Electrical Testing Calculators for UK Electricians

Earth loop impedance, known as Zs, is arguably the single most critical measurement you take during electrical testing. It determines whether the protective device on a circuit — be it an MCB, RCBO, or fuse — will disconnect the supply fast enough to prevent electric shock if a fault occurs. The entire safety chain of an installation depends on this value being within the limits set by BS 7671.

The earth fault loop path starts at the point of fault, passes through the circuit protective conductor (CPC) back to the consumer unit, through the main earthing terminal, down the earthing conductor to the means of earthing, through the earth return path of the supply network, through the secondary winding of the supply transformer, and back through the line conductor to the point of fault. Every connection, every joint, and every metre of conductor in that path adds impedance.

Zs is the sum of two components: Ze (the external earth fault loop impedance, measured at the origin of the installation with the main earthing conductor disconnected) and (R1+R2) — where R1 is the resistance of the line conductor from the origin to the furthest point of the circuit, and R2 is the resistance of the CPC over the same route. These values are measured at ambient temperature during testing, but conductors get hot under fault conditions. That is why BS 7671 requires you to apply a correction factor to account for conductor resistance at the maximum permitted operating temperature. The rule of thumb is to multiply the tabulated maximum Zs from BS 7671 by 0.8 to obtain the maximum permissible value for your on-site measurement at ambient temperature, although the precise factor depends on conductor type and installation conditions.

If your measured Zs exceeds the maximum value stated in BS 7671 — Table 41.2 for fuse-protected circuits at 0.4 s, Table 41.3 for circuit-breakers and RCBOs at 0.4 s or 5 s, or Table 41.4 for fuse-protected distribution and final circuits at 5 s — the circuit fails. For TT systems where an RCD provides fault protection, compliance is checked using the condition Ra × IΔn ≤ 50 V per Table 41.5 rather than a Zs table limit. A failed Zs test typically means the CPC has a poor connection somewhere, the circuit is too long for the cable size, or the external earth fault loop impedance is unusually high. Our Zs calculator lets you enter Ze and R1+R2, automatically applies the temperature correction, and instantly tells you whether the circuit passes or fails for every common protective device type and rating.

Every EICR and EIC requires you to record the prospective fault current at the origin of the installation. There are two values: the prospective short-circuit current (PSCC) — the maximum current that would flow in a dead short between line and neutral — and the prospective earth fault current (PEFC) — the maximum current that would flow in a fault between line and earth. These values matter because every protective device has a rated breaking capacity (often 6kA for domestic MCBs), and if the prospective fault current exceeds that rating, the device could fail catastrophically during a fault.

The calculation is straightforward. For PSCC, divide the nominal supply voltage (230V single phase, 400V three phase) by the impedance of the line-neutral loop at the origin. For PEFC, divide the supply voltage by Ze. In practice, you measure these directly with a loop impedance tester at the origin, or you calculate them from the measured Ze and Zn (neutral loop impedance) values. Our calculator accepts either direct measurements or calculated values and presents both PSCC and PEFC alongside the breaking capacity of common protective device families (BS 60898 MCBs, BS 61009 RCBOs, BS 88 fuses).

A common scenario on domestic EICRs is finding a prospective fault current of around 1-3kA at the origin of a standard 100A single-phase supply. This is well within the 6kA breaking capacity of most MCBs. But in commercial and industrial settings, or on supplies very close to the transformer, PSCC can reach 10kA, 16kA, or even higher. In those situations you need MCBs or MCCBs rated to at least that level, or the installation is non-compliant. Our calculator flags these issues automatically, saving you from having to cross-reference device data sheets manually.

Residual current devices (RCDs) are the last line of defence against electric shock. BS 7671 requires additional protection by a 30mA RCD for virtually all socket outlets, all circuits in bathrooms, all circuits in locations with increased shock risk, and all cables installed at a depth of less than 50mm in a wall without mechanical protection. Testing that these devices actually operate correctly is a non-negotiable part of every EICR and initial verification.

RCD testing involves applying a known residual current and measuring the disconnection time. At rated current (typically 30mA), the RCD must trip within 300ms. At five times rated current (5×IΔn — 150mA for a 30mA device), it must trip within 40ms. One important on-site nuance: GN3 Reg 5.6 states that for Type AC RCDs, a manufacturer may declare 250mA as the test current for the 40ms test instead of the standard 5×IΔn value. Where the manufacturer has declared 250mA, that figure takes precedence and your tester must be set accordingly. You also perform a half-rated (15mA) test — the RCD should not trip at this level, confirming it is not oversensitive. For each test, our calculator displays the expected trip time range, the pass/fail threshold, and explains what a failure at each level typically indicates.

