Electrical Diversity Factor Planner: Accurately Size Multiple Load Groups in Minutes

This article explains electrical diversity factor planning for accurately sizing multiple load groups quickly safely.

Engineers use diversity to estimate concurrent demand, optimize feeders, transformers, and protective device selection accurately.

Electrical Diversity Factor Planner – Calculate Diversified Demand and Feeder Current for Multiple Load Groups

Basic system parameters

System type Three-phase (3-wire) Single-phase

Nominal line voltage (V)

Global power factor (pu)

Load groups (basic)

Group 1 connected load (kW)

Group 1 demand factor (%)

Group 2 connected load (kW)

Group 2 demand factor (%)

Group 3 connected load (kW)

Group 3 demand factor (%)

Advanced options

Additional load groups (optional)

Group 4 connected load (kW)

Group 4 demand factor (%)

Group 5 connected load (kW)

Group 5 demand factor (%)

Group 6 connected load (kW)

Group 6 demand factor (%)

Design margins and checks

Design safety margin on current (%)

Measured main feeder maximum demand (kW)

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Enter system and load-group data to obtain diversified demand and design feeder current.

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Formulas used

For each load group i (i = 1…n):

Connected load:
P_conn_i (kW) = input value for group i.

Demand factor:
DF_i (%) = input demand factor for group i. If left blank, DF_i is assumed 100%.

Diversified demand per group:
P_dem_i (kW) = P_conn_i × DF_i / 100

Total connected load:
P_conn_total (kW) = Σ P_conn_i for all groups with P_conn_i > 0.

Total diversified demand:
P_dem_total (kW) = Σ P_dem_i for all groups with P_conn_i > 0.

Overall demand factor (per unit):
DF_overall (pu) = P_dem_total / P_conn_total (only if P_conn_total > 0).

If a measured main feeder maximum demand is available:

Overall diversity factor:
Diversity factor = Σ P_dem_i / P_measured, where P_measured is the measured main feeder maximum demand in kW.

Feeder current (three-phase, 3-wire):
I_nominal (A) = P_dem_total × 1000 / (√3 × V_LL × pf)
where V_LL is line-to-line voltage in V, pf is global power factor (pu).

Feeder current (single-phase):
I_nominal (A) = P_dem_total × 1000 / (V × pf)
where V is the phase-to-neutral or two-wire voltage in V, pf is global power factor (pu).

Design feeder current including safety margin:
I_design (A) = I_nominal × (1 + Safety_margin / 100)

Typical load group	Typical demand factor range (%)	Typical power factor range (pu)
Lighting circuits (office/commercial)	80–100	0.95–1.00
Socket outlet / small power	40–70	0.85–0.95
Motor loads (pumps, fans)	70–90	0.80–0.90
HVAC and chillers	60–85	0.80–0.90
General building main feeders	50–75 (overall)	0.85–0.95

Frequently asked questions

1. What is the difference between demand factor and diversity factor in this calculator?

The demand factor is applied per load group and represents the ratio between the maximum demand of that group and its total connected load. The diversity factor is calculated only if a measured maximum demand is provided and represents the ratio between the sum of individual group maximum demands and the measured maximum demand on the main feeder. The two concepts are related but not interchangeable.

2. How many load groups can I model and what if some groups are not used?

The planner allows up to six load groups. Any group with connected load equal to zero or left blank is ignored in the calculations. If a demand factor field is left blank while a connected load is entered, the calculator assumes a demand factor of 100% for that group.

3. How should I choose the safety margin on current?

The safety margin accounts for load growth, calculation uncertainties, and coordination with standard cable and breaker ratings. For main distribution feeders, design practice often uses 10–25% depending on the project requirements. For final circuits that are already conservative, the safety margin can be lower or even 0%.

4. Which voltage should I enter for three-phase systems?

For three-phase systems, enter the line-to-line voltage (for example 400 V, 415 V or 480 V). The calculator uses the three-phase power formula with this line voltage to obtain the feeder current. For single-phase systems, enter the actual operating voltage between the two conductors (for example 230 V or 240 V).

Principles of Electrical Diversity Factor and Demand Calculation

Effective electrical distribution design requires distinguishing between total connected load and expected maximum demand. Two related but distinct metrics are commonly used:

• Diversity factor (DF) — defined by standards and textbooks as the ratio of the sum of individual maximum demands to the maximum demand of the whole system. DF ≥ 1.
• Demand factor (k) — the reciprocal of diversity factor. It expresses expected maximum demand as a fraction of total connected load. 0 < k ≤ 1.

Formulas (HTML-only):

Variable definitions and typical ranges

• P_i_max — Maximum connected load of individual item or circuit i (kW or kVA). Typical values: lighting circuit 1–4 kW; socket circuit 2–6 kW; domestic cooker 6–10 kW.
• P_system_max — Estimated simultaneous maximum demand of the grouped loads (kW or kVA).
• DF_system — Often between 1.1 and 4.0 depending on load diversity; typical for residential blocks 1.5–3.5.
• k_system — Practical demand factors commonly 0.3–0.9 depending on load mix.

