How we calculate: electrics
Every formula and default figure our electrics calculators use. Each figure can be changed in its calculator to match your product. See how we calculate for the method behind all of them.
Cable size calculator
Open the cable size calculator
- Design current Ib = P ÷ (V × pf), or P ÷ (√3 × V × pf) for three phase
- Rating needed It = In ÷ (Ca × Cg × Ci × Cc): ambient, grouping, thermal insulation and device-type factors
- Choose the smallest size whose tabulated rating is at least It, then check Ib ≤ In ≤ Iz (Iz = tabulated rating × the factors)
- Voltage drop = mV/A/m × Ib × one-way length ÷ 1000, compared with the Appendix 12 limit
- Smallest protective conductor S = √(I² × t) ÷ k (adiabatic equation)
The 0.5 factor is the rule for a cable surrounded by thermal insulation for more than 0.5 m; shorter runs have their own factors in Appendix 4.
Indicative only: an electrician checks the design against BS 7671 and the cable maker's data, including fault protection and disconnection times.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Voltage drop limit: Custom circuit | 5 % | Regulation or HSE figure BS 7671:2018+A4:2026 Appendix 12 (other uses) Not yet checked against the source | Source to be added by the Researcher Brief v2 E2. Researcher to confirm against A4:2026. |
| Voltage drop limit: Lighting | 3 % | Regulation or HSE figure BS 7671:2018+A4:2026 Appendix 12 (lighting) Not yet checked against the source | Source to be added by the Researcher Brief v2 E2. Researcher to confirm against A4:2026. |
| Supply voltage: Single phase | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
| Supply voltage: Three phase | 400 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal three-phase line voltage (brief v2 E2). Researcher to cite the statutory source. |
| Device type factor: BS 3036 rewireable fuse | 0.725 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E1: Cc for a BS 3036 semi-enclosed (rewireable) fuse. Researcher to cite BS 7671:2018+A4:2026 Appendix 4. |
| Thermal insulation factor: Surrounded for more than 0.5 m | 0.5 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E1: the 0.5 rule for cable surrounded by thermal insulation for more than 0.5 m (Reg 523.9). Researcher to confirm against A4:2026. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Tabulated current-carrying capacity (It) by cable, reference method and size | BS 7671:2018+A4:2026 Appendix 4 (Table 4D5 for flat twin and earth; Researcher to confirm the tables for SWA and flex) | UK-wide | 0/75 values entered Not yet checked against the source |
| Voltage drop (mV/A/m) for flat twin and earth cable | BS 7671:2018+A4:2026 Appendix 4, Table 4D5 | UK-wide | 0/7 values entered Not yet checked against the source |
EV charger load calculator
Open the ev charger load calculator
- Charger current I = P ÷ V (single phase), or P ÷ (√3 × V_L) per phase (three phase), with no diversity
- Total = house maximum demand after diversity + charger current + other planned loads
- Fits when the total is within the main fuse; if it isn't but the house alone is, a load-managed charger or a DNO check is needed; if the house alone is over, ask your DNO
Most network operators ask to be told about a home charger (connect and notify); the installer usually does this.
PME earthing: an outdoor charging point needs the earthing arrangement in Reg 722.411.4.1. The electrician checks this.
Grants and funding schemes are not covered here.
Indicative only: an electrician or your DNO confirms the supply has room.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Supply voltage | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
| Line voltage | 400 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal three-phase line voltage (brief v2 E2). Researcher to cite the statutory source. |
Maximum demand and diversity calculator
Open the maximum demand and diversity calculator
- Connected current per circuit = its rating, or load (W) ÷ supply voltage
- Cooker: the first 10 A in full + 30% of the rest + 5 A if the control unit has a socket (On-Site Guide Appendix A)
- Socket circuits: the largest in full + 40% of each additional socket circuit (On-Site Guide Appendix A)
- Circuits with no rule entered yet are counted in full (no diversity), which overstates the demand
- Headroom = main fuse − maximum demand after diversity
Diversity rules are from the IET On-Site Guide, Appendix A; our transcription is still being checked against the current edition.
Indicative only: an electrician or your network operator (DNO) confirms the maximum demand before you add a big load.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Supply voltage | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Diversity allowances for household installations | IET On-Site Guide, Appendix A (allowance for diversity, household installations); Researcher to transcribe each rule with its page reference | UK-wide | 3/8 values entered Not yet checked against the source |
Voltage drop calculator
Open the voltage drop calculator
- Voltage drop (V) = mV/A/m × design current Ib (A) × one-way length (m) ÷ 1000
- As a percentage = voltage drop ÷ supply voltage × 100
- Longest run = (limit % × supply voltage ÷ 100) × 1000 ÷ (mV/A/m × Ib)
Tabulated mV/A/m figures assume the conductor is at its maximum operating temperature, so this is on the safe side for most circuits.
