BS7671 Voltage-Drop & Current Calculator

Refs (BS 7671:2018)
• Table 4F2B: Voltage Drop at 90°C.
• Table 4Ab: Max permitted voltage-drop: lighting ≤ 3%, other final circuits ≤ 5%.
     
*Quadratic only active if exactly one constant-power load.

Cable Selection & Resistance

         Ω
Rcable = Ω/m × length × Ct; clamped ≥ 1 μΩ. Or override manually.
#NameType R (Ω)P (W) I (A)V-drop (V)Action
0

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Resistive vs. Constant-Power Loads

Resistive Loads:
• Current ∝ Voltage (I = V/R).
• Examples: heaters, toasters, incandescent lamps.
• If voltage droops, they draw less current and power falls (P = V²/R).

Constant-Power Loads:
• Internal regulators (switch-mode PSUs, LED drivers, modern electronics).
• They try to keep P = V·I ≈ constant, so if V falls, I rises (until limits).
• Examples: PCs, TVs, phone chargers, LED lighting drivers.

Note: Motors & ballasts have their own nonlinear V–I behaviour and may need special treatment.

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Why Iterative Bisection?

• Our circuit is Vs → Rc → (parallel loads).
• Each load i has Iᵢ = Vload/Rᵢ (resistive) or Iᵢ = Pᵢ/Vload (constant-power), with
Vload = Vs – Rc·I_total.
• Summing gives h(I) = Σ Iᵢ(Vs – Rc·I) – I = 0.

• When there’s exactly one constant-power load (or all loads lumped into one P), h(I) becomes a quadratic:
a·I² + b·I + c = 0 → closed-form solution.
• With mixed or multiple loads, h(I) is higher-order or rational—no single quadratic applies.

• Bisection brackets the root I in [0, Vs/Rc] and halves the interval each pass,
guaranteeing convergence whenever h(0)>0 and h(Imax)<0.
• Each pass doubles the precision: ~20–30 passes → ~10⁻⁶ accuracy.

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Why 25 mm² Service Cables?

In ideal conditions, a 45 m run of 16 mm² Cu at 90 °C carrying 25 kW (≈107 A)
drops only about 7.11 V (2.96 % of 240 V), which meets the voltage-drop limit.

But BS 7671 compliance requires more than just voltage-drop checks:

Summary: Although 16 mm² can pass a single voltage-drop test at 107 A, real installations must also satisfy current rating, derating, fault-clearance, future proofing and DNO rules—hence the widespread use of 25 mm².