The project: a 5-story, 120,000 sq ft speculative office building on a 480Y/277V utility service. Every downstream step in this series — service sizing, feeders, voltage drop, fault current, coordination, arc-flash, the panel schedule — is built on the demand number this step produces.
Commercial load calculation isn't one lookup — it's characterizing each load category on its own terms and combining them correctly. Lighting is calculated per NEC Table 220.12, which assigns office occupancy a unit load of 3.5 VA/ft² for service and feeder sizing — a code-minimum figure, deliberately higher than a real installed LED design (which typically runs 0.8-1.0 W/ft² under ASHRAE 90.1), because Table 220.12 exists to size the service conservatively, not to predict actual lighting density. At 120,000 ft², that's 420,000 VA connected. Lighting in a continuously occupied office runs 3+ hours during business hours, so it's a continuous load under 210.20/215.2 and carries the 125% factor: 420,000 × 1.25 = 525,000 VA.
Receptacles don't get their own Table 220.12 entry the way lighting does. Commercial receptacle/general-power load is estimated either by outlet count (180 VA per strap under 220.14(I)) or, more practically at design-development stage, a VA/ft² design allowance — here, 1.0 VA/ft², a widely used rule of thumb distinct from and deliberately lower than the lighting figure: 120,000 ft² × 1.0 VA/ft² = 120,000 VA, treated as non-continuous with no 125% factor.
HVAC/mechanical is estimated at a standard commercial rule of thumb of 350 sq ft per ton of cooling: 120,000 ÷ 350 ≈ 343 tons, and at roughly 1.6 kVA/ton combined for chillers, air handlers, and pumps, connected mechanical load is approximately 343 × 1.6 ≈ 550,000 VA. Vertical transport — two 40 HP traction elevators — is sized per NEC 430.24's motor-summing method: sum of full-load currents plus 25% of the largest, 52 + 52 + 13 = 117A, equal to 1.732 × 480 × 117 ≈ 97,300 VA.
Total connected/demand load: 525,000 + 120,000 + 550,000 + 97,300 = 1,292,300 VA. At 480Y/277V three-phase: 1,292,300 ÷ (1.732 × 480) = 1,292,300 ÷ 831.36 ≈ 1,554.6A. That number is the one Step 2 checks the service size against.