Diversity Factor in Singapore HDB Electrical Installations: The Complete Technical Guide (2026)
If you have ever wondered why your HDB flat's main switch is rated at 40A yet your combined appliance load on paper adds up to far more, the answer lies in the diversity factor — one of the most important but least-discussed concepts in residential electrical design in Singapore.
This guide covers exactly what the diversity factor is, how it is calculated for HDB flats under Singapore Standard SS 638:2018+A1:2022 (the successor to SS CP5), worked numerical examples for 3-room through 5-room flats, and why getting this calculation right matters when you upgrade your DB box or rewire your flat.
1. What Is the Diversity Factor?
The diversity factor (DF) — also called the coincidence factor or simultaneity factor — expresses the ratio of the maximum demand actually recorded to the sum of all individual maximum demands of the loads connected to a circuit or feeder.
Or expressed as a percentage (coincidence factor):
Coincidence Factor (%) = (Maximum Demand of System ÷ Sum of Individual Max Demands) × 100
In IEC and SS 638 terminology, the diversity factor is always ≤ 1 (or ≤ 100%) when used as a coincidence factor. Some older references and the IEC electrotechnical vocabulary define it as the reciprocal (always ≥ 1), so always check which convention applies.
In Singapore, the SS 638 convention treats DF as ≤ 1, consistent with BS 7671 which SS 638 is harmonised with. This guide uses that convention throughout.
Why does it matter?
Not all appliances in a flat run at full load simultaneously. Your air-conditioner, water heater, oven, washing machine, dryer, and induction hob are never all running at once at their rated kW. The diversity factor quantifies this overlap — or rather, the absence of overlap — allowing engineers and LEWs to:
- Size DB boxes, MCBs, and RCCBs correctly without over-engineering
- Calculate the actual current demand on a riser cable or sub-main
- Determine whether an existing 40A or 63A main switch is adequate for a modern HDB flat
- Justify SP Group supply capacity requests for block upgrading
2. Singapore Regulatory Framework
Electrical installations in Singapore are governed by:
- SS 638:2018+C1:2020+A1:2022 — Code of Practice for Electrical Installations (replaces SS CP5:1998)
- Electricity Act 2001 and the Electricity (Electrical Installations) Regulations
- EMA licensing requirements — all electrical work must be carried out or supervised by an EMA-licensed LEW
- HDB renovation guidelines — additional requirements for works in HDB flats including permit thresholds
Under SS 638, Section 311 covers the assessment of general characteristics and Section 311.1 specifically addresses the determination of maximum demand, including the application of diversity factors. The standard references IEC 60364-1 and IEC 60050 for definitions.
3. Typical Appliance Loads in a Singapore HDB Flat
Before calculating the diversity factor, you need the individual load of each appliance. The table below lists typical values for a modern HDB flat in Singapore.
| Appliance | Typical rated load (W) | Typical running load (W) | Demand factor (individual) |
|---|---|---|---|
| Air-conditioner (1 HP split unit) | 900 | 700 | 0.78 |
| Air-conditioner (1.5 HP split unit) | 1,350 | 1,050 | 0.78 |
| Air-conditioner (2 HP split unit) | 1,800 | 1,400 | 0.78 |
| Instant water heater | 3,500 | 3,500 | 1.00 |
| Storage water heater | 3,000 | 3,000 | 1.00 (intermittent) |
| Induction hob (single) | 2,000 | 1,200 | 0.60 |
| Induction hob (double) | 3,500 | 2,000 | 0.57 |
| Refrigerator | 150 | 90 | 0.60 |
| Washing machine | 2,000 | 500 | 0.25 |
| Tumble dryer | 2,400 | 2,000 | 0.83 |
| Oven / microwave | 1,500 | 900 | 0.60 |
| LED lighting (whole flat) | 400 | 300 | 0.75 |
| TV / home entertainment | 300 | 250 | 0.83 |
| EV charger (Mode 2, 16A) | 3,680 | 3,680 | 1.00 |
| General power sockets (misc) | 1,000 | 500 | 0.50 |
Note: Rated load is the nameplate value. Running load is the typical sustained consumption during use. Demand factor = running load ÷ rated load.
4. Diversity Factor Worked Examples for HDB Flat Types
Below are step-by-step calculations for three common HDB flat configurations in Singapore. These follow SS 638 methodology and reflect typical HDB flat occupancy patterns.
Example A — 3-Room HDB Flat (post-1994, 40A main switch)
Assumed appliances:
- 2× air-conditioners (1 HP each) = 2 × 900W = 1,800W
- 1× instant water heater = 3,500W
- 1× induction hob (single) = 2,000W
- 1× refrigerator = 150W
- 1× washing machine = 2,000W
- LED lighting = 300W
- General sockets = 500W
Step 1 — Sum of all rated loads:
Step 2 — Convert to current at 230V single phase:
Step 3 — Apply diversity factor (0.5 for 3-room HDB):
Step 4 — Check against 40A main switch:
Conclusion: A 3-room HDB flat with standard appliances operates comfortably within the 40A (9.2 kW) supply capacity once diversity is applied. The apparent "overload" on paper (44.6A) never occurs in practice.
Example B — 4-Room HDB Flat (modern BTO, 40A main switch)
Assumed appliances:
- 3× air-conditioners (1.5 HP each) = 3 × 1,350W = 4,050W
- 2× instant water heaters (master + common bath) = 2 × 3,500W = 7,000W
- 1× induction hob (double) = 3,500W
- 1× refrigerator = 150W
- 1× washing machine = 2,000W
- 1× tumble dryer = 2,400W
- LED lighting = 400W
- General sockets = 1,000W
Step 1 — Sum of all rated loads:
Step 2 — Convert to current at 230V:
Step 3 — Apply diversity factor (0.45 for 4-room HDB):
Step 4 — Check against 40A main switch:
Conclusion: A fully-loaded modern 4-room BTO flat sits right at the edge of a 40A supply. If a dryer and two water heaters operate together with aircon, the MCB may trip. This is precisely why HDB and EMA allow upgrading to 63A supply for newer flats.
Example C — 5-Room / Executive HDB Flat with EV charger (63A upgraded supply)
Assumed appliances:
- 4× air-conditioners (2 HP each) = 4 × 1,800W = 7,200W
- 2× instant water heaters = 2 × 3,500W = 7,000W
- 1× induction hob (double) = 3,500W
- 1× refrigerator = 150W
- 1× washing machine = 2,000W
- 1× tumble dryer = 2,400W
- 1× EV charger (Mode 2, 16A) = 3,680W
- LED lighting = 500W
- General sockets = 1,500W
Step 1 — Sum of all rated loads:
Step 2 — Convert to current at 230V:
Step 3 — Apply diversity factor (0.4 for large HDB with EV):
Step 4 — Check against 63A upgraded supply:
Conclusion: A large HDB flat with EV charging genuinely requires a 63A supply upgrade. With the standard 40A, the diversity-adjusted demand of 48.6A would persistently overload the main switch, causing repeated tripping.