Load Summary Fundamentals — Demand Factors, Diversity, and Sizing Logic

What Is a Load Summary?

A load summary (also called a load schedule or load list) is the consolidated record of every electrical load in a facility — motors, lighting, HVAC, process equipment, and miscellaneous loads — organized by panel, switchboard, or distribution level. It is the foundation document used to size transformers, switchgear, generators, and cables, and to perform demand and diversity calculations.

Every downstream electrical calculation — short circuit, voltage drop, protection coordination, cable sizing — ultimately traces back to the load summary. An inaccurate or incomplete load summary propagates errors through the entire design.

Core Fields in a Load Summary

• Load tag / description — equipment name and unique identifier
• Connected load — nameplate rating (kW or kVA) of the equipment
• Load type — motor, lighting, heating, process, miscellaneous
• Power factor — for converting between kW and kVA
• Demand factor — the fraction of connected load expected to operate simultaneously
• Diversity factor — applied across groups of loads to account for non-coincident operation
• Voltage level and phase — supply voltage and single/three-phase designation
• Feeder / panel reference — which distribution board or switchgear feeds the load
• Duty cycle — continuous, intermittent, standby, or emergency

The Calculation Logic — Standard-Agnostic

The underlying math behind a demand load calculation is consistent regardless of which code governs the project:

• Connected Load — sum of nameplate ratings for all equipment in a group
• Demand Load = Connected Load × Demand Factor — the expected load at any given moment, since not all equipment runs simultaneously
• Diversified Demand Load = Sum of group demand loads × Diversity Factor — applied when combining multiple load groups (e.g. summing several panels into a main switchboard)
• Design Load (with margin) = Diversified Demand Load × (1 + spare capacity allowance, typically 15–25%) — used for equipment sizing to allow for future growth

This logic holds whether the project follows IEC, NEC, or a project-specific specification — what changes between standards is largely which demand factors are recommended for which load types, not the calculation structure itself.

Where Standards Diverge

• Demand factor tables — NEC Article 220 provides prescriptive demand factor tables for specific occupancy types (e.g. dwelling units, commercial lighting). IEC and most industrial/EPC practice instead rely on engineering judgment, historical data, or project-specific demand factor schedules rather than a single universal table.
• Terminology — Per IEEE Std 141 (Red Book), "demand factor" and "diversity factor" are sometimes used interchangeably in casual practice, but in rigorous usage demand factor applies within a single load group, while diversity factor applies across multiple groups being combined. This distinction is consistent in engineering theory but inconsistently applied in real-world documents — worth clarifying on every project.
• Power factor assumptions — IEC-based load summaries often default to project-specified power factor (commonly 0.8–0.85 lagging for motors), whereas NEC-based summaries may reference NEMA motor nameplate data directly. Always confirm the assumption rather than relying on a default.
• Spare/growth margin — not codified in any standard; this is a project or company-specific design philosophy, commonly ranging from 15% to 25% depending on the facility's expected lifespan and expansion plans.

Worked Example

A panel feeds three loads: a 30kW motor (demand factor 0.85), 10kW of lighting (demand factor 0.9), and 15kW of HVAC (demand factor 0.75).

• Motor demand load: 30 × 0.85 = 25.5 kW
• Lighting demand load: 10 × 0.9 = 9 kW
• HVAC demand load: 15 × 0.75 = 11.25 kW
• Total panel demand load: 25.5 + 9 + 11.25 = 45.75 kW
• With 20% growth margin: 45.75 × 1.2 = ~54.9 kW design load — used to size the panel feeder and upstream protection

Common Pitfalls

• Using connected load instead of demand load for equipment sizing — leads to significant oversizing and cost
• Applying demand factor twice — once at the load group, then again at the panel summary level, double-counting the reduction
• Ignoring power factor when converting between kW and kVA, leading to undersized transformers or switchgear
• Static load summaries not updated when equipment is added or changed late in design

Summary

• A load summary is the foundation document for all downstream electrical sizing calculations
• Demand and diversity factor logic is consistent across standards — only the recommended factor values differ
• Confirm power factor assumptions and growth margin explicitly rather than relying on defaults
• Keep the load summary live and synchronized with the single-line diagram throughout the project