Free BS 7671 Electrical Calculators

The Problem

Electrical design shouldn't depend on a spreadsheet

Electricians and electrical engineers spend too much time hunting through separate online calculators, recreating spreadsheet formulas, and cross-referencing BS 7671 tables. The Electrical Toolbox gives you one consistent place for every BS 7671 calculation you need - no setup, no cost.

Without the Toolbox

Adiabatic calculations done by hand or in unvalidated spreadsheets
Cable sizing across multiple disconnected tools with inconsistent results
Zs / Ze / R1+R2 calculations verified with a calculator and tables
Voltage drop checked from a separate tool with no unit control
Maximum Zs values looked up from BS 7671 appendices each time
Demand and diversity calculations done ad hoc for each project
Ring final circuit Zs worked out manually using the ring formula
Conduit fill checked against OSG tables by hand each time
Motor starting currents estimated from data sheets and tables
Insulation resistance pass/fail verified from BS 7671 Table 64
IP ratings looked up from the standard or product datasheets

With the Electrical Toolbox

Adiabatic equation (Reg 543.1.3) calculated with correct k values instantly
Full cable sizing with current capacity, voltage drop and Zs checks in one tool
Zs, Ze or R1+R2 from the other two, with a check against the device's maximum Zs
Voltage drop calculated per BS 7671 Appendix 4 with compliance check
Maximum Zs looked up instantly for circuit-breakers, fuses and RCDs, with 0.8 × values for measured readings
OSG diversity rules applied automatically by circuit description and premises type
Ring final circuit Zs verified with the ring formula and ring balance check
Conduit and trunking fill checked against IET On-Site Guide Appendix E cable factors instantly
Motor full-load and starting current calculated for DOL, star-delta, soft-start, auto-transformer and VFD starting
Insulation resistance values checked against BS 7671 Table 64 pass/fail automatically
IP rating codes looked up instantly with environment-specific minimum requirements

How It Works

Register once. Start calculating immediately.

The Electrical Toolbox is designed to be useful from the moment you log in. No configuration. No data to import. Just open the tool you need and get your answer.

Create your free account

Register in under a minute through the Construct Software ID system. Your account gives you immediate access to all 34 tools - no credit card, no trial period, no catch.

Open the tool you need

Pick from 34 purpose-built calculation tools covering adiabatic equations, cable sizing, maximum Zs lookup, voltage drop, demand & diversity, ring final circuit verification, motor starting, conduit fill, power factor correction, energy costing and more.

Enter your values, get your result

Each tool has a clean, engineer-friendly interface. Enter your known values and the result is calculated instantly - with full BS 7671 references throughout.

The Toolbox

34 tools. Every BS 7671 calculation you need.
Nothing you don't.

Adiabatic Calculator

Calculate the absolute minimum permitted circuit protective conductor size, or verify an existing protective conductor is adequately sized.

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.

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.

kVA Calculator

Convert to or from kVA, Voltage and Amps (Single Phase & Three Phase) or convert to or from kVA, Power Factor and kW.

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.

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.

Ohm's Law Converter

Calculate to and from Voltage, Current, Resistance and Power.

Prospective Fault Current Calculator

Calculate PEFC, PSCC and the Ipf to record from loop impedances or an instrument reading, including the three-phase estimate.

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.

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.

Watts, Amps, Volts Converter

Convert to or from Watts, Amps and Volts (Single Phase) or kW, Amps and Power Factor (Three Phase).

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

Power Factor Correction

Calculate the capacitor bank size (kVAr) required to correct power factor from an existing value to a desired target value.

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.

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).

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.

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.

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.

Energy & Running Cost

Calculate the energy consumption and running cost for an electrical load given its power rating, operating hours, and electricity unit rate.

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.

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.

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.

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.

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.

Discrimination Checker

An indicative check of whether two series-connected protective devices will discriminate, using recognised simple rules. Confirm with the manufacturers' selectivity data.

Who It's For

Built for anyone who designs,
installs or inspects electrical systems

The Electrical Toolbox covers the calculations that come up day-to-day across the full breadth of electrical contracting and engineering work.

Electrical Engineers & Designers

For engineers producing compliant designs

Quickly check cable sizing, calculate adiabatic minimums, confirm voltage drop compliance and verify Zs against maximum permitted values during design – without switching between separate tools or BS 7671 appendices.

Adiabatic equation (Reg 543.1.3) with correct k factor selection
Full cable sizing: current capacity, voltage drop and Zs in one tool
Maximum circuit length before voltage drop or Zs limits are exceeded
Power factor correction – capacitor bank kVAr from existing and target PF
Transformer fault current and motor starting current calculations
CPC, earthing and main bonding conductor sizing to BS 7671 Tables 54.1, 54.7 and 54.8
Load and diversity estimation for sub-main and final circuit design
Indicative discrimination check between series-connected protective devices

Electrical Contractors & Inspectors

For contractors installing and inspecting on site

On-site and design calculations for Zs / Ze / R1+R2, ring final circuit verification, insulation resistance checks, temperature correction of measured values, fault current analysis, conduit fill and discrimination checking – all available from any browser without installing anything.

