In short. A switchboard’s current rating is a thermal limit, not just a conductor size. Heat from busbars, breakers and joints must escape through the enclosure, and ambient temperature, ventilation, grouping and altitude all reduce what can be carried. Short-circuit forces set the mechanical limit.
Sources of heat
- Busbar resistive losses, which grow with the square of current.
- Breakers, contactors and fuses, each with a power loss at rated current.
- Joints and terminations, where poor contact adds resistance.
- Harmonic currents, which add losses and skin effect.
- Solar gain or hot room air.
Factors that limit the rating
| Factor | Effect |
|---|---|
| Ambient temperature | Hotter air reduces allowable current |
| Altitude | Thinner air cools less effectively |
| Enclosure and IP rating | Higher IP limits ventilation |
| Diversity of loading | Fully loaded sections heat more than lightly loaded ones |
| Busbar arrangement | Orientation and spacing affect cooling |
| Harmonics | Higher effective current heating |
Short-circuit effects
During a fault, busbars experience large forces and heating. Supports, spacing and clamping must withstand the peak current and its duration. The short-time withstand rating applies to a particular time, such as one second, and the protection must clear the fault within that. See fault levels.
Joints and maintenance
- Loose or oxidised joints are the most common hot spots.
- Tighten to specified torque, use correct washers and plated surfaces.
- Use infrared thermography under load to find hot joints, with consistent loading and emissivity settings.
- Keep ventilation filters clean.
Design checks
- Establish rated current of the assembly at site ambient.
- Apply derating for temperature, altitude and enclosure.
- Check heat dissipation versus room cooling.
- Check short-time withstand against the clearing time.
- Plan room ventilation or air conditioning.
Illustrative example
A 2,000 A board rated at a standard ambient is installed in a generator room at a higher temperature. The usable current falls below the generator breaker rating. Options include improving room ventilation, selecting a higher-rated assembly or reducing the connected load. A thermographic scan later finds one joint hotter than its neighbours, which is retightened.
Why switchboard thermal design matters
Busbars, joints and breakers produce heat from I²R losses. Enclosure ventilation must remove it. Hyderabad and Secunderabad summers push ambient temperatures high, so a board sized for a cooler assumed ambient can overheat in a closed electrical room.
Heat loss = I² × R; temperature rise depends on losses, surface area and ventilation
Derating factors
| Factor | Effect |
|---|---|
| High ambient | Reduces allowable current |
| Enclosure IP rating | More sealing means less airflow |
| Grouping of devices | Higher local temperature |
| Harmonic current | Extra heating, particularly in the neutral |
Busbar selection
- Choose busbar size for continuous current at the design ambient.
- Check short-circuit withstand, both thermal and mechanical.
- Provide enough support spacing for fault forces.
- Consider the neutral size for harmonic-rich loads.
Joints and connections
Loose joints are a leading cause of hot spots. Use correct torque, check after thermal cycling and use thermography as part of routine maintenance.
Cooling of electrical rooms
Provide ventilation or air conditioning, filtered against dust, and keep out monsoon moisture. Review room temperature on peak days.
See ambient temperature derating and harmonics.
Common mistakes
- Using catalogue ratings without site derating.
- No thermal survey.
- Blocked ventilation.
What to send us
Send board ratings, site ambient and loading. Send your application details. See also ambient derating.
Questions engineers ask
Why does a board overheat below its rated current?
High ambient, poor ventilation, harmonics or loose joints.
What is thermography?
Infrared imaging that finds hot spots in energised equipment.
Does altitude matter?
Yes, above certain altitudes, cooling and insulation both reduce.
How often should joints be checked?
At commissioning and at planned maintenance intervals, with thermal scans under load.
This guide gives general principles. Ratings come from the assembly manufacturer.