electrical academy · lesson 4 · about 12 minutes
Wiring inside a switchboard
Cables inside a panel sit in warmer, tighter spaces than cables in a room. This lesson covers how to size and fit them.
Learning material, not a design or safety instruction. Electrical work must be done by qualified people following the maker's instructions and local rules.
Size for the hottest place
- Inside a closed panel, the air can be well above room temperature. Planning rules often assume about 35 °C for the panel and as much as 55 °C where the wiring runs.
- Choose the cable cross-section for the hotter figure, not the room.
- If the room is warmer than the rule assumes, reduce the allowed heat loss to stay inside the limit.
Pick the right cable
Use heat-resistant insulation rated for high conductor temperatures. Halogen-free types give less smoke if there is a fire.
Thin conductors are usually laid in ducts and bundles. Heavier ones are laid in air, in small bundles.
Protect against two different problems
- Overload is a slow rise in current. The protection must match the cable's current rating.
- Short circuit is a sudden, very large current. The cable must survive until the protective device clears it, so check how long that takes.
- Where a device limits the fault current, the section between busbar and device can be sized for the smaller load.
Hold short wiring firmly
In a short circuit, conductors push apart with great force. Keep short runs tight, use cable supports, or run them in plastic ducts.
check yourself
Three quick questions
Think of your answer first, then open the question.
Which temperature should you size inside-panel wiring for?
The warmer temperature inside the device compartment, not the room.
What are the two faults wiring must be protected against?
Overload and short circuit.
Why support short wiring?
Short-circuit forces push conductors apart.
Learning material, not a design or safety instruction. Electrical work must be done by qualified people following the maker's instructions and local rules.