Choosing Protection Levels and Construction for Distribution Boxes

A distribution box is judged by what it keeps out and what it lets out. It has to keep water and dust away from live parts, let heat out of a space that is often too small for the load, and give a person a safe way to work inside it. Those three requirements pull against each other, and the box is where the compromise is designed.

Three Requirements That Pull Against Each Other

The design task is to find the version of all four that the site can live with.

Choosing the Protection Level

• Start from the environment, not from a number: indoor dry, indoor damp, outdoor sheltered, outdoor exposed, washdown.
• Remember that condensation can be a bigger problem than rain in a damp, unheated room.
• Account for the fittings: glands, vents, filters and door gaskets all affect the achieved class.
• Choose door gaskets and hinges that can keep the class over years of opening, not just at the factory.
• Where a vent is needed, protect it rather than removing it.

Indoor and Outdoor Installations

• Indoor, dry: the simplest box, provided the load and the access suit.
• Indoor, damp or unheated: expect condensation, and design for it.
• Outdoor, sheltered: weather protection on the exposed faces and a sensible entry.
• Outdoor, exposed: full weather protection, drainage, UV-stable finish and a raised entry.
• Washdown or wet process: material and finish chosen for the cleaning regime.

Door Types, Access and Stiffness

• Choose the door type from the board width and the frequency of access.
• A wide door needs stiffening, or it sags and seals badly.
• Provide the door opening angle the site needs, and remember that a wall can limit it.
• Provide locks and handles that suit the security and the gloves of the people who use it.
• Consider a removable centre section where a double door would otherwise be needed.

Cable Entries, Glands and Routing

• Provide a removable entry plate, sized for the cables plus spare.
• Prefer bottom entry outdoors, with a raised entry or a drip loop.
• Keep the cable route clear of terminals and of the door swing.
• Provide strain relief and keep cable weight off the terminals.
• Seal or brush the entry where dust or draughts matter.

Temperature Rise and Heat Dissipation

• Estimate the heat from the components at the actual load, not the nameplate maximum.
• Remember that a sealed box loses heat only through its surface, and that mounting against a warm wall reduces that.
• Use ventilation, a larger enclosure or a different mounting where the margin is thin.
• Keep the hottest devices low, where cooler air arrives.
• Where a vent is used, document the filter interval and make the filter accessible.

Earth and Neutral Bars, and Internal Metalwork

• Provide earth and neutral bar mounting as the design requires, with the bars accessible and labelled.
• Bond the door and removable panels with dedicated conductors.
• Mask bonding faces before coating.
• Keep the internal metalwork arranged so that live parts are not within reach when the door is open.
• Provide barriers between sections where the design calls for them.

Mounting and Installation Interface

• Provide a fixing pattern that suits the wall or the structure, and record it on the drawing.
• Allow for cable and conduit entry from the direction the installation actually uses.
• Consider weight when loaded, because a distribution box is heavier than its empty weight suggests.
• Provide clearances for the door and for working space.
• Keep the mounting points accessible for maintenance.

Checks on the Build

How We Approach Distribution Boxes

• We fabricate bodies, doors, entry plates, mounting plates, internal barriers and brackets in steel, stainless and aluminium.
• We hold the openings, entries, bar mounting and bonding points to the arrangement drawings you supply.
• We will tell you when a requested protection class, a requested internal layout and a requested thermal arrangement cannot all be met at once.
• We do not assemble or test the electrical components, we do not carry out the electrical design, and we do not certify the board to any standard.
• Every quotation states which operations are completed in our plant.

Send the environment, the board layout and the access requirement, and the box can be built to keep the weather out, let the heat out and let a person work safely inside it.

Frequently Asked Questions