(θ − 20))          α = 0.00393 /°C for copper
```

**Direct mV/A/m** — a manufacturer-supplied coefficient entered and used as-is.

## Voltage drop

```text
VD  = (mV/A/m × Ib × L) ÷ (1000 × runs)
VD% = VD ÷ Vn × 100
Vr  = Vn − VD
```

`L` is the one-way length, `runs` is the number of parallel runs, and `Vr` is the
receiving-end voltage.

## Maximum permissible length

```text
Lmax = (VDlimit% × Vn ÷ 100) × 1000 × runs ÷ (mV/A/m × Ib)
```

## Voltage-drop limit basis

The **Voltage-drop criterion** control sets `VDlimit%` from BS 7671 Table 4Ab, public LV
supply / final circuit:

| Basis                         | VDlimit         |
| ----------------------------- | --------------- |
| Public/final-circuit lighting | 3%              |
| Public/final-circuit other    | 5%              |
| Custom                        | value you enter |

Whenever a BS-referenced basis (not Custom) is selected, this scope note applies, verbatim:

> Private-supply whole-installation 6%/8% limits and optional >100 m uplift are not applied.

If your circuit falls under those provisions, select **Custom** and enter the percentage you
need — the tool does not derive it for you.

## Assumptions and limits

- **Copper conductors only.** There is no aluminium option anywhere in the tool.
- **BS 7671 Appendix 4 tabulated data** for ampacity and, where used, the tabulated
  voltage-drop coefficient.
- **Lagging power factor only** — leading power factor is not supported.
- **Balanced three-phase** loading; no unbalance or neutral-current modelling.
- **Parallel runs assume identical cables, routes, lengths, and terminations**, with equal
  current sharing between runs.
- **A pass means the implemented checks pass** — device coordination, corrected capacity, and
  voltage drop — not a full BS 7671 compliance declaration. See
  [Limits and what is not checked](/tools/voltage-drop-cable-checker/docs/limits-and-assumptions)
  for everything outside that scope.