How to apply diversity factors to size feeders and transformers

Design workflow:

1. Inventory connected loads and classify by load group (lighting, small power, HVAC, motors, cooking, lifts, etc.).
2. For each group, choose an appropriate demand factor k_group from standards, experience, or measured profiles.
3. Compute group design demand: P_group_design = ΣP_group_connected × k_group.
4. Sum all P_group_design values to get total demand. If necessary, apply a further system-level diversity allowance for non-coincident groups.
5. Convert power to current using the appropriate voltage and power factor, and apply correction factors for temperature, cable grouping, and harmonics when sizing conductors and protective devices.

Transformation formulas (3-phase and single-phase) useful for conductor and transformer sizing:

Variables for current conversion (typical values)

• P — Power in kW (use kVA if power factor unknown).
• V — Line-to-line voltage (typical low-voltage: 400 V three-phase, 230 V single-phase).
• pf — Power factor (typical values: lighting 0.9–1.0, motors 0.8–0.95, modern electronic loads 0.95 with PFC).

Standards, practical guidance and normative references

Designers should consult the primary standards and codes applicable in their jurisdiction when selecting diversity or demand factors. Key authoritative sources include:

• BS 7671 — Requirements for Electrical Installations (IET Wiring Regulations). See IET for guidance on demand and diversity. https://www.theiet.org/
• IEC 60364 — Electrical installations of buildings. https://www.iec.ch/
• NFPA 70 (NEC) — National Electrical Code; Article 220 covers load calculations and demand factors. https://www.nfpa.org/
• CIBSE Guides — Guidelines for building energy and electrical load estimation. https://www.cibse.org/
• IEEE standards on power system analysis and motor starting. https://www.ieee.org/

Note: Some countries provide tables of demand factors for specific building uses (e.g., BS 7671 Annexes, NEC Article 220 tables). Those tables should be used as primary references for permitted diversity.

Common pitfalls and verification

• Ambiguity between "diversity factor" and "demand factor": explicitly state which is being used and its numeric range.
• Overapplying diversity to safety-critical or continuous loads (medical equipment, emergency lighting) is not permitted—treat separately.
• Motor starting currents and inrush must be considered even when applying diversity; starting coincidence may require larger conductor and protection ratings.
• Always verify the final calculated maximum demand against measurements where available. Smart meter data and substation records provide real-world confirmation.

Checklist before final sizing

1. Confirm load categories and connected load documentation from client and vendor datasheets.
2. Select conservative demand factors for new designs; use measured/load studies for existing installations to refine values.
3. Check local code requirements for minimum feeder and service sizes; some codes prohibit reducing certain loads by diversity.
4. Design protection devices with adequate short-circuit and earth-fault ratings; apply diversity only to steady-state demand, not fault currents.

Worked Example 1 — Multi-unit residential building (20 apartments)

Problem statement: Size the main incoming transformer and primary feeder for a 20-apartment block using common engineering diversity factors. Assume three-phase 400 V supply and power factor 0.95.

Step 1: Inventory of connected loads per apartment

• Lighting per apartment: 1.5 kW
• Sockets/general-purpose outlets per apartment: 4.0 kW
• Electric cooker per apartment: 8.0 kW (fixed wiring)
• Water heater (instantaneous) aggregated at 10 apartments share demand: assume diversity

Step 2: Total connected load (all apartments)

Using HTML formulas:

Step 3: Select demand (diversity) factors

• Lighting k_ltg = 0.6 (typical residential diversity)
• Sockets k_sockets = 0.35 (many sockets not used simultaneously)
• Cooker k_cooker = 0.6 (high consumption but staggered)

Step 4: Compute design demand per group

Step 5: Aggregated design demand

Step 6: Convert to three-phase current (use pf = 0.95)

Compute numerator: 142,000. Denominator: 1.732 × 400 × 0.95 = 658.16 approximately.

I_total ≈ 142000 / 658.16 ≈ 215.8 A

Step 7: Select transformer rating

• Select the next standard transformer capacity exceeding the continuous demand and allow margin for diversity and future load growth; 250 kVA three-phase standard would be appropriate.
• Verify: 250 kVA at 0.95 pf gives kW = 250 × 0.95 = 237.5 kW which adequately covers 142 kW with ample margin.

Discussion and checks

• Confirm whether local regulations limit diversity on cookers or require dedicated metering—some jurisdictions disallow diversity on certain fixed appliances.
• Consider simultaneous water heater peaks or combined usage patterns at cooking times; if measured data suggests higher coincidence, increase k factors accordingly.
• Apply correction for ambient temperature and cable grouping when sizing conductors to carry the 216 A design current safely (round up from 215.8 A to next standard or per cable rating tables).

Worked Example 2 — Mixed-use commercial building with separate load groups

Problem statement: Determine feeder size and transformer rating for a three-floor mixed-use building: ground-floor retail, two upper floors office. Supply 400 V three-phase. Power factor assumed 0.9 for office loads and 0.95 for retail lighting.

