3.4 Connected Load, Maximum Demand & DEWA Diversity Factors

3.4 Connected Load, Maximum Demand & DEWA Diversity Factors

Study focus: Accurate load estimation dictates the sizing of Main Distribution Boards (MDBs), main incoming supply cables, and DEWA transformer capacity. Candidates must know DEWA's specific diversity rules: 66% for lighting, 100% first + 40% remaining for 13A socket circuits, 100% (zero diversity) for HVAC, and 100% duty / 0% standby for pumps.

Estimating electrical load is the first critical phase in designing any building distribution system in Dubai. Summing the total rated electrical capacity of every lamp, socket outlet, water heater, and air conditioner yields the Connected Load (CL). However, operating all electrical utilization equipment simultaneously at full rated capacity is practically impossible. Applying statutory DEWA Diversity Factors yields the realistic Maximum Demand (MD), establishing the true capacity required for incoming service feeders, switchgear, and supply transformers.

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Core Definitions & Load Terminology

1. Connected Load (CL): The sum total of continuous nameplate power ratings of all connected electrical equipment and sub-circuits, expressed in kilowatts ($kW$) or kilovolt-amperes ($kVA$).
2. Maximum Demand (MD): The highest integrated electric load drawn by the facility over a designated time interval (typically 15-minute or 30-minute peak demand window) during operating cycles ($MD \le CL$).
3. Demand Factor: The ratio of Maximum Demand to Connected Load: $$\text{Demand Factor} = \frac{\text{Maximum Demand (MD)}}{\text{Connected Load (CL)}} \le 1.0$$
4. Diversity Factor: The ratio of the sum of individual non-coincident peak demands of sub-circuits to the simultaneous maximum demand of the entire installation: $$\text{Diversity Factor} = \frac{\sum \text{Individual Peak Demands}}{\text{Simultaneous Maximum System Demand}} \ge 1.0$$

                    Load Pyramid: Connected Load vs Maximum Demand
  ┌─────────────────────────────────────────────────────────────────────────┐
  │ Connected Load (CL): Sum of ALL Nameplate Ratings (100% Capacity)       │
  ├─────────────────────────────────────────────────────────────────────────┤
  │                                    │                                    │
  │   Lighting (66%)   Socket Outlets (100%+40%)   HVAC (100% No Diversity)│
  │                                    │                                    │
  ├─────────────────────────────────────────────────────────────────────────┤
  │ Maximum Demand (MD): Realistic Peak Diversified System Demand           │
  └─────────────────────────────────────────────────────────────────────────┘

Economic & Operational Impact of Diversity

• Oversizing Risks: Ignoring diversity causes gross oversizing of transformers, main circuit breakers, main feeders, and busbars. This severely inflates initial capital expenditures, increases DEWA service connection fees, and forces transformers to run at light load, incurring poor efficiency and high fixed core losses.
• Undersizing Risks: Over-estimating diversity (under-estimating demand) leads to nuisance tripping of main incomer breakers during hot Dubai summer afternoons when HVAC units run continuously at peak ambient conditions.

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DEWA Statutory Diversity Allowances

DEWA Regulations for Electrical Installations prescribe standard diversity factors across different load categories:

1. General Interior Lighting Circuits

• Residential, Office & Commercial Spaces: $66\%$ ($0.66$) diversity factor applied to total connected lighting load.
• Hotel Guest Rooms & Apartment Bedrooms: $50\%$ to $66\%$ diversity.

2. 13A Ring & Radial Socket Outlet Circuits (BS 1363)

• First / Largest Socket Circuit: $100\%$ ($1.00$) of rated capacity (standard 32A ring circuit = $7.36\text{ kVA}$ at $230\text{ V}$).
• Remaining Socket Circuits: $40\%$ ($0.40$) of rated capacity for all additional socket sub-circuits connected to the distribution board.

3. HVAC & Air Conditioning Equipment

• Chillers, Package AC Units, DX Split Units, AHUs, FCUs: $100\%$ ($1.00$) demand factor — ZERO diversity allowed.
• Rationale: In Dubai, peak ambient air temperatures ($> 45^\circ\text{C}$) force all air conditioning and mechanical ventilation equipment to operate simultaneously at full continuous rated capacity.

4. Electric Water Heaters

• Individual / Residential Units: $100\%$ ($1.00$) demand factor.
• Multi-Unit Residential Developments: $100\%$ for largest water heater + $50\%$ for remaining units.

5. Water Pumps & Motor Loads

• Domestic / Transfer Water Pumps (Duty / Standby): $100\%$ for Duty Pump, $0\%$ for Standby Pump (mechanically or electrically interlocked to prevent simultaneous running).
• Elevators / Lifts: $100\%$ for largest lift motor + $75\%$ for 2nd lift motor + $60\%$ for remaining lift motors.

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Power Factor ($\cos\phi$) & Current Conversion

DEWA's allowable overall power-factor range is 0.90 lagging to unity at the point of supply, with 0.95 lagging recommended as a design target.