Indicative only: an electrician checks the design against BS 7671 and the cable maker's data.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Supply voltage: Single phase | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
| Supply voltage: Three phase | 400 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal three-phase line voltage (brief v2 E2). Researcher to cite the statutory source. |
| Voltage drop limit: Lighting | 3 % | Regulation or HSE figure BS 7671:2018+A4:2026 Appendix 12 (lighting) Not yet checked against the source | Source to be added by the Researcher Brief v2 E2. Researcher to confirm against A4:2026. |
| Voltage drop limit: Other uses | 5 % | Regulation or HSE figure BS 7671:2018+A4:2026 Appendix 12 (other uses) Not yet checked against the source | Source to be added by the Researcher Brief v2 E2. Researcher to confirm against A4:2026. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Voltage drop (mV/A/m) for flat twin and earth cable | BS 7671:2018+A4:2026 Appendix 4, Table 4D5 | UK-wide | 0/7 values entered Not yet checked against the source |
Amps, kW and kVA converter
Open the amps, kw and kva converter
- Single phase: P (W) = V × I × pf
- Three phase: P (W) = √3 × V_L × I × pf, where V_L is the line voltage
- Apparent power S (VA) = P ÷ pf
This converts figures; it doesn't size a cable or a device. Use the cable size calculator for that.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Voltage: Single phase | 230 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal single-phase supply voltage (brief v2 E2). Researcher to cite the statutory source. |
| Voltage: Three phase | 400 V | Our assumption Not yet checked against the source | Our choice: change it to suit your job UK nominal three-phase line voltage (brief v2 E2). Researcher to cite the statutory source. |
Consumer unit and RCBO planner
Open the consumer unit and rcbo planner
- All RCBOs: modules = circuits × RCBO width + main switch + SPD + spare ways × RCBO width
- Dual RCD: modules = RCDs × RCD width + circuits × MCB width + main switch + SPD + spare ways × MCB width
- Spare ways = circuits × spare allowance, rounded up
- Enclosure = the smallest size in the maker's range with at least that many usable modules
Regulation notes (non-combustible enclosures, SPDs, RCD protection for sockets, AFDDs) are being checked against BS 7671:2018+A4:2026 by our researcher.
Indicative only: the electrician designs the board, chooses the devices and checks discrimination and fault protection.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Spare ways | 20 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E6: spare ways allowance (assumption 20%). |
| Width of each RCBO | 1 modules | Common UK size Not yet checked against the source | From your product brief v2 E6 (CU1): single-module RCBO. Some RCBOs take 2 modules: check the maker's data. |
| Width of each MCB | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product Not stated in the brief. Researcher to fill from a consumer unit maker's data. |
| Width of each RCD | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product Not stated in the brief. Researcher to fill from a consumer unit maker's data. |
| Width of the main switch | 2 modules | Common UK size Not yet checked against the source | From your product brief v2 E6 (CU1): main switch width. |
| Width of the SPD | 2 modules | Common UK size Not yet checked against the source | From your product brief v2 E6 (CU1): surge protective device width. |
| Extra width per AFDD | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product Not stated in the brief. Researcher to fill from an AFDD maker's data. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Consumer unit enclosure sizes (usable modules) TODO: Researcher to add the maker or standard URL | Consumer unit makers' ranges (Researcher to fill from at least two UK makers' data sheets) | UK-wide | 0/6 values entered Not yet checked against the source |
Max Zs and expected Zs calculator
Open the max zs and expected zs calculator
- Expected R1+R2 = (R1+R2 per metre in mΩ/m) × length (m) × temperature multiplier ÷ 1000
- Expected Zs = Ze + expected R1+R2
- 80% figure = maximum Zs × 0.8: a rule of thumb for comparing a measured (cold) value with the tabulated maximum
- Longest run = (80% figure − Ze) × 1000 ÷ (R1+R2 per metre × multiplier)
Maximum Zs values come from BS 7671 Tables 41.2–41.4 for the device and disconnection time; ours are still being checked against A4:2026.
Indicative only: an electrician measures Zs and checks it against BS 7671.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Temperature multiplier | 1.2 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E5 (ZS1): multiplier taking R1+R2 at 20 °C to conductor operating temperature. Researcher to cite IET Guidance Note 3 / On-Site Guide. |
| Rule of thumb for measured values | 0.8 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E5: the 80% rule of thumb for comparing a measured Zs with the tabulated maximum. Researcher to cite. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| R1+R2 per metre at 20 °C for flat twin and earth (line/CPC) | IET On-Site Guide / Guidance Note 3 (R1+R2 per metre); prefer the cable maker's conductor resistance data | UK-wide | 0/7 values entered Not yet checked against the source |
| Maximum earth fault loop impedance (Zs) by protective device and rating | BS 7671:2018+A4:2026 Tables 41.2–41.4 (0.4 s and 5 s disconnection; Researcher to confirm the A4:2026 values and which table applies) | UK-wide | 0/46 values entered Not yet checked against the source |
Solar PV system size calculator
Open the solar pv system size calculator
- Panels per face = usable area ÷ panel area, rounded down (only whole panels fit)
- kWp = panels × panel power (W) ÷ 1000
- Generation (kWh a year) = kWp × kk × shading factor (MCS MIS 3002 method)
We don't estimate savings, payback or export payments: they depend on your tariff and how you use the power.
The inverter is sized by the installer to the array and the maker's data; an MCS installer also handles the DNO notification.
Indicative only: an MCS installer surveys the roof and gives the generation estimate.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Shading factor | 1 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 E7 (PV1): shading factor 1.0 = no shading. Researcher to cite the MCS shading method. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| kWh per kWp (kk) by postcode zone, orientation and pitch | MCS MIS 3002 / MGD003 irradiance data set (Researcher to confirm whether the tables may be reproduced, brief v2 E7 and §16 Q1) | UK-wide | 0/0 values entered |
Appliance running cost calculator
Open the appliance running cost calculator
- Energy a day (kWh) = power (W) × hours a day ÷ 1000
- Energy over the period = energy a day × days
- Cost = energy × your unit rate (standing charges are not included)
We don't hold any electricity prices: enter the unit rate from your own bill.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Home battery size calculator
Open the home battery size calculator
- Usable capacity = daily use (kWh) × share used after sunset
- Nominal capacity = usable capacity ÷ depth of discharge
- Energy to charge it = usable capacity ÷ round-trip efficiency
Batteries come in set sizes: pick the next size up from the maker's range.
BS 7671:2018+A4:2026 adds requirements for battery energy storage; the installer follows them and the maker's instructions.
Indicative only: an installer checks your usage data and the supply.
This calculator has no built-in defaults: every figure comes from you or your product's data.