Control Panel Enclosure Layout and Thermal Load: Back-Plate Planning and Cooling

Two decisions ruin more control panels than any others: the internal layout is drawn as if wiring takes no space, and the thermal load is estimated rather than calculated. Both are enclosure-side questions, and both are far cheaper to answer before the panel is built. This guide covers the layout rules and the way to approach the thermal numbers.

The Inputs a Panel Layout Needs

A layout drawn without the cable schedule and the dissipation figures is a sketch, not a design.

Back-Plate Layout and Spacing

• Place the largest and hottest devices first, and keep them low where the air is coolest in a naturally convecting panel.
• Leave spacing between heat-producing devices rather than grouping them into a hot corner.
• Provide space above and below the devices for the airflow path, not just around them.
• Keep devices that need front access within the door opening, not behind a structural member.
• Reserve a defined zone for future devices, because panels grow.
• Keep cable entry zones clear of devices, so the wiring can be routed rather than forced.

Wireways, Routing and Separation

• Draw the wireways at a realistic size and route them before placing the last devices.
• Allow for the bend radius of the largest conductor, which for a large power cable is greater than most layouts assume.
• Separate power and signal routing, and provide a barrier or a defined separation where the specification requires it.
• Provide a route for the protective conductor that does not run through signal space.
• Leave space for the ferrules, glands and terminals, which are frequently forgotten in the layout.
• Consider how the panel will be wired and tested; a layout that is dense everywhere is slow to assemble.

Approaching the Thermal Numbers

The calculation is the electrical designer's, but the enclosure side should be understood because it is where the passive capacity comes from.

• Sum the dissipation of all devices at worst case, not nominal.
• Add an allowance for power supplies, transformers and wiring losses.
• Estimate the passive dissipation of the enclosure, which improves with surface area, with a conductive material and with a larger temperature difference.
• Compare the two, and note the margin; a panel that is marginal in winter fails in summer.
• State the maximum internal temperature the devices allow, because that is the real limit.
• Decide whether the panel will be sealed or ventilated, because that decision changes the protection class.

Cooling Options Compared

The choice should follow the ambient and the environment first, and the load second, because the load can always be cooled if the air is available.

Condensation and Cold Starts

• Provide a thermostatically controlled heater where the panel is likely to fall below the dew point.
• Fit a pressure-equalisation element where a sealed panel experiences temperature cycles.
• Drain any cooling device so that condensate does not run into the panel.
• Consider start-up after a cold night, which is when condensation-related faults most often appear.
• Where a cooling unit is used, ensure it does not cool the panel below the dew point of the surrounding air.

Door-Mounted Devices and Cable Routing

• Keep door-mounted devices within a defined zone so the panel behind them stays usable.
• Provide a door bond and allow for the loom and its bend radius at the hinge.
• Keep heavy door-mounted devices off a lightly constructed door.
• Provide a door stop so that the door and the loom are not strained when opened fully.
• Where a display is fitted, set the viewing height and angle deliberately.

Documentation, Labels and As-Built Records

• Keep an as-built layout drawing, because the panel will be modified during its life.
• Label devices consistently with the drawing, and leave space for future labels.
• Record the cooling device type and its maintenance interval.
• Record the protection class and the assumptions behind it.
• Keep the enclosure dimensional record, particularly the mounting plate and the gland plate.

What to Give the Fabricator

1. Enclosure size, mounting type and depth. 2. Device layout with dimensions and mounting heights. 3. Dissipation figures and the required internal temperature. 4. Cable schedule with entry sizes and positions. 5. Environment, protection class and finish. 6. Cooling device selection, or the aperture and mounting face it needs. 7. Volumes and the delivery schedule.

Our Approach to Panel Layout

• We fabricate enclosures around a supplied layout, and we check the internal depth against the devices, the wiring space and the cooling provision.
• We cut the mounting plate pattern, the gland plate entries and the cooling apertures to the layout drawing, so the panel builder assembles rather than adapts.
• We will say when a proposed layout will not fit once realistic wiring space is allowed for.
• We do not calculate the electrical design, select the protective devices or certify the finished panel; that is the electrical designer's scope.
• Every quotation states which operations are completed in our plant.

Send the device layout, the cable schedule and the ambient conditions, and the enclosure can be built to fit the panel rather than the panel being fitted to the enclosure.

Frequently Asked Questions