Manufacturer override — MEP Tools

How to replace the tabulated BS 7671 voltage-drop coefficient with manufacturer resistance/reactance or a direct mV/A/m value.

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) 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 — the installation-condition factors that still apply to Iz under any coefficient source.
• Cable & installation selection — where the Coefficient source control sits within stage 02.
• Reading the results — how the effective coefficient and the disabled comparison table appear in the result panel.