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Shortlec — Sistemas Elétricos

How is an electrical switchboard sized?

Sizing a switchboard is not picking a box and filling it with breakers. It is translating the project’s scheme into concrete decisions — which gear, which busbars, which enclosure — so the board meets the standard, withstands the reality of the network, and is easy to install and use. This guide explains how a panel builder does that work.

The starting point: the project’s scheme

Everything starts from the single-line diagram or the specification — that is where the circuits, currents and requirements the board must serve live — and which type of switchboard it is. The panel builder sizes the board from that scheme; it does not replace it. If you do not have a design yet, a list of outgoing circuits (how many, what currents) and where the board will be installed are enough to start the conversation.

What gets sized in a board

  • The gear — protections, contactors and the rest, circuit by circuit, with the right technical characteristics (the rated current is not enough — see why “identical” gear is not identical);
  • The busbars and connections — sized for the board’s current and for heating in continuous operation;
  • The enclosure — dimensions, material and ingress-protection (IP) rating suited to the environment, with room for the gear to breathe;
  • The spare capacity — space and capacity left free for the circuits yet to come. A board without spare capacity becomes too small at the first plant upgrade.

A simple example

Picture an industrial hall fed at 630 A from the transformer station, with four motors, three sub-boards and the lighting. Sizing the main board follows these steps (the numbers change from project to project; the method does not):

  • Incomer — a 630 A main disconnect and busbars sized for that current in continuous operation;
  • Outgoing circuits — one protection per circuit, sized to the current and characteristics of each load: motors with protection suited to their starting, sub-boards and lighting with distribution protections;
  • Spare capacity — two or three free outgoing ways and around 20% of free space in the enclosure, for the extensions to come;
  • Enclosure — dimensions that fit the gear with room for cable connections and heat dissipation, with the ingress protection suited to the site;
  • Check — with everything defined, we present the proposal and validate the configuration with you before manufacturing.

The project as a whole — installation and use

This is what separates a well-sized board from a merely compliant one: at Shortlec, we think about the project as a whole, not just the supply of the board.

When sizing, we consider who will install it: access and room to connect the cables, cable entries positioned for the site’s real routing, a gear layout that makes the electrician’s work easier. And we consider who will use it: clear organisation, complete labelling of circuits and protections, and a board logic that keeps day-to-day operation and maintenance simple — years after the board leaves our workshop.

A board that arrives on site easy to install saves electrician hours; a board that is easy to read saves downtime and mistakes for its whole life. It is a cost invisible in the quote — and enormous in practice.

Validation before manufacturing

With the sizing done, we present the technical and commercial proposal — scope, gear, lead time and price — and validate the configuration with you before moving forward. Only then does the board go to manufacturing, wiring and testing to IEC 61439, delivered with a test report and CE marking.

Frequently asked questions

Do I need the electrical design done first?

Ideally the designer’s single-line diagram exists — it is the basis of the sizing. Without it, a list of outgoing circuits and the installation location are enough to start; the board is always sized to what the project requires.

What is “spare capacity” in a switchboard?

It is the space and capacity left free for future extensions — more circuits, more protections. It costs little at manufacturing time and avoids replacing the board at the first expansion. In industrial environments it is one of the best decisions you can make.

Is a bigger board always better?

No. The right amount of spare capacity is an investment; oversizing everything is paying more for no real gain. The balance comes from looking at the board’s real use and growth plans — exactly the conversation we have with you before manufacturing.