# Manufacturer override

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

The **Manufacturer override** panel (stage 02, Cable selection) controls where the
voltage-drop coefficient (mV/A/m) used in the voltage-drop calculation comes from, via the
**"Coefficient source"** control.

## Coefficient source options

### BS table values (default)

Uses the tabulated BS 7671 Appendix 4 mV/A/m value for the selected cable. For cables whose
table entry is split into resistive and reactive components, the effective coefficient is:

> mV/A/m = r·cosφ + x·sinφ

where r and x are the table's resistive and reactive mV/A/m components and φ is derived from
the circuit's power factor.

### Manufacturer R + X

Enter the conductor's AC resistance (**"AC resistance per conductor"**) and reactance (**"AC
reactance per conductor"**), both in Ω/km per conductor. The effective coefficient is
computed from these depending on the circuit type:

| Circuit      | Effective coefficient        |
| ------------ | ---------------------------- |
| DC           | mV/A/m = 2R                  |
| Single-phase | mV/A/m = 2(R·cosφ + X·sinφ)  |
| Three-phase  | mV/A/m = √3(R·cosφ + X·sinφ) |

The reactance field is disabled for DC circuits.

**Resistance temperature basis** — a second control, **"Resistance temperature basis"**,
sets how the entered R is interpreted:

- **"At conductor operating temperature"** (default) — R is used as entered, at the
  conductor's rated operating temperature (70/90/60 °C, from the selected cable family).
- **"At 20°C"** — R is treated as R20 and corrected to the conductor's operating temperature
  using:

  > R(θ) = R20 × (1 + α(θ − 20))

  with α = 0.00393 /°C for copper. When this basis is selected, the panel shows a read-only
  **"Effective AC resistance per conductor"** field with the corrected value used in the
  calculation.

### Direct mV/A/m

Enter the effective voltage-drop coefficient directly in the **"Effective coefficient"**
field (mV/A/m) — the only mode in which that field is editable. The UI notes a caveat for
this mode: a single mV/A/m value can't be split back into separate R and X.

## Consequences of using an override

Selecting either **Manufacturer R + X** or **Direct mV/A/m** (i.e. anything other than **BS
table values**) has two effects:

1. **A warning is added** stating that the manufacturer voltage-drop coefficient is
   user-entered and must be verified against the project cable data. The tabulated
   current-capacity check (Iz, from the [correction factors](/tools/voltage-drop-cable-checker/docs/correction-factors))
   is unaffected and continues to run against the BS 7671 tabulated values regardless of the
   coefficient source.
2. **The cable-size comparison table is disabled.** Because a manufacturer coefficient
   applies only to the specific cable it was measured for, the tool cannot use it to evaluate
   other sizes in the family. The result panel shows: "Comparison is unavailable because a
   manufacturer coefficient applies only to the selected cable."

## Next

- [Correction factors](/tools/voltage-drop-cable-checker/docs/correction-factors) — the
  installation-condition factors that still apply to Iz under any coefficient source.
- [Cable & installation selection](/tools/voltage-drop-cable-checker/docs/cable-and-installation) —
  where the Coefficient source control sits within stage 02.
- [Reading the results](/tools/voltage-drop-cable-checker/docs/results) — how the effective
  coefficient and the disabled comparison table appear in the result panel.