Which standard does this calculator follow?

It follows BS 7671:2018+A2:2022 principles and diversity guidance from the IET On‑Site Guide (7th Edition, 2022), Table A2.

Is diversity in BS 7671 mandatory?

Diversity factors are guidance (not prescriptive) to estimate demand. Designers must apply engineering judgement for the specific installation.

How is a cooker demand calculated?

Demand current Id = 10 A + 30% of (Itotal − 10 A) + 5 A if a socket-outlet is incorporated in the cooker control unit, per IET OSG Table A2.

How are 3‑phase results shown?

The tool assumes an even distribution of 1‑phase loads across phases and reports per‑phase current. 3‑phase loads are added by line current.

Does power factor matter?

Yes. For loads entered as power (kW), the current depends on the assumed PF. Resistive loads (lighting, heating) are near 1.0; motors can be lower.

Can this be used for DNO applications?

It provides a defendable estimate. Some DNOs have their own forms or assumptions; always verify and use their required method where applicable.

BS 7671 Maximum Demand Calculator

Estimate the maximum demand of a UK electrical installation using BS 7671 principles and IET On‑Site Guide diversity guidance. This tool is designed for electricians, designers, and inspectors to quickly apply diversity to common loads (lighting, cooker, socket circuits, showers, EV, etc.) with full accessibility and mobile‑first UX.

Interactive Calculator

Add each load and choose the matching diversity method or preset. Fields marked * are required.

Supply type *

Single-phase

Three-phase

Nominal voltage (V) *

Frequency (Hz)

Default power factor

Loads

Results

Summary updates as you type. Values are rounded to 2 decimals.

Maximum demand

0.00 A

System apparent power

0.00 kVA

Estimated active power

0.00 kW

Assumptions

Single-phase

Data Source and Methodology

Authoritative data source: IET Wiring Regulations, BS 7671:2018+A2:2022 and IET On‑Site Guide, 7th Edition (2022), Table A2 — Diversity. Official IET links: BS 7671 overview and On‑Site Guide.

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

Diversity values are guidance to help estimate demand; they are not prescriptive. Designers must consider usage patterns, simultaneity, and manufacturer instructions.

The Formulas Explained

Single‑phase current from power:

$$ I_{1\phi} = \frac{1000 \cdot P_{\mathrm{kW}}}{V \cdot \mathrm{PF}} $$

Three‑phase current from power (balanced):

$$ I_{3\phi} = \frac{1000 \cdot P_{\mathrm{kW}}}{\sqrt{3} \cdot V_{\mathrm{LL}} \cdot \mathrm{PF}} $$

Cooker demand (IET OSG Table A2):

$$ I_{\mathrm{d}} = 10 + 0.3 \cdot \max\!\big(I_{\mathrm{total}} - 10,\,0\big) + 5\cdot s $$ where s = 1 if a socket‑outlet is incorporated in the cooker control unit, else 0.

Socket‑style “first 10 A + 30% remainder” rule:

$$ I_{\mathrm{d}} = 10 + 0.3 \cdot \max\!\big(I_{\mathrm{rated}} - 10,\,0\big) + \Delta $$ with optional addition Δ (e.g., +5 A) where applicable.

Lighting diversity (IET OSG Table A2):

$$ I_{\mathrm{d}} = 0.66 \cdot I_{\mathrm{total}} $$

Custom diversity factor f (0–1):

$$ I_{\mathrm{d}} = f \cdot I_{\mathrm{total}} $$

Glossary of Variables

V: Nominal voltage in volts. Single‑phase: 230 V typical; three‑phase line‑to‑line: 400 V typical.
PF: Power factor (0–1). Ratio of real to apparent power.
P: Load power in kilowatts (kW).
I_total: Calculated load current before diversity.
I_d: Diversified current (the demand used in summation).
kVA: Apparent power, calculated from current and voltage.
kW: Estimated active power using the assumed PF.
Δ: Optional additive current (e.g., +5 A for a cooker control unit socket).
f: Diversity factor (e.g., 0.66 for 66%).

How It Works: A Step‑By‑Step Example

Suppose a single‑phase dwelling at 230 V with:

Lighting: 900 W total → I_total = 900 / 230 = 3.91 A → I_d = 0.66 × 3.91 = 2.58 A
Cooker: 8 kW, no socket → I_total = 8000 / 230 = 34.78 A → I_d = 10 + 0.3 × (34.78 − 10) = 17.43 A
Socket circuit (rule 10 A + 30% remainder): 32 A → I_d = 10 + 0.3 × (32 − 10) = 16.6 A
Shower: 9.5 kW at PF=1 → I_total = 9500 / 230 = 41.30 A → I_d = 1.0 × 41.30 = 41.30 A

Summed maximum demand ≈ 2.58 + 17.43 + 16.6 + 41.3 = 77.91 A.
Apparent power ≈ 230 × 77.91 / 1000 = 17.92 kVA. With PF≈1, active power ≈ 17.92 kW.

Frequently Asked Questions (FAQ)

Is this calculator compliant with BS 7671?

It applies BS 7671 principles and the IET On‑Site Guide (Table A2) diversity guidance. Always verify with the latest edition and project‑specific requirements.

Can I use custom diversity factors?

Yes. Choose “Fixed current” or “Power (kW)” and set a custom diversity percentage to reflect usage patterns or manufacturer data.

How do I enter EV chargers?

Enter as “Power (kW)” at PF≈1 and use 100% diversity unless a smart load management system justifies a lower value.

What about heat pumps and motors?

Use “Power (kW)” with an appropriate PF (e.g., 0.9) and apply diversity if justified by simultaneity or control systems.

Are results per phase for 3‑phase supplies?

Yes. The calculator assumes single‑phase loads are evenly spread across phases and reports a per‑phase maximum demand. Three‑phase loads are included by their line current.

Why do my results differ from DNO calculators?

DN0s may enforce specific assumptions or simultaneity factors. Use their required method when applying for supply upgrades.

Tool developed by Ugo Candido. Content verified by the CalcDomain Editorial Team. Last reviewed for accuracy on: 14 September 2025.