BS7671 Voltage-Drop & Current Calculator
• Table 4F2B: Voltage Drop at 90°C.
• Table 4Ab: Max permitted voltage-drop: lighting ≤ 3%, other final circuits ≤ 5%.
Cable Selection & Resistance
Ω
| # | Name | Type | R (Ω) | P (W) | I (A) | V-drop (V) | Action |
|---|---|---|---|---|---|---|---|
| 0 | – | – | – | – | – |
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.
• 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.
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:
- Current-carrying & Derating:
• 16 mm² base rating ~87–100 A in free air, but in conduit/bundles it derates to ~50–60 A.
• 25 mm² remains lawful after grouping & ambient-temperature corrections. - Protective Coordination & Fault Withstand:
• Larger CSA handles higher fault currents (I²t) and resists damage until the breaker clears. - Future Loads & DNO Rules:
• Meter tails/main feeds are often mandated at 25 mm² to allow EV chargers, heat pumps, etc. - Temperature & Longevity:
• Oversizing keeps conductors cooler, reducing insulation ageing.
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.