Warm wires

[Brian]

Following on from the "Terminal blocks" posting yesterday, I've now removed the terminal blocks and soldered the wire joins.

I thought I'd better test everything so decided to turn on/plug in everything I owned. Thus the dishwasher, washing machine, kettle, oven (yup - into the ring main, courtesy of the kitchen fitters), fan heater, halogen heater and everything else was operating at the same time. The electricity meter was spinning around pretty quick.

Well, I'm still here, so I guess it all worked fine.

One slight problem I thought I'd raise... Because some of the ring main cable is exposed, I thought I would feel this to see if it was getting warm. I also felt my new joints (which are safely wrapped in electrical tape).

One of the cables felt warm (the solder joint part seemed fine). Only very slightly warm, to the extent that I could be imagining it, but it was certainly warmer than the other ring main cable next to it. BUT! The warm-ish cable came from the socket that the fan heater was plugged into. I ought to point out that the mains cable leading out of the back of the fan heater gets warm too. I don't know what its rating is - 2KW?

I THINK I'm right in saying that there's nothing to be worried about here. But I thought I'd better check the experts of this group who have been so helpful so far.

[bigcat]

getting warm.

cable will get warm if you put enough currents through it. PVC cable is rated to 60 or 70C (?), so if it burnt you its too hot.

NT

[John Rumm]

Nothing to worry about. You could in some cases have a cable sitting there at 60 degrees C and it still be considered safe and within designed limits.

Circuit design is based on the concept of a temperature "budget". The top limit is set by what the cable insulation will stand (70 deg C for PVC, 90 for XLPE and some LSOH cables).

Things like anticipated ambient temperatures (general and local to the cable), its length (i.e. its resistance), proximity to other cables and to thermal insulation etc, all eat into that budget. What remains dictates the maximum amount of current the circuit can carry (and hence the power it can dissipate as heat generated by the current flowing through the cable resistance).

An allowance is also made for "diversity" i.e. the assumption that the situation you have artificially created with all appliances on at once is very unlikely to happen in reality - so a certain amount of theoretical overloading is permitted.

[Dave Liquorice]

Was that "other ring main cable" part of the same ring with the warm cable or part of another ring? Both sides of the same ring getting marginally warm under fairly hefty loads like you applied I wouldn't be too concerned about. A noticeable difference in temp I'd be a bit more worried about and start looking for a break in the ring, though it depends a bit on where the load is applied to the ring. In the middle the load will be evenly distributed on the two halves, right at one end the short leg will supply more of the current and thus have more I^2R losses related to the resistance difference between the long and short paths.