Electrical Contract Works · Question

What is a single-line diagram and why should I ask for one?

Short answer

A single-line diagram is a simplified drawing that shows how electricity flows through an installation, from the incoming supply through main panels and distribution boards to the circuits they feed, using one line to represent what is physically several conductors. You should ask for one because it is the clearest document for checking, comparing and later maintaining the design.

In more detail

The drawing leaves out wiring detail and instead shows the structure. Each panel, breaker, meter, transformer or backup source appears as a symbol, linked by a single line, with labels for ratings and what each branch serves. A reader can follow the path of power from the supply point down to a single machine.

What it tells you:

How the installation is divided into sections, and what switches off what.
Which protective devices guard each branch.
Where the supply authority's meter and any backup source connect.
How room has been left for future additions.

Asking for one is worthwhile for three reasons. First, it makes quotations comparable. If two contractors price from the same diagram, differences in price can be traced to specific items rather than hidden assumptions. Second, it helps the owner follow progress, since the finished panels can be checked against the drawing. Third, it becomes the reference for maintenance, modification and safety work years later, when the original team may be unavailable.

Ask for the as-built version too

Designs change during construction. Ask the contractor to update the diagram to match what was actually installed and hand it over with the test records.

The diagram is prepared and checked by a qualified designer, and the competent authority may need to see it with your application. You do not need to read every symbol. Ask the designer or contractor to walk you through it once, and keep a printed copy near the main panel and a digital copy in your records.

Key points

It is a simplified map of the electrical installation from supply point to final circuits.
It shows panels, protective devices, meters and how the system is divided.
Quotations built on the same diagram are far easier to compare.
Request an updated as-built copy at handover for future maintenance.
A qualified designer prepares it, and the contractor should explain it to you.

Who this applies to

Owners about to award an electrical contract for a new or expanded building.
Facility managers who will maintain the installation after handover.
Anyone comparing several electrical quotations for the same project.

Last reviewed 3 October 2026

This answer is general information, not advice on your particular case. Terms, eligibility, and requirements change, so check the current position with the relevant institution or authority.