#How to Calculate Maximum Demand (BS 7671 Guide with Diversity Tables)

Add up every breaker in a modern consumer unit and you'll get a number that would blow the cut-out fuse before breakfast — 32A per ring, 40A for the shower, 32A for the cooker, and suddenly a three-bed semi "needs" 200A. It doesn't, and maximum demand is how you prove it. This guide covers what maximum demand actually is, how diversity works, the typical allowances electricians use, and two worked examples you can follow on your next job.

#What maximum demand is and why it matters

Maximum demand is the realistic peak current an installation will draw in service, after allowing for the fact that loads don't all run at once. BS 7671 requires the designer to assess it — but deliberately leaves the method to engineering judgement, which is where the familiar diversity allowances come in.

You need a maximum demand figure whenever you're:

• Sizing a consumer unit, main switch and tails — the assessed demand, not the sum of the breakers, is what the main switch and 25mm tails need to carry
• Checking supply capacity — UK domestic cut-outs are commonly 60A, 80A or 100A; if the assessed demand approaches the fuse rating, you need the DNO involved before adding load
• Installing an EV charger — a 7.4kW charger is a sustained 32A. Most charge point installs now stand or fall on the maximum demand assessment, which is why the OZEV-style load calculations all start here
• Adding a shower, heat pump or extension — same question, same maths

#Connected load vs maximum demand

Connected load is everything added together at full rating: every luminaire, the full cooker plate load, every circuit at its device rating. It's a real number, but a useless one for design — nobody runs every load simultaneously.

Maximum demand is the connected load with diversity applied: an assessment of what the installation will plausibly draw at its busiest. It's always lower than connected load, and it's the figure that drives supply and switchgear decisions.

#Diversity explained

Diversity is the recognition that loads don't coincide. The kettle and the oven run together sometimes; every light, every socket, the shower and the cooker all at full load at the same instant — effectively never.

Some loads get generous diversity because usage is intermittent (lighting, cooking). Others get none, because a thermostat or the user can hold them at full load for long periods: electric showers, immersion heaters and EV chargers are all assessed at 100%. That's not caution for its own sake — a 7kW charger genuinely does draw 32A for hours.

#Typical diversity allowances (single household)

The table below shows typical allowances based on the IET On-Site Guide approach for an individual domestic installation. Treat them as a starting point for judgement, not a substitute for it.

So a 45A cooker assesses at 10 + (0.3 × 35) = 20.5A, or 25.5A if the cooker unit includes a socket. Two ring finals on 32A devices assess at 32 + (0.4 × 32) = 44.8A, not 64A.

#Worked example 1: domestic 100A single-phase house

A house on a 100A single-phase supply: total lighting 10A connected, a 45A cooker (no socket in the unit), two ring finals, an 8.5kW shower and a proposed 7.4kW EV charger.

The assessed demand comfortably exceeds the 100A cut-out — and that's a genuinely useful answer. Without the charger the house assesses at about 108.9A, which is already tight; the diversity method is conservative and the DNO fuse won't blow the moment a calculation says 108A, but adding a guaranteed 32A continuous load on top is a different matter. The realistic options are a charger with load curtailment (a CT clamp on the tails so the charger throttles when household demand rises), or an application to the DNO for a supply upgrade. This is exactly the assessment charge point installers are expected to make — and why "just fit it on a 40A breaker" isn't a design.

Run your own figures in seconds with the maximum demand calculator.

#Worked example 2: small commercial three-phase

A small workshop on a 400V three-phase (TPN) supply. Commercial premises use different, generally tighter allowances than a household — the percentages below are typical judgement values for illustration:

Assessed demand ≈ 31.3 kW. For a balanced three-phase load, I = P / (√3 × 400 × pf); taking an overall power factor of about 0.9 gives 31,300 / (1.732 × 400 × 0.9) ≈ 50A per line — comfortable on a 100A TPN supply, and worth balancing the single-phase loads across phases so one line doesn't carry the lot.

#When judgement overrides the tables

The allowances are assessments, not physics. Override them when you know better:

• Two electric showers in one house won't diversify against each other in a family household at 7:30am — assess both at 100%
• Electric heating as the primary heat source runs for hours; the "other fixed loads at 100%" line matters more than any table row
• Holiday lets and HMOs behave nothing like a single household — the domestic table doesn't apply
• Flats and blocks need after-diversity maximum demand (ADMD) figures per dwelling and diversity across dwellings — a different exercise entirely
• Where the assessed figure lands close to the cut-out rating, measure: a data logger on the tails for a week beats any table

BS 7671 puts the assessment on the designer. The table gets you a defensible starting number; the judgement is what you're being paid for.

#A note for Australian and New Zealand readers

Maximum demand in Australia and New Zealand is calculated to AS/NZS 3000 Appendix C, which uses its own load groups (A through H), per-point allowances and multipliers for multiple domestic installations. The IET On-Site Guide percentages in this article do not apply there — the structure of the calculation is different, not just the numbers.

Once you have a demand figure, the related checks follow naturally: prospective fault current at the origin for device breaking capacity, and maximum Zs for each circuit's disconnection times.

Values and allowances in this guide are for guidance only. Always verify against the current edition of BS 7671, the IET On-Site Guide and manufacturer data for the actual installation.

#Do it faster with TradePlanr

The free maximum demand calculator applies these diversity allowances for you — enter the lighting, cooker, shower, rings and other loads and read off the assessed demand, no sign-up needed. TradePlanr is a job management app built for electricians: quotes, job scheduling, invoices and certificates alongside a full set of BS 7671 calculators. The free plan covers the tools, and the full app is a flat £9.99 a month — no per-user pricing, no surprises.