Maximum Demand: The 20-Unit Apartment Block — 14% Gap Between Standards
20 apartments at 10kW each. NEC says 95kW design load. IEC says 115kW. That 14% gap changes your transformer size, main switch rating, and project cost.
Maximum demand calculation is where engineering standards diverge the most — not because the physics differs, but because the statistical assumptions about human behaviour differ between countries. How much of a building's installed load will actually be used simultaneously? The answer depends on where you are.
The Scenario
A residential apartment building:
- 20 apartments, each with 10kW installed load (oven, hot water, general power, lighting, A/C)
- Common areas: 15kW (lifts, corridor lighting, fire systems, car park ventilation)
- Supply voltage: 400V three-phase
- No gas heating — all-electric building
Total installed load: 20 × 10kW + 15kW = 215kW
Nobody expects all 20 apartments to draw maximum load simultaneously at 10pm on a weeknight. The question is: what fraction of that 215kW should we design the supply for?
Side-by-Side Results
Scenario
20 apartments × 10kW each + 15kW common areas, 400V three-phase, all-electric
| Parameter | AS/NZS | BS 7671 | IEC 60364 | NEC |
|---|---|---|---|---|
Diversity method | Table C1 lookupPrescriptive diversity factorsAS/NZS 3000, Table C1 | Engineer judgmentNo prescriptive diversity tableBS 7671, Appendix A | Annex A factorsGuideline diversity valuesIEC 60364-3, Annex A | Article 220 OptionalDemand factor tablesNEC 220.84 |
Diversity factor (apartments) | 52%Table C1 for 20 unitsAS/NZS 3000, Table C1 | 50-60%Typical engineering estimateCIBSE Guide, ACE 114 | 55%For 20 dwellingsIEC 60364-3, Annex A | 40%Optional method for dwellingsNEC 220.84 |
Common area demand | 15kW (100%)Common loads at full demand | 15kW (100%)Essential loads at full demand | 15kW (100%)Common loads at full demand | 15kW (100%)House loads at full demand |
Design maximum demand | 119kW200 × 0.52 + 15 | 110-130kWEngineer's assessment range | 125kW200 × 0.55 + 15 | 95kW200 × 0.40 + 15 |
Supply current (A) at 400V | 172A119k / (√3 × 400) | 159-188ARange based on judgment | 180A125k / (√3 × 400) | 137A95k / (√3 × 400) |
Transformer size | 200kVANext standard size up from 119kW | 200kVAConservative selection | 200kVANext standard size up from 125kW | 150kVA95kW at 0.85 PF = 112kVA |
Main switch rating | 200AStandard frame size | 200AStandard frame size | 200AStandard frame size | 150AOr 200A if derated |
Why the Numbers Differ
Different Statistical Bases
Each standard's diversity factors are derived from different population studies:
AS/NZS 3000 (Table C1): Based on Australian Bureau of Statistics energy surveys and utility metering data from Australian distribution networks. Reflects Australian living patterns — high air conditioning use in summer, relatively low heating loads (mild climate in most population centres).
IEC 60364 (Annex A): Based on European metering data, primarily from continental European countries. Reflects higher heating loads (colder climates) and different appliance usage patterns.
NEC Article 220: Based on US utility data. The Optional Method in Article 220.84 uses a demand factor that reflects American building patterns — larger dwellings on average, different HVAC profiles, and 120V/240V split-phase supply.
BS 7671: Deliberately does not provide a prescriptive diversity table. The IET Wiring Regulations leave diversity assessment to the designer's judgment, supplemented by guidance documents (ACE 114, CIBSE Guide). This is philosophically different — the standard trusts the engineer rather than prescribing a value.
Why BS 7671 Has No Diversity Table
The British approach is that diversity depends on the specific building, its occupancy, and its equipment. A luxury apartment with underfloor heating, a home gym, and an EV charger has a fundamentally different demand profile than a social housing unit. One table cannot capture this variation — so BS 7671 expects the engineer to assess each project individually.
The NEC Optional Method
NEC 220.84 is notably permissive. For dwelling units served by a single feeder or service:
- First 10kVA at 100%
- Remainder at 40%
This produces the lowest design demand of any standard. The reasoning: American homes are typically larger with more installed load per dwelling, but the actual simultaneous usage (as measured by utilities) is proportionally lower. Americans have more installed capacity than they use simultaneously.
What About Electric Vehicles?
None of the traditional diversity tables account for EV charging. A 7kW EV charger per apartment adds 140kW of installed load — and if all residents arrive home at 6pm and plug in, the actual diversity may be much higher than historical tables suggest.
| Scenario | Traditional Max Demand | With 20 × 7kW EV | Impact |
|---|---|---|---|
| AS/NZS (52%) | 119kW | 119 + (140 × 0.5) = 189kW | +59% |
| NEC (40%) | 95kW | 95 + (140 × 0.4) = 151kW | +59% |
| IEC (55%) | 125kW | 125 + (140 × 0.45) = 188kW | +50% |
All standards are currently updating their diversity guidance for EV charging. AS/NZS 3000:2018 Amendment 2 includes specific EV diversity provisions. NEC 2023 added Article 625 requirements. BS 7671 Annex F provides EV demand factors.
EV Charging Changes Everything
A 20-unit apartment block designed in 2020 with a 200kVA supply may need 315kVA when all units get EV chargers. Transformer replacement costs $50,000-$100,000. Design new buildings with EV capacity from day one.
Cost Impact of Getting It Wrong
Undersizing (Using Too-Low Diversity)
If the NEC 40% factor is applied to a building where actual demand is closer to IEC's 55%:
- 150kVA transformer overloads → reduced lifespan, possible failure
- Main cable overheats → voltage drop issues, potential fire risk
- Network operator supply rejection — utility will not connect an undersized installation
Oversizing (Using Too-High Diversity)
If the IEC 55% factor is applied where NEC's 40% is more realistic:
- 200kVA transformer instead of 150kVA → $15,000 extra cost
- Larger main cable → $5,000 extra
- Higher standing charges from the utility for maximum demand tariff
Neither error is harmless, but undersizing has safety consequences while oversizing only has cost consequences. This is why most prudent designers round up.
Practical Guidance
- Use the standard required by your jurisdiction — not the one that gives the cheapest answer
- For international projects, IEC 60364 Annex A is a reasonable middle ground
- Never use NEC diversity factors for non-US buildings — American usage patterns don't transfer
- Include EV charging capacity in any new residential design, regardless of current demand
- Measure actual demand on similar existing buildings if possible — real data beats any table
Key Takeaways
- 14% difference between NEC (95kW) and IEC (125kW) for the same 20-unit building
- NEC is most permissive (40% diversity) — produces the smallest supply requirement
- IEC is most conservative (55% diversity) — produces the largest supply
- BS 7671 avoids prescriptive tables entirely — trusts the engineer
- EV charging invalidates all historical diversity tables — design for the future
Related Resources
- Auckland CBD Crisis: Maximum Demand Calculation — How underestimated demand caused a city-wide power failure
- Maximum Demand: Why Adding Up All Your CBs Overestimates — The diversity factor trap explained
- Cable Sizing: The 50m Office Feeder — Cable sizing flows from demand — see how the standards diverge
- Motor Starting: The 75kW Pump — Motor loads are often the largest contributor to maximum demand
- View all worked examples →
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Lead Electrical & Instrumentation Engineer
18+ years of experience in electrical engineering at large-scale mining operations. Specializing in power systems design, cable sizing, and protection coordination across BS 7671, IEC 60364, NEC, and AS/NZS standards.