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Adiabatic Calculator
Calculate the absolute minimum permitted circuit protective conductor size, or verify an existing protective conductor is adequately sized.
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Cable Size Calculator
Find the minimum cable size for a circuit from BS 7671 Appendix 4, applying ambient, grouping and thermal insulation factors, and check voltage drop, earth fault loop impedance (Zs) and the protective conductor thermal limit.
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Demand/Diversity Calculator
Estimate the maximum demand of an installation from its connected load, applying the IET On-Site Guide diversity allowances for the type of premises and circuit.
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kVA Calculator
Convert to or from kVA, Voltage and Amps (Single Phase & Three Phase) or convert to or from kVA, Power Factor and kW.
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Max Zs for RCD Protection
Look up the maximum earth fault loop impedance where an RCD provides fault protection (BS 7671 Table 41.5), and check a measured value.
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Uo 55 V & 63.5 V Zs Values
Look up the maximum Zs for circuit-breakers and fuses on reduced low voltage systems (Uo 55 V and 63.5 V), with 0.8 x values for comparison with measured readings.
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Ohm's Law Converter
Calculate to and from Voltage, Current, Resistance and Power.
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Prospective Fault Current Calculator
Calculate PEFC, PSCC and the Ipf to record from loop impedances or an instrument reading, including the three-phase estimate.
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R1+R2 Zs Ze Calculator
Calculate Zs from Ze and R1+R2, or either of those from the other two, with an optional check of a measured Zs against the maximum for the protective device.
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Voltage Drop Calculator
Calculate cable voltage drop from the BS 7671 Appendix 4 mV/A/m values for single-phase and three-phase circuits, and check it against the limits for public and private supplies.
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Watts, Amps, Volts Converter
Convert to or from Watts, Amps and Volts (Single Phase) or kW, Amps and Power Factor (Three Phase).
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Limiting Temperature Calculator
Calculate how long a fault current can flow before the live conductors reach their limiting temperature (BS 7671 Table 43.1), and check a protective device disconnection time against it.
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CPC Size Calculator
Calculate the minimum circuit protective conductor size from the line conductor size (BS 7671 Table 54.7), including CPCs of a different material or insulation to the line conductor.
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Earth & Bonding Size Calculator
Size the earthing conductor and main protective bonding conductors for the supply earthing arrangement, including PME supplies (BS 7671 Tables 54.1, 54.7 and 54.8).
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R1 (Line) to R2 (CPC) Calculator
Estimate the CPC resistance (r2) from a measured line conductor resistance (r1) and the conductor sizes, and the resulting R1+R2 for radial and ring final circuits.
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Max Zs Converter
Convert maximum Zs values from older editions of BS 7671 (16th and 17th Edition) to the current basis, applying the Cmin voltage factor, and compare with the current tables.
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Zs Values - Common Devices
Look up the maximum Zs for circuit-breakers and fuses (BS 7671 Tables 41.2 to 41.4) at 0.4 s and 5 s, with 0.8 x values for comparison with measured readings.
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r1 r2 to R1+R2 Converter
Convert end-to-end r1, r2 and rn readings to R1+R2 for ring final and radial circuits, with ring continuity checks and values corrected to conductor operating temperature.
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Current Carrying Capacity
Look up the tabulated current-carrying capacity of a cable from BS 7671 Appendix 4, and apply ambient temperature, grouping and thermal insulation factors to find its installed rating.
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Ring Final Circuit Verification
Verify a ring final circuit — calculate Zs at the worst-case mid-point using the ring formula and check compliance with the maximum permitted Zs for the protective device.
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Maximum Circuit Length
Calculate the maximum permitted circuit length for a given cable type, size, and protective device before the voltage drop limit or maximum Zs limit is exceeded.
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Power Factor Correction
Calculate the capacitor bank size (kVAr) required to correct power factor from an existing value to a desired target value.
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Motor Starting Calculator
Calculate full-load current, starting current and starting kVA for single-phase and three-phase motors with direct-on-line, star-delta, soft-start, auto-transformer and VFD starting.
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Earth Electrode Resistance (TT)
Calculate the maximum earth electrode resistance for a TT installation protected by an RCD, and check that the touch voltage stays within 50 V (BS 7671 Reg 411.5.3).
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Short Circuit Current (Remote End)
Calculate the maximum and minimum earth fault or short-circuit current at the far end of a cable run from the fault current at its origin, the cable type, size and length.
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Conduit & Trunking Fill
Check whether a mix of single-core cables will fit in a conduit or trunking, using the cable factor method of the IET On-Site Guide Appendix E.
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Insulation Resistance Check
Check whether a measured insulation resistance value meets the minimum requirements of BS 7671 Table 64 for the relevant circuit type and test voltage.
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Energy & Running Cost
Calculate the energy consumption and running cost for an electrical load given its power rating, operating hours, and electricity unit rate.
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Cable Impedance (First Principles)
Calculate conductor resistance and R1+R2 from first principles using resistivity and temperature coefficients — useful for non-standard cables not in the BS 7671 tables.
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IP Rating Reference
Look up the meaning of IP (Ingress Protection) rating codes for solid particle and liquid ingress protection, with the minimum IP ratings BS 7671 requires in special locations such as bathrooms and swimming pools.
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Cable Colour Code Reference
Look up the old (pre-2004) and current harmonised cable identification colours for single-phase, three-phase and DC systems, and when a warning notice is needed for mixed colours.
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Transformer Fault Current
Calculate the prospective short-circuit current at the secondary terminals of a transformer from its kVA rating, percentage impedance, and secondary voltage.
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Voltage Unbalance
Calculate the voltage unbalance of a three-phase supply from the three line-to-line voltages, as defined in EN 50160, and check it against the 2% limit.
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Discrimination Checker
An indicative check of whether two series-connected protective devices will discriminate, using recognised simple rules. Confirm with the manufacturers' selectivity data.