Unbalanced final ring

[mainline]

it is best to run on leg 1 4mm cable to the cluster of high current drawing sockets and then 2.5mm for the rest.

The best way is to design it properly in the first place.

 

[Steve-S]

The best way is to design it properly in the first place.

What I wrote was quite clear.

 

[Pretty Mouth]

I also ran the numbers for another example ring, 101m long, 20A cable rating, 20A spot load at exactly 1m along one leg. I won't reproduce all of the working outs, but initially: 19.802A through the short leg, and once the cable had heated: 19.763A. Almost nothing in it.

 

[DefyG]

19.763A. Almost nothing in it.

Unless method 101 for example, where max ccc permitted for 2.5 t&e is 17Amps, it becomes an issue!

 

[Pretty Mouth]

Unless method 101 for example, where max ccc permitted for 2.5 t&e is 17Amps, it becomes an issue!

For clarity, my posts in this thread focus on the original question, regarding the effect cable heating may have on the flow of current through an unbalanced ring. My examples show how the current may be distributed through the circuit under certain unusual loading situations, before and after cable heating, and my last example shows that under the stated specific circumstances, cable heating has almost no effect on that.

 

[Steve-S]

What has alerted me, to some surprise, is how out of balance a final ring can be, which must be more typical in British homes than we think as heavy current drawing appliances are clustered. This was not a big problem in the 1960/70s. I measured 82% on one leg while Efixx measured 83%. Previously most I measured were 70-30 at most, giving 22.4 amps on one leg - just inside the max for the cable for the majority of installations. Fitting a ring with sockets in round robin (1st socket on leg 1, 2nd socket on leg 2, 3rd socket on leg 1, etc) will solve a balance issue, but this method can be quite impractical and a heavier in use of cable increasing costs.

This points us to that final rings should be all in 4mm, or 4mm on one leg to the clustered heavy current drawing sockets and then 2.5mm for the rest at least, for many reasons. In a normal house/flat I would lean to put as many appliances as possible on a full 4mm ring protected by an AFDD; diversity taken into account of course. A 3kW-ish oven & low current drawing induction hob on a 4mm radial and of course the immersion, if one on premises, on its own 2.5mm radial. Maybe more economical having the immersion on 4mm as final ring and oven/hob will be on 4mm so cable being cheaper all on the same purchased drum. At TLC a 100m drum of 4mm is £147 (£1.47 p/m), with 100m drum of 2.5mm £72 (£0.72 p/m), about twice the price, but £1.03p on a 25m drum. Costs of 4mm could drop in cable costs as only using one final ring in 4mm. Worth thinking about.