timb
Statesman
Posts: 512
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Post by timb on Mar 24, 2019 21:38:23 GMT
Just for clarity, the current carying capacity for domestic twin and earth cable varies with installation type and the length of circuit. As you will not necessarily know the installation you will have to assume the worst case scenario. This would be for a single cable (spur) at 2.5mm square section fitted using method 103 (regs) ie in a stud wall with thermal insulaton with the cable NOT fully touching the wall (heatsink). The maximum current allowed is 13.5A for a 240VAC main, hence why all UK domestic plugs are rated at 13A max.
The reason for this limit is that the thermoplastic insulation on the cable is rated to 70deg C, if the cable is subjected to a current higher than 13.5A for a period of time without the necessary heatsink then it could exceed this temperature, the insulation will then break down causing a fire.
The MCB's role in the circuit is protect the cable, not the device. The MCB will be rated at 32A for most ring circuits with this stepping down to 20A for series spurs. The increased current is to allow a number of loads at different points in the circuit, all fused differently with a max of 13A per load (this can change depending on demand eg for a kitchen). This is known as diversification. As you can imagine if the 13A rule is disregarded then the MCB will not trip until a short is detected, long after a fire would have started in the above scenario.
You are right to be cautious!
Tim
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