Thermal Emissivity Explained: Designing Cooler, More Efficient Switchboards
Select Switchboards │ Last Updated: 01/08/2025
When it comes to keeping a switchboard cool, most people immediately think of airflow: fans, vents, and grilles.
However, one of the most effective cooling methods, both functionally and financially, to manage the temperature of your switchboard is to choose the right enclosure surface material.
Materials shed heat by infrared radiation in a process called thermal emissivity.
Different materials have drastically different levels of thermal emissivity, which is determined by the material type, surface finish, and coatings.
For switchboard enclosures, you can also help increase your switchboard's thermal emissivity through the use of add-ons such as sun shields.
Selecting the best enclosure material for your switchboard can be the difference between a derating sticker and a worry-free installation that meets AS/NZS 61439 temperature-rise requirements without oversized busbars or forced ventilation.
At Select Switchboards, our expert switchboard engineers have created this blog to help understand the role of thermal emissivity in switchboard performance and how to build smarter, more efficient boards from the outside in.
What Is Thermal Emissivity and Why Does It Matter?
Emissivity (ε) is the ratio of heat a real surface radiates compared to an ideal black body (ε = 1).
According to the Stefan-Boltzmann law, radiated heat increases in proportion to ε × T⁴—meaning that doubling the emissivity nearly doubles the rate of radiative cooling at a given temperature.
In sealed switchboards, where airflow and convection are limited, radiation can account for 30–50% of total heat rejection.
This is even more important in outdoor switchboards that are exposed to direct sunlight, as emissivity becomes even more critical due to additional solar gain.
AS/NZS 61439 Temperature Rise Standards
Clause 10.10 of the AS/NZS 61439 standard requires verification of temperature rise within the switchboard assembly.
This ensures that all components operate safely and stay within their designated thermal limits under load. Critical for preventing overheating, prolonging component lifespan, and ensuring reliable performance.
Designing a switchboard enclosure with a high surface emissivity allows you to stay within compliance limits, without the need for forced cooling, oversized busbars, or costly ventilation systems.
This makes emissivity not only an engineering advantage by ensuring compliance, but also a practical and cost-saving strategy in comparison with designs that use powered cooling systems.
Enclosure Material Matters: Emissivity by Base Material
The choice of metal for your switchboard enclosure makes a big difference. Here’s how common materials compare:
Aluminium Sheet
Surface: Bright rolled / commercial
Emissivity (ε): 0.04 – 0.09
Takeaway: Very low emissivity. Acts like a heat mirror. Poor at radiating internal heat unless anodised or painted.
Mild Steel
Surface: Cold-rolled
Emissivity (ε): 0.20 – 0.32
Takeaway: Medium emissivity. Even a thin primer improves heat dissipation.
Stainless Steel (304/316)
Surface: Polished (new), weathered
Emissivity (ε): 0.07 – 0.10 (new), ~0.85 (weathered)
Takeaway: Very reflective when new, but emissivity improves significantly over time or with brush finish.
Important Note:
Wall thickness (1.6–3 mm typical) does not affect emissivity directly, but a thicker plate stores and conducts more heat. This promotes an even surface temperature and reduces any localised hotspots. Altering the wall thickness can be an important consideration when designing a switchboard that is near the limits of AS/NZS 61439 temperature-rise compliance.
Enclosure Coatings and Finishes That Improve Cooling
Surface treatments can turn low-emissivity metals into effective radiators:
Black Epoxy or Polyester Powder Coat
Typical ε: 0.89 – 0.96
Benefits: Offers the highest radiative cooling. However, it also absorbs solar heat, best suited for indoor switchboards or shaded outdoor areas.
Light-Grey RAL 7035 (Common OEM Colour)
Typical ε: ≈0.87 - Solar absorption: ≈0.50
Benefits: A solid all-rounder—radiates heat effectively while keeping solar gain to a moderate level.
