A single-line diagram (SLD) is the primary document in a commercial electrical permit package. It shows how power flows from the utility service through the building’s electrical distribution system to the individual loads. Building department plan checkers use the SLD to verify that the electrical system is designed correctly, and contractors use it during installation. Our commercial electrical engineering team produces SLDs for projects in all 50 states.
Why It Is Called a Single-Line Diagram
Commercial electrical systems use three-phase power, which means power flows through three separate conductors (phases) from the utility to the building. A complete wiring diagram showing all three conductors for every circuit would be extremely complex and difficult to read. The single-line diagram represents the entire three-phase system using a single line, with annotations indicating the number of conductors, wire sizes, voltage, and phase configuration at each point in the system.
This simplification makes the distribution hierarchy legible while still conveying all the information needed to verify that the system is correctly designed. Once you understand the conventions, a single-line diagram tells you everything about how power flows from the utility to the final loads.
Reading the SLD from Top to Bottom
Utility Service
The SLD starts at the top with the utility service. The utility transformer symbol appears at or near the top of the diagram, followed by the service entrance conductors connecting the transformer to the building’s main electrical equipment. The transformer rating (in kVA), the primary voltage, and the secondary voltage are annotated at the transformer symbol. For EV charging projects, the transformer capacity is a critical element because the EV load determines whether the existing transformer can serve the new demand.
Main Disconnect and Meter
Below the transformer, the single line typically shows the utility meter and the main disconnect. The meter measures the building’s total electrical consumption. The main disconnect is the device that can de-energize the entire building electrical system. For commercial buildings, the main disconnect is typically a main circuit breaker in the main distribution panel (MDP) or a main disconnect switch ahead of the MDP. The main breaker amperage rating appears next to the symbol.
Main Distribution Panel and Switchboard
The MDP or switchboard is the central distribution point for the building. From the MDP, feeder circuits branch out to sub-panels and major equipment. The MDP is typically shown as a bus bar on the SLD with multiple circuit breakers or fusible switches branching from it. Each feeder circuit is labeled with the breaker size, conductor size, conduit size and type, and the load it serves. Voltage, fault current at the MDP bus, and the available interrupting capacity of the protective devices may also be noted.
Sub-Panels and Distribution Boards
Sub-panels appear below the MDP in the hierarchy. Each sub-panel serves a specific area or system of the building: the mechanical room, the lighting panel for a floor, the panel serving a specific tenant space. The feeder from the MDP to the sub-panel is labeled on the single line with conductor size, conduit size, and circuit breaker or fuse rating at both ends.
What Information Must Appear for Permit Approval
A permit-ready SLD must include the service voltage and number of phases, the main service breaker or disconnect rating, the fault current available at the service entrance (from the utility), the fault current available at each panel (calculated by the engineer), the ampacity rating of all conductors (wire size per NEC 310), the circuit breaker ratings and interrupting capacity at each protective device, the rating of all transformers and major equipment, and the load type served by each feeder circuit. Our engineers follow this checklist for every commercial electrical package we produce.
How the SLD Relates to Other Documents
The SLD is one document in a complete electrical permit package. Panel schedules list every individual circuit in every panel with the load, breaker size, and wire gauge for each circuit. Load calculations document how the total connected and demand loads were calculated. Equipment specifications describe the make and model of major equipment. The SLD shows the relationship between all of these elements at the system level.
Contractors use the SLD to understand the distribution system layout before installation begins. Field superintendents use it to verify that installed equipment matches the approved design. Inspectors use it during the final inspection to confirm the as-built electrical system matches the approved permit documents. Learn more about our engineering capabilities for commercial projects.
Frequently Asked Questions
What is the difference between a single-line diagram and a schematic diagram?
A single-line diagram shows the distribution hierarchy of an electrical system at a high level, using one line to represent three-phase conductors and standard symbols for equipment. A schematic diagram shows the exact wiring of a specific control circuit or device, including every conductor and every connection. SLDs are used for power distribution design; schematics are used for control system design.
Is an SLD required for all commercial electrical permits?
Yes, in virtually all commercial jurisdictions. The SLD is the primary document the plan checker reviews to evaluate the electrical system design. Without an SLD, the plan checker cannot verify that the system is designed to code and the permit application will be rejected as incomplete.
Can a general contractor read and interpret an SLD without electrical engineering training?
A general contractor with commercial construction experience can learn to read SLDs well enough to understand the basic distribution hierarchy and to verify that equipment in the field matches the approved documents. Interpreting the technical calculations and verifying code compliance requires electrical engineering training. For questions about the design intent or code basis of specific elements, the electrical engineer of record is the right resource.
Get Your Commercial Electrical Package
We produce permit-ready commercial electrical engineering packages including SLDs for projects in all 50 states. Submit at rightangleeng.com/get-started.
References
National Fire Protection Association. (2023). NFPA 70: National Electrical Code. Quincy, MA.
Institute of Electrical and Electronics Engineers. (2021). IEEE Std 141-1993 (Reaffirmed): IEEE Recommended Practice for Electrical Power Distribution.
International Building Code. (2021). Chapter 27: Electrical. International Code Council.