Reading electrical drawings and specifications

Electrical drawings show where equipment goes and how it is connected; the specifications describe what the equipment must be and how it must be installed. Together they are the instructions a contractor prices and builds from. You can read most of what matters by starting with the legend, then the single-line diagram, then the plans and panel schedules, following circuit tags from each device back to its panel.

Owners and property managers see electrical drawings at tender, during construction and years later when planning a renovation. Being able to find your way around them saves time in meetings with designers and contractors and helps you spot when something important is missing.

What is in a typical set

• Legend, abbreviations and general notes. The key to every symbol and short form used in the set, plus notes that apply everywhere. Read this first.
• Site plan. Utility service route, exterior lighting, site power such as EV chargers and pole lights.
• Single-line and riser diagrams. How power flows from the service to every panel, with ratings.
• Power plans. Receptacles, equipment connections, panels and circuit numbers on each floor.
• Lighting plans. Fixtures, switches, sensors and control zones, with fixture type tags.
• Systems plans. Fire alarm, communications, security and other low-voltage systems, sometimes on separate sheets or by separate designers.
• Schedules. Panel schedules, fixture schedules, equipment and mechanical connection schedules.
• Details. Mounting, grounding, equipment pads and anything that needs a closer view.

Reading a power plan

Power plans are drawn on the architectural floor plan with electrical symbols on top. Each device has a symbol from the legend, and each group of devices on the same circuit is tagged with the panel name and circuit number. Lines between devices show that they are on the same circuit; they do not show the exact route the cable takes. An arrow pointing away from the last device, called a home run, means "this circuit runs back to the panel", and the tag next to it tells you which panel and circuit. You then find that circuit in the panel schedule.

Lighting plans work the same way, with extra tags for fixture type, control zone and switch or sensor. The fixture type tag points to the fixture schedule, which describes the product, lamp or LED output, colour temperature, voltage and mounting.

Things that are easy to miss

• General notes that apply to every sheet, such as mounting heights or a requirement for a separate neutral on every circuit.
• Keynotes, the numbered tags that point to a note on the same sheet.
• Equipment connections for mechanical units, which may be shown on a separate schedule rather than on the plan.
• Demolition sheets in renovation sets, which show what is to be removed or made safe.
• Revision clouds and the revision block, which show what changed in each issue.

Reading the specifications

The specifications, often called the specs, are written documents organised by section. Electrical work is commonly in Division 26, with communications in Division 27 and electronic safety and security, such as fire alarm, in Division 28. Many Canadian specifications follow the National Master Specification structure. Each section is usually split into three parts:

1. General. Scope, related sections, submittals required, quality requirements and warranties.
2. Products. The equipment and materials, with ratings, standards and acceptable manufacturers.
3. Execution. How the work is installed, tested and commissioned.

Specifications matter most when something goes wrong. If the drawings show a panel but the specification says it must have copper bus and a certain short-circuit rating, the specification is what makes that requirement enforceable. The contract usually sets out which document governs if the drawings and specifications disagree.

How the documents change through a project

During construction the contractor submits shop drawings and product data for review, asks questions through requests for information, and records changes. At the end, those changes are captured in record drawings. Insist on getting them, in an editable format as well as PDF; they are the starting point for every future project.

Questions to ask at design review

Owners do not need to check calculations, but a review meeting is the cheapest point to raise operational issues. Useful questions include:

• How much spare capacity and how many spare breaker positions will each panel and the switchboard have?
• Are electrical rooms large enough for future equipment, with working space kept clear of other services?
• Which loads are on emergency or standby power, and does that match how we run the building?
• Are equipment names consistent with our existing naming system?
• Can lighting controls be adjusted by our staff, and is the sequence written down?
• How will shutdowns be phased if the building stays occupied?
• What record drawings and operating information will we receive at the end?

A typical case: planning a renovation from old drawings

A property manager wants to know whether a vacant floor can take a new tenant with a lab. The original drawings are twenty years old. Reading them shows two panels on the floor, fed from a transformer in the electrical room, and the panel schedules list a handful of spares. But the floor has had two renovations since, and nobody has record drawings for them. The sensible next step is a site survey to compare the panels and breakers with the old schedules before relying on the spare capacity they show. The drawings tell you where to look; the survey tells you what is there. Once the survey is done, the updated drawings become the new base for the tenant's design, and they should be kept current from then on so the next manager does not have to repeat the exercise.

How ehZee can help

We prepare electrical drawings and specifications for new work and renovations, sealed for permit where required, and can review existing documents against what is installed. See our electrical design service, and read single-line diagrams explained and electrical design for tenant renovations.

Common questions