Electrical calculations with the working shown
Max Zs, R1 + R2, fault current, voltage drop and more. Every result shows its formula and your numbers, and the testing values are checked against BS 7671 and the IET’s tables.
23calculators
155London suppliers
152with opening hours
Max Zs lookup
BS 7671 Table 41.3, 230 VMaximum Zs
1.37 Ω
Test reading at 10 °C1.10 Ω
0.95 × 230 V ÷ (5 × 32 A) = 1.365625 Ω. Full table and calculator
Calculators
Grouped by what they are for. Each one states what it covers, shows its formula and works through your numbers.
Testing & protection
- R1 + R2Calculate R1 + R2 for standard copper conductor sizes, or from your own resistance per kilometre.
- Max ZsMaximum earth fault loop impedance for type B, C and D circuit-breakers and RCBOs in TN systems, with the value to compare test readings against.Zs ≤ U0 × Cmin ÷ Ia
- Earth fault loopAdd Ze and R1 + R2 to get Zs, then compare it with the maximum for a type B, C or D circuit-breaker or RCBO in a TN system.Zs = Ze + (R1 + R2)
- Ring finalExpected readings for the ring final cross-connection test, from the end-to-end resistances r1, rn and r2.
- Earth conductor sizeThe minimum copper protective conductor area that passes the adiabatic check, from fault current and disconnection time or from a manufacturer's let-through energy.S = √(I² × t) ÷ k
Power & current
- Ohm’s lawFind voltage, current, resistance and power in a DC resistive circuit.V = I × R
- Watts to ampsConvert electrical input power to current, with an explicit supply type.I = P ÷ (V × PF)
- kW to ampsConvert kilowatts to amps for DC, single-phase or balanced three-phase.I = kW × 1,000 ÷ (V × PF)
- Amps to kWCalculate real electrical power from current, voltage and power factor.kW = V × I × PF ÷ 1,000
- kVA ↔ kWConvert between apparent and real power using your power factor.kW = kVA × PF
- Prospective fault currentCompare line–neutral and line–earth fault-current estimates from separate loop impedances.
- Power factor correctionCalculate theoretical reactive compensation for a sinusoidal, lagging load.
- Three-phase imbalanceCompare three RMS line currents using maximum deviation from their average.
Cables & circuits
- Voltage dropCalculate voltage drop using a supplied circuit coefficient, current and route length.
- AWG to mm²Convert American Wire Gauge to nominal conductor diameter and cross-sectional area.
- Conduit cross-sectionCompare round cable cross-sectional area with a conduit’s usable internal area.
- Tray & trunking areaCompare round cable area with a specified rectangular containment cross-section.
Energy & cost
Planning & estimates
How the numbers are checked
The testing and protection calculators use values from BS 7671 and the IET’s guidance. Each value is checked two ways.
- Worked out from its formula, such as Zs ≤ U0 × Cmin ÷ Ia for maximum Zs.
- Compared with the printed tables: all 49 entries of Table 41.3, and the IET’s corrected test-reading tables.
- Limits stated on every page, including what a calculator leaves out, such as fuses and TT systems.
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