MEP Engineering · Engineering Glossary
A one-line diagram (also called a single-line diagram) is a simplified electrical drawing that uses single lines to represent multi-conductor circuits and standard symbols to show electrical components — service entrance, transformers, switchboards, panels, and large equipment — and their interconnections. It is the primary electrical drawing used for permit submittals, utility coordination, and construction.
The one-line diagram shows the complete power distribution system from the utility service entrance through the main switchboard, distribution panels, sub-panels, and motor control centers to all major loads. Component ratings — ampacity, interrupting capacity, voltage — are shown for each device. The structural engineer and contractor use the one-line to understand the scope and routing of main feeders. Utility companies review the one-line diagram when approving service entrance applications and utility interconnections. Code compliance for fault current ratings, selective coordination, and ground fault protection can be verified on the one-line. Apex Grid produces one-line diagrams as standard deliverables on all electrical engineering projects.
Apex Grid Engineering maintains this glossary as a resource for architects, contractors, developers, and property owners who encounter unfamiliar engineering terminology on their projects.
View All Engineering TermsApex Grid's licensed PEs provide mep engineering services across multiple jurisdictions — with fast quote turnaround and permit-ready documents.
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