What is the NEC 220 load calculation for a commercial building?

NEC Article 220 tallies the building load — general lighting from Table 220.12, 180 VA per receptacle, continuous loads at 125%, the largest motor at 125% plus others at 100%, and special loads such as kitchen equipment with their demand factors. Part III governs feeders and services. The calculated demand in VA is converted to amperes to size the service.

How do you size a commercial electrical service in amps?

Convert the calculated demand load to current with I = VA / (V x 1.732) for three-phase, then round up to a standard service size with 20–25% spare capacity. For example, a 350 kVA demand at 480Y/277V is about 421 A, so a 600 A service is selected. The equipment must also be rated for the utility's available fault current.

Why do commercial buildings use 480V with step-down transformers?

Distributing at 480Y/277V reduces current for a given power, allowing smaller conductors and lower losses, and it directly feeds 277V lighting and large HVAC. Dry-type transformers then step down to 208Y/120V for receptacles and small loads, sized per NEC 450.3, keeping the high-current low-voltage runs short.

Electrical·13 min read·June 15, 2026

⚡ How to Design a Commercial Electrical System (NEC 220)

A step-by-step guide to designing a commercial electrical distribution system using NEC Article 220 load calculations — sizing the service, switchboard and MDP, step-down transformers, panelboards, and the single-line diagram.

📖Illustrated Guide to NEC 2026 Electrical Inspection (Full Access)

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How Commercial Power Distribution Is Organized

A commercial electrical system moves power in a tree: the utility feeds a service entrance, the service feeds a switchboard or main distribution panel (MDP), the MDP feeds feeders to step-down transformers and panelboards, and panelboards feed branch circuits to loads. Designing it well means sizing each layer from the loads up using NEC Article 220, then documenting the whole tree on a single-line diagram. This guide follows that build-up.

Topics covered

commercial electrical designelectrical panel designNEC 220 load calculationelectrical distribution designswitchboardsingle-line diagrampanelboardservice entrance