Convert Active Power ($P$ in $kW$) to Apparent Power ($S$ in $kVA$):

$$S_{\text{kVA}} = \frac{P_{\text{kW}}}{\cos\phi}$$

Three-Phase Demand Line Current ($I_{\text{demand}}$):

$$I_{\text{demand}} = \frac{S_{\text{kVA}} \times 1000}{\sqrt{3} \times 400\text{ V}} = \frac{P_{\text{kW}} \times 1000}{\sqrt{3} \times 400\text{ V} \times \cos\phi}$$

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Step-by-Step Worked Numerical Example

Problem Statement

Perform a comprehensive load estimation and main incomer breaker sizing for a commercial building floor in Dubai with the following connected loads:

1. Lighting Load: $18\text{ kW}$ total connected LED lighting (Power Factor = $0.90$ lagging).
2. Socket Outlet Circuits: $10$ ring socket sub-circuits (32A rated @ 230V = $7.36\text{ kVA}$ each, Power Factor = $0.95$).
3. HVAC Air Conditioning: $60\text{ kW}$ total connected DX packaged cooling units (Power Factor = $0.85$ lagging).
4. Electric Water Heaters: $6$ units @ $2\text{ kW}$ each = $12\text{ kW}$ total (Power Factor = $1.00$).
5. Domestic Water Pumps: $2$ pumps (1 Duty, 1 Standby) @ $7.5\text{ kW}$ each (Power Factor = $0.85$ lagging).

Calculate the Total Connected Load, Total Diversified Maximum Demand, System Demand Factor, and Main Circuit Breaker (MCCB) Rating.

Step 1: Compute Connected Load (CL)

• Lighting Connected $kVA$: $\frac{18\text{ kW}}{0.90} = 20.00\text{ kVA}$
• Sockets Connected $kVA$: $10 \times 7.36\text{ kVA} = 73.60\text{ kVA}$
• HVAC Connected $kVA$: $\frac{60\text{ kW}}{0.85} = 70.59\text{ kVA}$
• Water Heaters Connected $kVA$: $\frac{12\text{ kW}}{1.00} = 12.00\text{ kVA}$
• Water Pumps Connected $kVA$: $2 \times \frac{7.5\text{ kW}}{0.85} = 17.65\text{ kVA}$

$$\text{Total Connected Load (CL)} = 20.00 + 73.60 + 70.59 + 12.00 + 17.65 = 193.84\text{ kVA}$$

Step 2: Apply DEWA Diversity Factors to Calculate Maximum Demand (MD)

1. Diversified Lighting: $$P_{\text{light}} = 18\text{ kW} \times 0.66 = 11.88\text{ kW} \quad \Rightarrow \quad S_{\text{light}} = \frac{11.88\text{ kW}}{0.90} = 13.20\text{ kVA}$$
2. Diversified Socket Outlets:
   • 1st circuit @ 100%: $7.36\text{ kVA}$
   • 9 remaining circuits @ 40%: $9 \times 7.36\text{ kVA} \times 0.40 = 26.50\text{ kVA}$
   • Total Socket Demand: $7.36 + 26.50 = 33.86\text{ kVA}$
3. Diversified HVAC (100% No Diversity): $$S_{\text{hvac}} = 70.59\text{ kVA} \quad (60.00\text{ kW})$$
4. Diversified Water Heaters (100%): $$S_{\text{wh}} = 12.00\text{ kVA} \quad (12.00\text{ kW})$$
5. Diversified Water Pumps (1 Duty @ 100%, 1 Standby @ 0%): $$S_{\text{pump}} = 1 \times \frac{7.5\text{ kW}}{0.85} + 0 = 8.82\text{ kVA}$$

$$\text{Total Diversified Maximum Demand (MD)} = 13.20 + 33.86 + 70.59 + 12.00 + 8.82 = 138.47\text{ kVA}$$

Step 3: Calculate System Demand Factor

$$\text{Demand Factor} = \frac{\text{Maximum Demand}}{\text{Connected Load}} = \frac{138.47\text{ kVA}}{193.84\text{ kVA}} = 0.714 \quad (71.4\%)$$

Step 4: Compute Three-Phase Demand Line Current ($I_{\text{demand}}$)

$$I_{\text{demand}} = \frac{S_{\text{demand}} \times 1000}{\sqrt{3} \times 400\text{ V}} = \frac{138.47 \times 1000}{692.82} = 199.86\text{ A}$$

Step 5: Select Main Distribution Board Breaker & Feeder Cable

• Main Incomer Protection Rating ($I_n$): Select standard MCCB rating immediately above $I_{\text{demand}}$ ($199.86\text{ A}$):
  • Select $250\text{ A}$ 3-Pole MCCB ($I_n = 250\text{ A}$).
• Incoming Cable Sizing: Select $150\text{ mm}^2$ 4-core Cu/XLPE/SWA/LSZH cable ($I_z \ge 250\text{ A}$).