How to Calculate Maximum Demand (with Diversity)
Add up every breaker in a modern consumer unit and you'd blow the cut-out before breakfast — maximum demand, with diversity applied, is how you prove the installation draws far less than the sum of its circuits.
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.
Key takeaways
- Maximum demand is the realistic peak current after allowing that loads don't all run at once.
- Connected load is everything at full rating; maximum demand is connected load with diversity applied.
- Lighting ~66%; rings = 100% of the largest + 40% of each additional; cooker = first 10A + 30% remainder (+5A if it has a socket).
- Showers, immersions and EV chargers get no diversity — assessed at 100%.
- BS 7671 leaves the method to judgement; the On-Site Guide allowances are a defensible starting point, not a rule.
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 figure whenever you're sizing a consumer unit, main switch and tails (the assessed demand, not the sum of the breakers, is what they carry); checking supply capacity (UK cut-outs are commonly 60A, 80A or 100A — if demand approaches the fuse rating, involve the DNO); installing an EV charger (a 7.4kW charger is a sustained 32A, so most charge point installs stand or fall on this assessment); or adding a shower, heat pump or extension.
Connected load, maximum demand and diversity
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 at once. Maximum demand is that connected load with diversity applied: an assessment of what the installation will plausibly draw at its busiest. It's always lower, and it's the figure that drives supply and switchgear decisions.
Diversity is the recognition that loads don't coincide. 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 draws 32A for hours.
Typical diversity allowances (single household)
Based on the IET On-Site Guide approach for an individual domestic installation — a starting point for judgement, not a substitute for it. Lighting: 66% of total connected lighting current. Ring final / socket circuits: 100% of the largest circuit + 40% of each additional circuit. Cooker: first 10A at 100% + 30% of the remainder, plus 5A if the control unit has a socket-outlet.
Water heating (immersion, thermostatic): 100% — no diversity. Instantaneous water heaters / electric showers: 100% — no diversity. EV charger: 100% — sustained load, no diversity. Other fixed loads (heating, motors): 100% unless there's a clear case for less. So a 45A cooker assesses at 10 + (0.3 × 35) = 20.5A (25.5A with a socket); two ring finals on 32A devices assess at 32 + (0.4 × 32) = 44.8A, not 64A.
No diversity means no diversity
Showers, immersions and EV chargers are assessed at the full load. A thermostat or a user can hold them there for hours — the maths must reflect that.
Worked example 1: domestic 100A single-phase
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. Lighting: 10A × 0.66 = 6.6A. Cooker: 10 + 0.3 × (45 − 10) = 20.5A. Ring finals (×2): 32 + 0.4 × 32 = 44.8A. Shower (8.5kW): 8500 / 230 = 37.0A. EV charger (7.4kW): 32.0A. Total = 140.9A.
The assessed demand comfortably exceeds the 100A cut-out — a genuinely useful answer. Without the charger the house assesses at about 108.9A, already tight; the method is conservative and the DNO fuse won't blow the moment a calculation says 108A, but adding a guaranteed 32A continuous load is a different matter. The realistic options are a charger with load curtailment (a CT clamp on the tails so it 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.
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 — these are typical judgement values for illustration. Lighting: 2.0 kW connected at 90% = 1.8 kW. Socket circuits: 12 kW at 100% largest + 50% remainder = 8.5 kW. Three-phase machine: 15 kW (pf 0.85) at 100% = 15 kW. Water heater: 6 kW at 100% = 6 kW.
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 won't diversify against each other in a family household at 7:30am, so assess both at 100%. Electric heating as the primary heat source runs for hours, so the "other fixed loads at 100%" line matters more than any table row. Holiday lets and HMOs behave nothing like a single household. Flats and blocks need after-diversity maximum demand (ADMD) per dwelling and diversity across dwellings — a different exercise entirely. Where the assessed figure lands close to the cut-out, measure: a data logger on the tails for a week beats any table.
A note for Australian and New Zealand readers: maximum demand there is calculated to AS/NZS 3000 Appendix C, which uses its own load groups (A through H), per-point allowances and multipliers. The IET On-Site Guide percentages in this article do not apply — the structure of the calculation is different, not just the numbers.
Guidance only
Values and allowances here 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.
Frequently asked questions
What diversity applies to a cooker?
The first 10A at 100%, plus 30% of the remainder, plus 5A if the cooker control unit includes a socket-outlet. So a 45A cooker assesses at 10 + 0.3 × 35 = 20.5A, or 25.5A with a socket.
Do EV chargers and showers get diversity?
No. Electric showers, immersion heaters and EV chargers are all assessed at 100% — no diversity — because a thermostat or the user can hold them at full load for long periods. A 7.4kW charger is a sustained 32A.
What diversity applies to ring final circuits?
100% of the largest socket circuit plus 40% of each additional one. So two 32A ring finals assess at 32 + 0.4 × 32 = 44.8A, not the 64A you'd get by simply adding the breakers.
From guidance to action
Related guides
What Is Prospective Fault Current (PFC)?
PSCC vs PEFC, how to measure PFC, and why breaking capacity has to exceed it.
Max Zs Values Table (BS 7671 18th Edition)
Full maximum Zs tables for Type B/C/D MCBs and RCBOs, the Cmin formula and the 80% rule.
How to Test R1+R2 on a Ring Main
The three-step ring final continuity test, cross-connections and the readings that prove it.
