Low-voltage electrical design

The Portuguese names appear on the documents you receive, so they are worth knowing.

• Memória descritiva e justificativa. The written description: type of supply, planned power, make-up of the switchboards, protection criteria, and the reasons for the choices made.
• Calculations. The power of each circuit, conductor sizes, ratings of the protective devices, voltage drops.
• Peças desenhadas (drawings). Floor plans showing switchboards, lighting, sockets and equipment, plus the switchboard diagrams. The single-line diagram (esquema unifilar) shows, on one line, the path of the energy from the intake to each circuit. The switchboard is built from it, and it is usual practice to leave a copy beside the board.
• Ficha eletrotécnica. A form on a model approved by DGEG that identifies the installation and summarises the planned power. E-REDES, the distribution network operator, asks for it in grid connection requests, including requests for more power addressed to it.
• Termo de responsabilidade pelo projeto. The statement, signed by the designer, taking responsibility for the design.

The rule is in article 5 of Decreto-Lei 96/2017. For installations supplied at low voltage, the common case is this: a design by a qualified designer is mandatory when the total power to be supplied from the grid, in the building, is above 10.35 kVA. That covers most buildings with several units, and houses or commercial spaces that need more than 10.35 kVA, which already means a three-phase supply. The law also requires a design, whatever the power, in venues for public shows, in locations with an explosion risk and in campsites and marinas. DGEG also notes that electrical installations covered by Decreto-Lei 101-D/2020, on the energy performance of buildings, likewise require a design.

The sum has its own rules: in residential buildings, for example, simultaneity factors are not applied. So we confirm which case applies at first contact, using the data of the installation.

Where the law does not require it, a design is still useful. In a refurbishment it is how you decide on paper, rather than in the wall, how many circuits are needed, whether the switchboard has room and whether the contracted power (potência contratada) is enough. If the works go through the town hall (câmara municipal), tell us what the process asked for.

We design low-voltage electrical installations for homes, condominiums and commercial spaces: new installations, refurbishments, changes of power and the supply to EV charging points. The design can be commissioned on its own, to be built by another installer, or together with the installation work.

The scope is low voltage. Installations supplied at medium or high voltage are outside this service.

• The floor plans, as PDF or DWG. If there are none, say so: a survey can be made.
• A list of the higher-power equipment: hob, oven, heat pump, air conditioning, water heater, EV charger.
• A recent electricity bill, if the installation exists, so we can see the contracted power.
• Photographs of the switchboard and the meter.
• Whatever the town hall, the architect or the builder asked for, if that applies.

In general terms, it starts with gathering data: what exists, what is wanted, which equipment will be connected. Sizing and drawing follow. Choices that affect the works or the bill (power to contract, single-phase or three-phase, number of switchboards) are discussed with you before the design is closed.

A complete, signed design: written description, calculations, drawings, ficha eletrotécnica and the design responsibility statement. The documents are written in Portuguese. The drawing on this page is an illustrative example of the kind of sheet we deliver.