Connected loads (given)

• Retail (ground): Lighting 8 kW, Small power 18 kW, HVAC 20 kW
• Offices (per upper floor): Lighting 12 kW, Small power 35 kW, HVAC 40 kW (per floor)
• Two office floors => double office loads
• All connected loads sum to: Retail 46 kW; Offices total (per floor) 87 kW × 2 = 174 kW; Grand total connected = 220 kW

Select demand factors per group

• Retail lighting k = 0.8; retail small power k = 0.7; retail HVAC k = 0.9
• Office lighting k = 0.85; small power k = 0.6; office HVAC k = 0.9

Group design demands

Conversion to current

Because pf varies by group, compute equivalent three-phase kVA or convert approximate combined pf. Conservative approach: compute three-phase current using overall pf = 0.92 (weighted approximation).

I = (P × 1000) / (√3 × V × pf) = (171.4 × 1000) / (1.732 × 400 × 0.92)

Select transformer rating

• Transformer apparent power S = P / pf = 171.4 / 0.92 ≈ 186.3 kVA. Select standard rating 200 kVA.
• Check inrush and motor starting of HVAC units; if simultaneous starting risk exists, consider auto-transformer or soft-start measures rather than oversizing transformer.

Additional considerations and detailed checks

1. Apply diversity between building groups if measured data or occupancy profiles justify further reductions; e.g., retail may peak at different hours to office zones.
2. For power factor correction, evaluate bank size to raise pf near unity and reduce transformer apparent loading.
3. Assess protective device coordination, selective discrimination, and harmonic filters for electronic loads.

Automating calculations: The Electrical Diversity Factor Planner concept

An effective planner tool automates the steps above, enabling sizing of multiple load groups in minutes. Essential functions:

• Load database with categorized items and vendor-specified maximums.
• Configurable default demand factors by category with override capability per jurisdictional rules.
• Simultaneity matrix allowing users to model coincidence between groups and motors starting sequences.
• Automatic conversion to current and standard equipment sizing (cables, busbars, transformers) with checks for temperature, grouping, and derating factors.
• Report generation with calculation witness values and normative references embedded for audit and sign-off.

Key algorithmic formulae for a planner (HTML)

Data inputs and recommended defaults

1. Unit counts and individual nameplate loads (kW or kVA).
2. Default k values per load category (see tables earlier).
3. Target power factor for conversion (0.9–0.95 usual).
4. System voltage and earthing arrangement.
5. Local regulatory constraints (forbidden diversity allowances on fixed appliances or emergency loads).

Field validation and iterative refinement

Even when using a planner, field validation is essential. Installation loads and behavior can diverge significantly from nameplate data.

• Use temporary metering and data logging at distribution panels during representative periods to capture diversity and coincidence in situ.
• Run a day/week measurement campaign covering known peak dates or operational cycles (e.g., production lines, HVAC seasonal peaks).
• Iteratively update planner defaults with measured demand factors and adjust feeder and transformer sizing accordingly.

Measurement best practices

1. Use Class A power quality meters capturing 3-phase currents, voltages, energy, and harmonics.
2. Log for at least one typical week; include business-critical periods and off-hours.
3. Correlate measured peaks with operational events (shift changes, maintenance, start-ups).

Regulatory and safety notes

When applying diversity, designers must ensure safety and compliance. Some items cannot be reduced by diversity due to safety or contractual reasons:

• Emergency lighting, life-safety circuits, and medical equipment typically require full capacity without diversity reductions.
• Mandatory fixed appliance allowances in certain national regulations (e.g., dedicated circuits for cookers or water heaters may not be diversified).
• Always document the rationale and normative reference when applying non-standard demand factors.

References and further reading

• BS 7671: Requirements for Electrical Installations (IET Wiring Regulations). IET. https://www.theiet.org/
• IEC 60364 series — Electrical installations of buildings. International Electrotechnical Commission. https://www.iec.ch/
• NFPA 70: National Electrical Code, Article 220 — Branch-Circuit, Feeder, and Service Calculations. NFPA. https://www.nfpa.org/
• CIBSE Guide A: Environmental Design — useful for estimating building loads and timings. https://www.cibse.org/
• IEEE Std 141 (Red Book) — Recommended Practice for Electric Power Distribution for Industrial Plants. https://www.ieee.org/

Summary of practical recommendations

• Always separate the concepts of diversity factor (DF ≥ 1) and demand factor (k ≤ 1) to avoid calculation errors.
• Start with conservative demand factors for initial sizing; refine with measurement data where feasible.
• Document assumptions, selected k values, and normative references for auditability and safety.
• Use an automated planner to accelerate sizing across many load groups, but validate outputs with field data and local codes.

Applying these engineering procedures and checks enables accurate sizing of multiple load groups in minutes when using a properly configured Electrical Diversity Factor Planner. Careful definition of terms, adherence to codes, and field verification ensure reliability and safety in the final design.