Supply details
Enter the existing supply for a demand vs. capacity check
If the DNO cut-out is not accessible, use 60A as a conservative estimate for most UK homes. Call your DNO to confirm if needed.
Circuits
Toggle circuits on/off · Enter installed load in watts
| Circuit type | Qty | Load (W) | Diversity | Demand (W) |
|---|
Total after diversity
0 W
Demand exceeds existing supply. The calculated max demand exceeds the existing main fuse rating. A supply upgrade via your DNO may be required. Confirm with the network operator before proceeding.
Results — IET On-Site Guide · BS 7671:2018+A4:2026
Max demand
0
Amps at 230V
Total load (after diversity)
0
Watts
Recommended main fuse
—
Next standard size up
Supply headroom
—
A remaining
Standard fuse sizes
UK DNO cut-out ratings
40A
45A
60A
80A
100A
Highlighted = recommended size for your calculated demand. Contact your DNO for supply upgrade requests — typically 4–6 weeks lead time. A3/A4 BS 7671 Section 131 applies.
For guidance only. This calculator provides indicative guidance based on domestic diversity allowances in the IET On-Site Guide to BS 7671:2018+A4:2026. It does not constitute electrical design, a maximum demand assessment, or professional advice, and must not be relied upon as such. Responsibility for verifying maximum demand, applying engineering judgement and ensuring compliance rests entirely with the competent person carrying out the work. VoltScore Ltd accepts no liability for any loss, damage or design decision arising from use of this tool, to the fullest extent permitted by law. For safety-critical or complex installations, always commission a formal design from a qualified electrical engineer.
Common questions
Domestic diversity allowances from the IET On-Site Guide, applied to BS 7671:2018+A4:2026. These are the standard diversity allowances for domestic installations in the UK. Thermostatic space heating, hot water, immersion heaters and floor warming carry no diversity — 100% of rated load per circuit/unit is used in all cases. No diversity is applied to EV chargers or battery storage systems either, per current guidance.
EV chargers draw a sustained, predictable load over several hours — typically overnight. Unlike sockets or lighting, the load does not vary unpredictably. BS 7671:2018+A4:2026 and OZEV guidance both treat EV charger load at 100% for sizing purposes. Battery storage (Chapter 57, A4:2026) is similarly treated at full rated charge/discharge current.
Enter the cooker's rated wattage (usually on the data plate — typically 6,000–14,400W for an electric range). The OSG Table 1B diversity rule for cooking appliances is: 10A + 30% of the remainder above 10A (at 230V), plus 5A if there is a socket outlet on the cooker control unit. This calculator applies that rule automatically.
You will need a supply upgrade from your DNO. Contact them to request an increase — standard residential upgrades from 60A to 80A or 100A are routine but take 4–6 weeks and may involve a cost to the customer. Always confirm the existing supply rating with the DNO before designing an installation that relies on the full 100A.
The calculator supports a 400V supply input for three-phase connections, which affects the amps result. Three-phase domestic supplies are rare in the UK — most homes are single-phase 230V. If designing for three-phase, ensure loads are balanced across phases and apply BS 7671 Section 131 correctly — this calculator is a guide only.