The Basis for Electrical Service Sizing
An electrical load calculation is the process of totaling all the electrical loads in a building — every circuit, appliance, system, and piece of equipment — and determining the total demand that the electrical service must be sized to handle. The result drives the service entrance size (100A, 200A, 400A, 800A, etc.), the main panel capacity, and the feeder sizes throughout the building.
What Goes Into the Calculation
Load calculations follow the National Electrical Code (NEC), which defines standard demand factors for different load types. The calculation includes:
- Lighting loads — calculated at a per-square-foot basis by occupancy type, plus actual connected load from the lighting schedule
- Receptacle loads — general purpose outlets and dedicated equipment circuits
- HVAC equipment — air handlers, condensing units, exhaust fans, and associated controls
- Kitchen and cooking equipment — ovens, fryers, dishwashers, refrigeration compressors
- Special loads — EV chargers, welding equipment, medical equipment, data center loads, elevator motors
- Demand factors — the NEC allows diversity factors for certain load categories (not all loads run simultaneously at full rated value)
Why Getting It Right Matters
An undersized electrical service creates immediate problems: breakers trip, equipment doesn't start, and the building can't support its intended use. An oversized service costs more to install and sets up a higher utility demand charge. Accurate load calculations — done by a licensed electrical engineer who understands both the NEC requirements and the building's actual usage patterns — result in a service that's right-sized for the project.
Load Calculations and Utility Coordination
The utility provider reviews the electrical load calculation when approving service upgrades. If you're increasing electrical service to a building, the utility needs to know the requested service size and the basis for it. An electrical engineer's load calculation provides this documentation in a format the utility can act on.