BS 7671:2018+A4:2026 Reg 411.3.4 introduced a new requirement for domestic premises: all AC final circuits supplying luminaires (lighting circuits) shall now have additional protection by a 30mA RCD. This is a mandatory obligation and a common EICR failure point on pre-A4 installations where lighting circuits were historically left unprotected. When running our RCD testing calculator, ensure you have verified and recorded RCD protection for every lighting circuit in domestic properties — any unprotected lighting circuit requires an EICR observation.

The landscape of RCD types has expanded significantly. Type AC RCDs detect sinusoidal AC residual currents — these are the traditional devices. Type A RCDs also detect pulsating DC residual currents, which are generated by electronic equipment such as EV chargers, heat pumps, and variable speed drives. BS 7671 now requires Type A protection for these specific loads. Type B RCDs detect smooth DC residual currents as well and are required for certain three-phase EV chargers. Type F RCDs are designed for frequency-controlled equipment. Our testing calculator covers all four types with appropriate thresholds and explains which circuits require which type.

Getting conduit and trunking fill calculations wrong leads to cables overheating, insulation damage, and potentially fires. The IET On-Site Guide provides cable factors for every standard cable type and size, and conduit capacity factors for every standard conduit size. The rule is simple: the sum of all cable factors must not exceed the conduit factor. For trunking, IET guidance limits the cable fill to 45% of the internal cross-sectional area.

Our conduit fill calculator supports all standard round conduit sizes from 16mm through to 50mm, and both rigid PVC and steel conduit. You select the conduit type and size, add the cables you plan to install, and the calculator instantly shows your percentage fill against the maximum. It also shows the derating factor that applies based on the number of circuits — because grouping cables together reduces their current carrying capacity, per BS 7671 Table 4C1.

The maximum demand calculator is essential when you are designing a new installation or assessing whether an existing supply is adequate. For domestic premises, Table 1A in the IET On-Site Guide provides diversity allowances for different types of load. For example, cooking appliances have a diversity of 10A plus 30% of the remainder, while socket outlets use 100% of the largest circuit plus 40% of the remaining circuits. For commercial and industrial premises, Table 1B provides different diversity factors. Our calculator walks you through both tables, letting you add each load type and automatically calculating the diversified maximum demand. This is the figure you put on an application for a new or upgraded supply from the Distribution Network Operator (DNO).

Power factor is the ratio of true power (watts) to apparent power (VA). A power factor of 1.0 means all the current drawn is doing useful work. In practice, inductive loads like motors, transformers, and fluorescent lighting cause the current to lag behind the voltage, reducing the power factor to 0.7-0.85 in many commercial installations. A low power factor means higher current draw for the same amount of useful work, leading to larger cable sizes, more losses, and potentially DNO surcharges. Our power factor calculator computes true power, apparent power, and reactive power from any two of these values plus the power factor, and calculates the capacitor size needed to correct the power factor to a target value, typically 0.95.

Voltage drop must be checked for every circuit to ensure the voltage at the load is sufficient for equipment to operate correctly. BS 7671 limits voltage drop to 3% for lighting circuits (6.9V on a 230V supply) and 5% for all other circuits (11.5V). The volt drop depends on the cable length, the cable size, the design current, and the cable type. Our volt drop calculator uses the millivolt-per-ampere-per-metre values from BS 7671 Appendix 4 Table 4Ab and similar tables, covering both single-core and multicore cables in thermoplastic and thermosetting insulation. Enter the circuit details and the calculator shows the actual volt drop in volts and as a percentage, with a clear pass or fail indication.

Cable sizing brings together several of these calculations. You start with the design current, apply the correction factors for ambient temperature (Ca from Table 4B1), grouping (Cg from Table 4C1), thermal insulation (Ci from Table 52.2), and the type of protective device (Cf). The required current carrying capacity is the design current divided by the product of all these factors. You then select a cable from the appropriate table in Appendix 4 whose tabulated current rating meets or exceeds this value. Finally, you verify that the cable also satisfies the adiabatic equation for fault protection, and that the volt drop is within limits. Our cable sizing calculator performs all of these steps in one go, showing the full working and the selected cable size.