16A on Ring

I think you need to go back to the 15th edition to find 15A switch fuses or 16a MCBs being suggested as suitable for unregulated use  a ring final (perhpas as a way of keeping a  few of the old 15A round pin sockets), at about the same time this clause was removed, the advice for things like fixed electric fires and immersion heaters also changed to having a dedicated circuit recommended.

Of course under some conditions you can have a 16 A point load on  a ring, but to do so by design means it is not a standard circuit and you cannot fall back on 'no maths or thinking  needed' as you can if you use one of the pre-pack designs such as a 32A ring on 2.5mm or a 32A radial in 4mm as described in various annexes and the On Site Guide

So what are the correct design considerations?

Firstly there should be enough slack in the current load on the ring for the new additional load  not to be an issue - so a fully loaded ring with the washing machine and the tumble drier also on it would be likely to lead to overload. A heat pump is likely to run for some hours at a time, so really there is only 16A left for the rest of the circuit for much of the time - is that really enough ? (if you want to think about that, if you made the 32A breaker at the board into a 16A one, would it really never trip ?) This is not a short duration load like an oven that is likely to be turned off most of the time, or to ping on and off with a thermostat I suspect it will run night and day unless the pool is very small.

Secondly, and this is has become more of an issue as houses get better insulated,  and is also the reason for the slight changing of the reference designs over the years, if the load is not more or less half way round the ring, then the current share is unequal, and you need to be sure the cable  of the short leg will not be overloaded. A 2.5mm cable dangling in free air  or nailed to an uninsulated brick wall can carry 27A, and if this is how it is built then there is scope for surviving quite a significant current imbalance.

But if you route it in a partition or roof space with glass fibre wool or similar the maximum rating falls sharply. Indeed
Method 103: Surrounded by thermal insulation including in a stud wall with thermal insulation with cable not touching the wall.   suggests that the 2.5mm cable should be de-rated to about 13A, so not even enough for half a ring, even with absolutely perfect current balance between the two sides !! (it also suggests is is not enough for  2,5mm socket radial on 16A breaker either - but better routed a 2.5mm radial can be commonly up to 20A)

 Very few rings are perfectly balanced and hopefully very few are fully loaded in the few cases where the cable cooling is as poor as example 103. Those that are should probably be fitted with a lower rated breaker than the standard 32A.

How much do you really know about the ring you are proposing to tap into - its not your design, do you know the cable routes, the insulation, where there is grouping and of course the lengths to your proposed tap points.

Are you happy to become design authority ?  Even if you do, for ever inspection thereafter, every other sparks who sees it is going to worry it may not be quite enough and that may be a problem for the owner.


In summary, it may well work OK really, but would need some serious thought to be sure,  while a run of 2.5mm or 4mm back to the board is the dead cert safe answer.

I think you need to go back to the 15th edition to find 15A switch fuses or 16a MCBs being suggested as suitable for unregulated use  a ring final (perhpas as a way of keeping a  few of the old 15A round pin sockets), at about the same time this clause was removed, the advice for things like fixed electric fires and immersion heaters also changed to having a dedicated circuit recommended.

Of course under some conditions you can have a 16 A point load on  a ring, but to do so by design means it is not a standard circuit and you cannot fall back on 'no maths or thinking  needed' as you can if you use one of the pre-pack designs such as a 32A ring on 2.5mm or a 32A radial in 4mm as described in various annexes and the On Site Guide

So what are the correct design considerations?

Firstly there should be enough slack in the current load on the ring for the new additional load  not to be an issue - so a fully loaded ring with the washing machine and the tumble drier also on it would be likely to lead to overload. A heat pump is likely to run for some hours at a time, so really there is only 16A left for the rest of the circuit for much of the time - is that really enough ? (if you want to think about that, if you made the 32A breaker at the board into a 16A one, would it really never trip ?) This is not a short duration load like an oven that is likely to be turned off most of the time, or to ping on and off with a thermostat I suspect it will run night and day unless the pool is very small.

Secondly, and this is has become more of an issue as houses get better insulated,  and is also the reason for the slight changing of the reference designs over the years, if the load is not more or less half way round the ring, then the current share is unequal, and you need to be sure the cable  of the short leg will not be overloaded. A 2.5mm cable dangling in free air  or nailed to an uninsulated brick wall can carry 27A, and if this is how it is built then there is scope for surviving quite a significant current imbalance.

But if you route it in a partition or roof space with glass fibre wool or similar the maximum rating falls sharply. Indeed
Method 103: Surrounded by thermal insulation including in a stud wall with thermal insulation with cable not touching the wall.   suggests that the 2.5mm cable should be de-rated to about 13A, so not even enough for half a ring, even with absolutely perfect current balance between the two sides !! (it also suggests is is not enough for  2,5mm socket radial on 16A breaker either - but better routed a 2.5mm radial can be commonly up to 20A)

 Very few rings are perfectly balanced and hopefully very few are fully loaded in the few cases where the cable cooling is as poor as example 103. Those that are should probably be fitted with a lower rated breaker than the standard 32A.

How much do you really know about the ring you are proposing to tap into - its not your design, do you know the cable routes, the insulation, where there is grouping and of course the lengths to your proposed tap points.

Are you happy to become design authority ?  Even if you do, for ever inspection thereafter, every other sparks who sees it is going to worry it may not be quite enough and that may be a problem for the owner.


In summary, it may well work OK really, but would need some serious thought to be sure,  while a run of 2.5mm or 4mm back to the board is the dead cert safe answer.