Ring final circuit verification – ring formula Zs with ring balance check
Zs / Ze / R1+R2 calculator, checking measured Zs against 0.8 × the maximum
Insulation resistance pass/fail to BS 7671 Table 64
R1+R2 temperature correction (×1.20 at 70°C / ×1.28 at 90°C)
Conduit and trunking fill per IET On-Site Guide Appendix E
Indicative discrimination check for series-connected protective devices
PSCC and PFC calculations for fault level records and schedule values
Maximum measured Zs values (0.8 × tabulated) for Schedule of Test Results checks

Construct Software Ecosystem

Part of a broader suite of
specialist electrical tools

The Electrical Toolbox sits alongside a growing suite of Construct Software products covering cable calculation, test and certification, compliance management, facilities management and more - all built specifically for the electrical contracting industry.

SafetyCert

Compliance management for PAT testing, Minor Works, EICRs and distribution board documentation

CableSpec Pro

BS 7671 cable calculation for single circuits and multi-circuit projects with full project reports

ElectraCert

BS 7671 test & certification - EICs, EICRs, Minor Works certificates with PDF generation

ContractorCompliance

Contractor insurance, accreditation and competency management for procurement teams

FacilitiesMaster

Full-stack FM platform for PPM, assets, reactive jobs and compliance documentation

InterimReport

Structured electrical interim inspection reports and checklists

TESCert

BS 7909 temporary electrical system certification for events and media installations

More coming

The Construct Software suite continues to grow with new specialist tools for the electrical industry

Learn more at our About page or visit constructsoftware.co.uk.

FAQ

Common questions answered

Everything you need to know before getting started.

Who is the Electrical Toolbox for?

The Electrical Toolbox is designed for anyone working in electrical contracting, electrical design or facilities maintenance who regularly needs BS 7671 calculations. This includes electrical engineers, design and build contractors, approved electricians, inspection and testing engineers, apprentices learning the 18th Edition and anyone who needs to cross-check a calculation quickly. If you work with circuits, cables, protection devices or earthing systems, the Toolbox has tools you will use.

Is it really free?

Yes. The Electrical Toolbox is offered entirely free of charge as part of the Construct Software ecosystem. There is no trial period, no subscription, no credit card required and no premium tier. All 34 tools are available to every registered user at no cost. The Toolbox is funded by Construct Software's broader product portfolio.

Do I need to install anything?

No. The Electrical Toolbox is a cloud-based web application. You access it through any modern web browser on a desktop, laptop or tablet - no installation, no plugins, no app store. Just log in and start calculating. An internet connection is required.

What edition of the wiring regulations do the tools follow?

The tools are built around BS 7671:2018+A4:2026 (the 18th Edition, Amendment 4). Maximum Zs values, disconnection times, k values, cable current ratings and voltage drop, CPC, earthing and bonding conductor sizes and insulation resistance minimums are all referenced to the current edition, with diversity and conduit fill taken from the IET On-Site Guide. Tools that check measured values compare them with 0.8 × the tabulated maximum Zs, as recommended in BS 7671 Appendix 3. The Max Zs Converter converts 16th and 17th Edition values to the current basis, applying the Cmin voltage factor.

How accurate are the calculations?

The calculations use the same established equations and published data that you would use manually - the adiabatic equation (Reg 543.1.3), current ratings and voltage drop from BS 7671 Appendix 4, maximum Zs from Tables 41.2 to 41.5, k values from Tables 43.1 and 54.2 to 54.6, CPC, earthing and bonding sizes from Tables 54.1, 54.7 and 54.8, insulation resistance minimums from Table 64, and diversity and conduit fill factors from the IET On-Site Guide. Where a situation falls outside the published tables, the tools say so rather than estimate. The tools are intended to support engineering judgement and design work, not replace it. Always validate results against project-specific conditions and the current edition of BS 7671.

Is my account data secure?

Account authentication is handled by the Construct Software ID system, which uses cryptographically hashed credentials, secure session management and cross-subdomain cookie validation. The Electrical Toolbox itself stores only a small amount of user preference data (your most recently viewed tool and reference page). No sensitive personal or project data is stored within the Toolbox application.

Can I use it on a phone or tablet?

The Electrical Toolbox is designed primarily for desktop and tablet use, where you can comfortably enter values and read results on a full-size screen. It is accessible on mobile but the calculation interfaces work best on a screen of 768px width or wider.

Free BS 7671 electrical calculators,right in your browser.