Every formula the calculator uses to derive design current, corrected capacity, voltage drop, and maximum length under BS 7671 Appendix 4.
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.
If you enter the load directly as a current, that value is used unchanged. Otherwise Ib is derived from the load and circuit type:
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.
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 for what each
term (Ca, Cs, Cd, Cg, Ci, Cf, Cc, Ccores) corrects for:
Ctotal = Ca × Cs × Cd × Cg × Ci × Cf × Cc × Ccores
The required tabulated capacity for the protective device to be covered is:
It ≥ (In / runs) ÷ Ctotal
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
for what that does and does not verify.
The coefficient can come from three sources, controlled by 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:
mV/A/m = r · cosφ + x · sinφ
Manufacturer R + X, entered in Ω/km:
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:
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.
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.
Lmax = (VDlimit% × Vn ÷ 100) × 1000 × runs ÷ (mV/A/m × Ib)
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.