Fill in any two values — the third is calculated.
Fill in any two values — the third is calculated.
Fill in any two to get the third.
Calculates assessed demand using BS 7671 / IET OSG diversity for cookers, then recommends MCB and cable size, plus whether it needs a dedicated circuit.
| Requirement | Guidance | Source |
|---|---|---|
| Dedicated circuit | Required when appliance exceeds 3kW (13A limit). Under 3kW can be supplied from a 13A socket on a ring main, though a fused spur is better practice for sustained loads above 2kW | OSG / BS 7671 good practice |
| Control unit | A cooker control unit (with or without socket) is required within 2m of the appliance, accessible and not above the cooker | Reg 553.1.6 / OSG |
| Typical protection | 30A or 32A MCB (Type B) is standard for most domestic cookers | OSG Table A1 |
| Typical cable | 6mm² T&E covers most single ovens/hobs up to ~32A; larger ranges may need 10mm² | Verify via cable sizing calc |
| Induction hobs | Often lower current draw than stated wattage due to diversity within the hob itself — check manufacturer data | Manufacturer spec |
| Socket outlet | A single socket may be incorporated into the control unit — this adds 5A to the diversity calculation | OSG Appendix A |
Calculates minimum cable CSA using BS 7671 Appendix 4 correction factors. Covers T&E (flat twin), SWA, and flex.
BS 7671 requires cables to fill no more than 40% of the conduit internal CSA to allow cables to be drawn in without damage.
Cable groups (number of each)
A standard 32A ring final circuit in a domestic property. Check total load and floor area compliance.
Check if a spur off a ring circuit is within the limits of BS 7671 and GN3.
Estimate the maximum demand for a domestic or small commercial installation.
Estimate how long a vehicle will take to charge from a given state.
Calculate the design current and minimum cable size for a home EV charger circuit.
Check if adding an EV charger overloads the existing supply — critical for DNSP / DNO assessment.
Long garage / driveway runs often fail on voltage drop — check before you pull the cable.
| Charger Type | Current | Supply | Typical Cable | MCB |
|---|---|---|---|---|
| 3-pin (granny) | 10A | Single phase | 2.5mm² T&E | 16A Type B |
| 7kW home unit | 32A | Single phase | 6mm² T&E / SWA | 40A Type B |
| 11kW unit | 16A/phase | Three phase | 2.5mm² 5-core | 20A Type B (×3) |
| 22kW unit | 32A/phase | Three phase | 6mm² 5-core | 40A Type B (×3) |
| 50kW DC rapid | — | Three phase | Manufacturer spec | Per design |
All domestic EV charger installations in the UK require OZEV-approved equipment and must comply with BS 7671 Amendment 2 (Regulation 722). Smart charging capability is mandatory on new domestic installations.
Estimate annual and daily energy generation for a UK solar PV system.
Size the inverter to match the array — typically 80–100% of total panel kWp.
Calculate the AC current from the inverter to size the cable and protection device back to the consumer unit.
Determine whether the system requires a G98 notification or a full G99 application to the DNO.
Estimate annual bill savings and export income — useful for customer quotes.
| Item | Detail | Notes |
|---|---|---|
| G98 notification | ≤3.68kW single phase (16A) | Notify DNO within 28 days of commissioning |
| G99 application | >3.68kW or >16A/phase | Pre-approval required before installation |
| MCS certification | Required for SEG payments | Also required for planning exemptions |
| Typical UK yield | 900–1100 kWh/kWp/yr | Varies by region, pitch and orientation |
| String fuse (DC) | Per manufacturer spec | Required if >2 strings in parallel |
| AC cable (inverter) | Size per inverter output current | Treat as continuous load — apply 125% |
| RCD protection | Type A minimum (Type B if no isolation transformer) | Check inverter manufacturer requirements |
| Isolation | AC isolator adjacent to inverter | DC isolator required at array and inverter |
| Formula | Description | Variables |
|---|---|---|
| V = I × R | Ohm's Law — Voltage | V=Volts, I=Amps, R=Ohms |
| P = V × I | Single phase power | P=Watts |
| P = I² × R | Power from current & resistance | — |
| I = P / (√3 × V × PF) | 3-phase full load current | √3 ≈ 1.732 |
| Vd = (mV/A/m × Ib × L) / 1000 | Voltage drop (BS 7671 method) | L in metres |
| It ≥ In / (Ca×Cg×Cc×Ci) | Cable tabulated current needed | Correction factors |
| Isc = Vp / Zs | Prospective fault current | Zs = loop impedance |
| kW = kVA × PF | Real vs apparent power | PF = power factor |
| E = P × t / 1000 | Energy (kWh) | P=Watts, t=hours |
| cos φ = kW / kVA | Power factor angle | φ = phase angle |
| Device | Rating | Max Zs (Ω) — 0.4s | Max Zs (Ω) — 5s |
|---|---|---|---|
| Type B MCB | 6A | 7.67 | — |
| Type B MCB | 16A | 2.87 | — |
| Type B MCB | 20A | 2.30 | — |
| Type B MCB | 32A | 1.37 | — |
| Type B MCB | 40A | 1.15 | — |
| Type C MCB | 16A | 1.44 | — |
| Type C MCB | 32A | 0.72 | — |
| Type D MCB | 16A | 0.72 | — |
| 30mA RCD | Any | 1667Ω max | — |
BS 88 fuse Zs values are not shown here as they vary between fuse standards (BS 88-2, BS 88-3) and disconnection time — always check BS 7671 Table 41.4 or manufacturer data directly for fuse-protected circuits.
| CSA | 2-core (A) | 3-core (A) | Max V-drop (mV/A/m) |
|---|---|---|---|
| 1.0mm² | 14.5 | 13.5 | 44 |
| 1.5mm² | 18.5 | 17.5 | 29 |
| 2.5mm² | 25 | 23 | 18 |
| 4.0mm² | 32 | 30 | 11 |
| 6.0mm² | 43 | 38 | 7.3 |
| 10mm² | 57 | 52 | 4.4 |
| 16mm² | 75 | 68 | 2.8 |
| 25mm² | 96 | 87 | 1.8 |
| 35mm² | 119 | 107 | 1.3 |
| 50mm² | 150 | 134 | 0.95 |
| 70mm² | 185 | 168 | 0.65 |
| 95mm² | 232 | 210 | 0.49 |
| Phase/Line Conductor | Min CPC (same material) | Min CPC (different material) |
|---|---|---|
| ≤16mm² | Same CSA as phase | Equivalent conductance |
| 16mm² – 35mm² | 16mm² | Equivalent conductance |
| >35mm² | Half the phase CSA | Equivalent conductance |