Ohm's Law & Power
Ohm's Law
V = I × R
Fill in any two values — the third is calculated.
Result
Power (Watts)
P = V × I
Fill in any two values — the third is calculated.
Result
kVA ↔ kW Converter
kW = kVA × PF
Fill in any two to get the third.
Result
Current & Circuits
Single Phase Current
I = kW×1000 / (V×PF)
Full Load Current
Amps (A)
Three Phase Current
I = kW×1000 / (√3×V×PF)
Full Load Current
Amps (A) per phase
MCB / Fuse Size
In ≥ Ib
Minimum Breaker
Cable Sizing BS 7671 · 18th Edition
Cable Size Calculator
It ≥ In / (Ca×Cg×Cc×Ci)
Calculates minimum cable CSA using BS 7671 Appendix 4 correction factors. Covers T&E (flat twin), SWA, and flex.
Recommended Minimum Cable Size
Voltage Drop BS 7671 · 3% limit
Voltage Drop Calculator
Vd = (mV/A/m × I × L) / 1000
Voltage Drop
Conduit Fill BS 7671 · Space Factor 40%
Conduit Fill Calculator
Fill ≤ 40% of conduit CSA
BS 7671 requires cables to fill no more than 40% of the conduit internal CSA to allow cables to be drawn in without damage.
Cable groups (number of each)
Fill Result
Ring Final Circuit BS 7671 · Reg 433.2
Ring Main Load Check
Ib ≤ 20A · Floor Area ≤ 100m²
A standard 32A ring final circuit in a domestic property. Check total load and floor area compliance.
Ring Circuit Assessment
Spur Assessment
Sockets on spur ≤ 1
Check if a spur off a ring circuit is within the limits of BS 7671 and GN3.
Spur Assessment
Fault Current & Protection
Prospective Fault Current
Isc = V / Zs
Prospective Fault Current
Motor Starting Current
Istart ≈ FLC × Mult
Starting Current
Power & Energy Costs
Running Cost Calculator
Cost = kWh × Rate
Annual Running Cost
Max Demand Calculator
Pmax = Σ(P × DF)
Estimate the maximum demand for a domestic or small commercial installation.
Maximum Demand
Quick Reference Tables
Common Formulas
| Formula | Description | Variables |
|---|---|---|
| V = I × R | Ohm's Law — Voltage | V=Volts, I=Amps, R=Ohms |
| P = V × I | Single phase power | P=Watts |
| P = I² × R | Power from current & resistance | — |
| I = P / (√3 × V × PF) | 3-phase full load current | √3 ≈ 1.732 |
| Vd = (mV/A/m × Ib × L) / 1000 | Voltage drop (BS 7671 method) | L in metres |
| It ≥ In / (Ca×Cg×Cc×Ci) | Cable tabulated current needed | Correction factors |
| Isc = Vp / Zs | Prospective fault current | Zs = loop impedance |
| kW = kVA × PF | Real vs apparent power | PF = power factor |
| E = P × t / 1000 | Energy (kWh) | P=Watts, t=hours |
| cos φ = kW / kVA | Power factor angle | φ = phase angle |
BS 7671 Max Zs Values — Common Devices
| Device | Rating | Max Zs (Ω) — 0.4s | Max Zs (Ω) — 5s |
|---|---|---|---|
| Type B MCB | 6A | 7.67 | — |
| Type B MCB | 16A | 2.87 | — |
| Type B MCB | 20A | 2.30 | — |
| Type B MCB | 32A | 1.44 | — |
| Type C MCB | 16A | 1.44 | — |
| Type C MCB | 32A | 0.72 | — |
| Type D MCB | 16A | 0.72 | — |
| BS 88 fuse | 32A | 1.09 | 1.84 |
| BS 88 fuse | 63A | 0.47 | 0.82 |
| 30mA RCD | Any | 1667Ω max | — |
Standard Cable Current Ratings (70°C PVC, Clipped Direct)
| CSA | 2-core (A) | 3-core (A) | Max V-drop (mV/A/m) |
|---|---|---|---|
| 1.0mm² | 14.5 | 13.5 | 36.2 |
| 1.5mm² | 18.5 | 17.5 | 24.0 |
| 2.5mm² | 25 | 23 | 15.0 |
| 4.0mm² | 32 | 30 | 9.5 |
| 6.0mm² | 43 | 38 | 6.4 |
| 10mm² | 57 | 52 | 3.8 |
| 16mm² | 75 | 68 | 2.4 |
| 25mm² | 96 | 87 | 1.5 |
| 35mm² | 119 | 107 | 1.15 |
| 50mm² | 150 | 134 | 0.87 |
| 70mm² | 185 | 168 | 0.63 |
| 95mm² | 232 | 210 | 0.47 |
Minimum Earth Conductor Sizes (BS 7671 Table 54.7)
| Phase/Line Conductor | Min CPC (same material) | Min CPC (different material) |
|---|---|---|
| ≤16mm² | Same CSA as phase | Equivalent conductance |
| 16mm² – 35mm² | 16mm² | Equivalent conductance |
| >35mm² | Half the phase CSA | Equivalent conductance |