How Are Electrical Load Calculations Done for a Building?

An electrical load calculation is the NEC-based analysis that determines the required capacity of electrical services, feeders, and branch circuits. It sums connected loads by category, applies code demand factors reflecting realistic simultaneous use, accounts for continuous loads at 125%, and produces the service size, feeder sizes, and panel schedules the design is built on. Think of it as the building's electrical budget: every load accounted for, realistic usage patterns applied, and the infrastructure sized to serve it all safely with room to grow.

The engineer starts with the connected load inventory — every piece of electrical equipment, organized by NEC load category: general lighting, receptacles, HVAC motors, kitchen equipment, elevators, and any special loads. Nameplate data, mechanical equipment schedules, and the lighting design all feed this list, which is why the load calc can't be finished until the other disciplines have made their selections. Then the code math: demand factors applied per category, continuous loads at 125%, motor loads with their starting-current considerations, and the largest motor's extra allowance on feeders. The result flows into the one-line diagram — service size, main breaker, feeder sizes, panel schedules — and out to the utility company, which uses it to size the transformer and service lateral. When equipment changes late in design, the load calc has to follow; I've seen services undersized because a late mechanical equipment swap never made it back into the electrical numbers.

Load calculation errors are expensive in both directions: undersized means a service that can't be legally or safely expanded, oversized means paying for gear and utility capacity you don't need. The mistakes I see repeat are always the same handful. Here's what I verify before signing off on a load calc.

• Complete inventory: every load category represented — missing EV charging or kitchen equipment is common
• Demand factors per NEC article: the right table for each load type, applied correctly
• Continuous loads at 125%: lighting and HVAC classifications checked, not assumed
• Spare capacity included: room for growth designed in now, not wished for later
• Calc matches the one-line: service, feeders, and panels all trace back to the same numbers