Overheating Cable

[sdcoomber]

I have a 2.5 t/e feed from the CU to the immersion cupboard. The cable goes to a 13amp DP switch. An unswitched feed goes to a water pump which draws 2.5amps when in use. A switched feed goes to an immersion timer, then to the element via a junction box. The immersion is used daily for several hours. The water pump just when water in one bathroom is used.

The cable close to the element overheated and melted the insulation. Having replaced it, the same thing happened, so I double up 2 x 2.5 t/e cables from the element to the junction box and all seemed well.

I have had cause to change the element and today noticed that inside the DP switch, the cables have got hot. All insulation is now black! The connectors were all tight.

The cable length from the CU is less than 15 meters, the current draw from the element is 12.8 amps. It seems to defy logic asI understood a 2.5t/e should be good for 18/20 amps. Any ideas?

 

[electronicsuk]

Ok, a few observations. You say an unswitched feed goes to your water pump. Is this feed supplied via a socket or hardwired? If the latter, it should be connected through a Fused Connection Unit fitted with 3A fuse.

Secondly, you say you used T+E cable to replaced the damaged cable on the immersion heater. Twin & earth is not suitable for connection between the timer and element, you should be using butyl flex or some other form of heat resistant cable suitable for the job.

You also mention that you have cause to replace the element. Was it found to be faulty? If all terminations inside the DP switch were tight then there seems to be no good reason for the insulation inside the switch to have melted. This certainly suggests an overload, which could be due to a faulty element. Furthermore, it also suggests that you do not have an appropriate fuse/breaker protecting this circuit to prevent this type of overload.

The current handling capacity of 2.5mm^2 T+E depends on the installation method, do not simply assume it will take 18/20A.

 

[bernardgreen]

Heat from the element is conducted along into the copper of the cable.

The heat from the tank raises the ambient temperature.

The current in the hot copper adds more heat to the copper whose resistance has aready been increased by the conducted heat.

In short the cable must be heat resistant and of a size to take the required current when at 70 degrees C or higher.