High-Reflectivity White Polyurethane
Typical ε: ≈0.97 Solar absorption: ≈0.14
Benefits: Excellent for hot, sunny conditions. Combines very high emissivity with minimal heat absorption.
Clear Anodising on Aluminium
Typical ε: ≈0.77
Benefits: Increases emissivity while maintaining the clean, metallic aesthetic of aluminium.
Bright Zinc Plating (Galvanised New)
Typical ε: ≈0.23
Benefits: Low emissivity—ineffective for radiative cooling. Apply a top coat if the enclosure dissipates more than 200 W/m².
Important Tip:
Gloss level matters. Using a high-gloss finish can lower emissivity by up to 10% which makes using a matte or textured powder coat finish a quick win to improve enclosure emissivity.
Beating the Sun: Solar Gain Mitigation Tactics
In Australia, outdoor switchboards in tropical and arid zones can face an additional 80–120 W/m² solar load. Here are two low-cost solutions to manage thermal emissivity in these conditions:
Sun-Shields
Using double-skin panels or clip-on awnings creates a ventilated air gap that both reflects short-wave solar radiation and radiates remaining heat back to ambient. Testing on a 3.5 m² enclosure showed:
- Top shield only: 25% internal temperature reduction
- Top + side shields: 46% reduction
Orientation and Colour
A white roof panel over a darker body blocks the high-noon component, cutting peak internal temperatures by about 8 °C in Australian summer tests.
Installation Tip:
Maintain a 20–25 mm stand-off air gap between the shield and enclosure surface for maximum effect.
Design Checklist for Passive Thermal Performance
Quantify Heat
- Sum internal power losses (in watts) plus estimated site solar gain (use 90 W/m² as a worst-case figure).
- If the total heat load per enclosure surface area is ≤150 W/m², passive cooling measures usually suffice.
Pick a Base Material
- Balance weight, corrosion resistance, and cost.
- Aluminium is a good option for coastal locations, but it must be anodised or coated to improve emissivity.
Select Surface Finish
- Indoor: any high-emissivity (high-ε) paint
- Outdoor: use light-coloured finishes or white over primer - Marine environments: 316 stainless steel, brushed or painted
Address Thickness and Structure
- Thicker panels act as thermal mass, distributing heat more evenly.
- However, the benefit diminishes beyond 3 mm thickness.
Add Enhancements
- Plan early for passive features like solar shields, ventilated roof panels, or louvres to improve cooling performance.
Verify Compliance
- For assemblies rated ≤1600 A, you can often use the calculation method to verify temperature-rise compliance, provided you have documented emissivity data for your chosen finish.
Effective Switchboard Enclosure Design: Cooling a Board in Katherine, NT
Here’s how switchboard design choices make a real difference:
Scenario:
- 200 W of devices inside a 1200 × 800 × 300 mm (2.9 m² surface) outdoor board
- Summer ambient temperature: 45 °C
- No shading
Step-by-Step:
- Solar load: 90 W/m² × 2.9 = 261 W
- Total heat: 200 + 261 = 461 W
- Paint choice: Switching from bare aluminium (ε ≈0.05) to RAL 7035 (ε ≈0.87) increases radiative loss by about 17×, cutting steady-state internal temperature by ~11 °C.
- Adding top and side sun-shields: Drops solar gain by 46%, reducing total load to 341 W.
- Result: Meets AS/NZS 61439 limits without forced cooling.
Need Help Designing a Cooler, Compliant Switchboard?
At Select Switchboards, we design and manufacture custom switchboards built to thrive in Australia’s harshest conditions. From material specification to factory-finished coatings and integrated sun shields, our expert team helps you achieve AS/NZS 61439 compliance and cost-efficient performance without unnecessary over-engineering.
Whether you're looking to Design A New Switchboard System, Upgrade An Existing Switchboard, or simply seek advice on temperature regulation and AS/NZS 61439 compliance for your next project, Get In Touch With Our Team At Select Switchboards Today!