Cables
Heat-Resistant Flex (3 or 5 cores)
In short
Heat-resistant flex is a flexible cable whose insulation stands more heat than ordinary flex.
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
What it is
HEAT-RESISTANT FLEX is a flexible cable whose insulation stands more heat than ordinary flex. Choose it where the part gets hot: an immersion heater, a boiler or a pump.
It is drawn black here, so you can tell it from ordinary flex. On site, read the print on the sheath: the colour of a sheath is not its rating.
On the canvas it comes short, with its cores loose at both ends.
- Take a core with the Wire Tool, and click a terminal inside its reach. When both ends of a core are on terminals, that core is a wire.
- The Wire Tool also drags an end or a bend by its handle, and drags the sheath to move the whole cable.
- Bend adds a bend.
- Erase lifts a core off its terminal, or cuts back one that is not needed.
What the meter reads
It reads and works as ordinary flex does: a core with both ends on terminals is a wire.
3 cores: brown and blue may typically be the live (L) and the neutral (N), and green-and-yellow is the earth (E). That is enough for an immersion heater or a pump.
5 cores: brown, black, grey, blue and green-and-yellow. That is enough for a boiler that takes a permanent live, a switched live (SL) and a pump live (PL).
A motorised valve comes with a heat-resistant flex of its own.
Where it connects
Lessons that use it
- An immersion heater on its own circuit
Intermediate · Protection and extras
Practise it in the simulator.
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