This free split-screen designer pairs a 3D model of a multi-story commercial building with its electrical single-line diagram, so you can see how power flows from the utility service all the way down to branch circuits. Click equipment in either view to cross-highlight the utility service, main switchboard, step-down transformers, floor panelboards, mechanical feeders, and the emergency/standby system. It's a visual learning tool for electrical designers, engineers, electricians, and students learning commercial power distribution per the NEC.
The designer covers the full commercial distribution path, from the utility down to the load:
• Utility service — the incoming medium- or low-voltage service, metering, and service entrance. • Main switchboard / switchgear — the service-entrance equipment with the main overcurrent device that distributes power to feeders. • Step-down transformers — dry-type transformers that step service voltage (e.g., 480V) down to 208Y/120V for receptacles and lighting. • Floor panelboards — distribution and branch-circuit panelboards on each level feeding lighting and receptacle circuits. • Branch circuits — the final wiring serving lighting, receptacles, and equipment loads. • Mechanical feeders — feeders to HVAC equipment, rooftop units, pumps, and motor loads. • Emergency / standby systems — a generator, automatic transfer switch (ATS), and fire pump arranged per NEC Articles 700, 701, 702, and 695.
Power enters from the utility through the service entrance into a main switchboard or switchgear, which holds the main overcurrent protective device and splits power into feeders. Larger buildings use a radial distribution: the switchboard feeds step-down transformers and floor panelboards, which in turn feed branch-circuit panelboards and individual circuits. Mechanical loads such as HVAC and pumps are fed by dedicated motor feeders. A separate emergency and standby system — a generator with one or more automatic transfer switches — keeps life-safety loads (egress lighting, fire alarm, fire pump) energized when utility power fails, with the legally required emergency system (NEC 700) generally separated from optional standby (NEC 702) and the fire pump fed per NEC 695.
1. Drag to orbit the building, scroll to zoom, and pan to move around the 3D model. 2. Click any piece of equipment to highlight it and cross-reference it on the single-line diagram. 3. Trace power from the utility service → switchboard → transformers → panelboards → branch circuits, and follow the separate emergency/standby path from the generator through the ATS to the life-safety loads.
A switchboard is large, free-standing service-entrance or main distribution equipment that takes the incoming service and divides it into feeders, typically with high ampere ratings. A panelboard is a wall-mounted enclosure with branch-circuit breakers that distributes power to the final circuits. In a commercial building the switchboard usually feeds multiple panelboards.
An ATS is a device that automatically switches a load between two power sources — normally the utility and a backup generator. When it senses the utility has failed, it signals the generator to start and transfers the load to it; when utility power returns, it transfers back. ATS equipment is central to emergency and standby systems under NEC Articles 700, 701, and 702.
Emergency systems (NEC 700) are legally required by code or authority and supply loads essential to life safety — egress lighting, exit signs, fire alarm — with power restored within 10 seconds. Legally required standby systems (NEC 701) serve loads whose failure could create hazards or hamper rescue, with a 60-second restoration. Optional standby systems (NEC 702) protect business or process loads (data centers, refrigeration) where outage is an inconvenience or financial loss rather than a life-safety issue.
A single-line or one-line diagram is a simplified drawing that represents a three-phase electrical system with a single line, showing how power flows from the service through switchboards, transformers, and panelboards to the loads. It is the primary document for understanding, designing, and coordinating overcurrent protection in a distribution system.
NEC Article 695 requires a reliable power source for electric fire pumps, often a dedicated service tap or feeder ahead of the building main, sometimes backed by an on-site generator. The fire-pump circuit is sized to carry locked-rotor current and is protected and arranged so that it remains energized as long as possible during a fire, separate from the normal building distribution.