Electrical Contract Works · Question

What is an electrical load calculation and why is it done first?

Short answer

An electrical load calculation is the designer's estimate of how much electrical demand a building or plant will place on its supply, built up from every light, socket, motor and machine. It is done first because cables, panels, protective devices and the supply request all take their size from that estimate. Without it, every later choice is a guess.

In more detail

Think of the calculation as the brief for the whole electrical design. A qualified electrical designer lists each item of equipment, notes what it draws when running, and then considers how much of it will realistically run at the same time. That second step matters, because adding up every nameplate rating usually overstates what the building will ever demand together.

The result drives several decisions that are expensive to reverse:

the capacity and type of supply you request from the supply authority
the size of incoming cables, main panels and distribution boards
the rating of protective devices in each circuit
whether a dedicated transformer or backup source is worth planning for

Cable routes, shafts and panel rooms are usually fixed during construction. If the load estimate arrives late and turns out larger than assumed, the owner may face wider trenches, a bigger room or a supply upgrade after walls are closed. Starting with the estimate lets the architect and civil team leave room in the right places.

A good estimate also leaves sensible headroom for growth, since a workshop that adds one machine next year should not need the whole installation rebuilt.

A small packaging unit

A packaging unit lists its sealing machines, compressors, lighting and office equipment. The designer finds that the compressors and sealers start together every morning, so the estimate reflects that peak rather than a loose average. The supply request and panel sizing follow from this figure.

The designer, not the owner, applies the method and the applicable standards. Your role is to supply an honest equipment list and expected growth.

Key points

The calculation turns a list of equipment into a demand figure that guides the whole design.
Designers consider which equipment runs together, not only the sum of ratings.
Supply requests, cables, panels and protective devices are sized from this estimate.
Doing it late can force costly changes to rooms, trenches and supply arrangements.
A qualified designer prepares it; owners provide equipment lists and growth plans.

Who this applies to

Owners planning a new factory, workshop, warehouse or office building.
Promoters adding machinery to an existing unit.
Project managers comparing electrical quotations that assume different demand.

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.