Step 1's calculation produced a demand load of roughly 1,554.6A. This step turns that number into an actual main switchboard selection — and shows why the right answer isn't just "the smallest breaker that clears the math."
The calculated demand of approximately 1,554.6A sizes the main switchboard selection directly. A 1600A switchboard — the next standard size up from the calculated load — would clear it with only about 3% spare capacity, which is too tight for a speculative office building: at design-development stage, tenant fit-out loads (what specific future tenants will actually install for their own equipment, workstations, and supplemental cooling) aren't finalized. Most commercial designers target 20-25%+ spare capacity above the calculated demand specifically to absorb that unknown without an expensive service upgrade mid-lease-up.
This design selects a 2000A, 480Y/277V, three-phase four-wire main switchboard instead — a standard commercial equipment size — giving roughly 445A, or about 29%, of spare capacity over the calculated 1,554.6A demand. That margin is the clearest illustration of a principle that runs through the rest of this series: the calculated demand load is a floor, not a target. The gap between "clears the calculation" and "a defensible commercial design" is exactly the spare-capacity margin a designer builds in deliberately.
From the 2000A switchboard, distribution splits into three parallel paths, worth naming explicitly since every later step in this series refers back to them: 480V direct to mechanical/HVAC (dedicated panels and motor control centers feed the chiller, air handlers, and pumps — lower current at higher voltage is exactly why commercial mechanical equipment is specified at 480V wherever possible); 277V direct to lighting (branch lighting panels feed straight off the 480Y bus's line-to-neutral voltage, zero step-down transformers required for the building's largest single continuous load); and 208Y/120V, stepped down, to receptacles and general power — the one load category genuinely too low-voltage to serve directly off a 480Y/277V bus. The elevators run as a fourth, standalone 480V path straight from the switchboard, sized per NEC 430.24 (see Step 1) and kept independent per NEC 620's life-safety/recall requirements.
✨ Steps 3-8 are part of this Project. Feeder & conductor sizing, voltage drop, fault current, selective coordination, arc-flash documentation, and the final panel schedule unlock with a one-time $4.99 purchase.