16A on Ring

There’s potentially a lot to consider here, I would really be looking at installing a new dedicated circuit, rather than hooking it into a socket ring circuit.


It is possibly not the best job for you to take on.


Andy B.

There is no such thing as a 16A RCD. RCDs detect leakage (not overload) and are commonly 30mA. I guess you are thinking of a 16A MCB. But no you cannot spur off a ring with a 16A MCB. The largest spur you can take from the ring is a 13A FCU.


A new 16A circuit is required from the CU whether it is long and messy or not. New circuits are notifiable to Building Control at your local council. You have to notify before you start and pay a (hefty) fee. In practice it often works out cheaper and less hassle to employ a sparks who can self notify.

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.

it might be possible to split the "lower socket ring" into two lower rated radial circuits, and create a dedicated supply to the heat pump by making the break at the nearest socket outlet to the CU on one leg of the original ring. Although I agree with all the above replies, need serious thought and both new circuits would be notifiable.

Thanks for all the replies


Given its now been removed from the latest editions, and doing so would result in none standard installation, I will look at ways to run a dedicated circuit.


Ive been looking at the specs of the clients heat pump. As mentioed, they said they had been running it on a 13A plug for some time. There doesnt seem to be any overloading signs at the plug terminal for heat, although it was midly warm to the touch when I arrived to have a look (they still had it running)


Under the specs of the P12 given in the link below, it states a running current of 7.8A, and reccommends a slow fuse of 15A. I assume this is to account for surges during the compressor pump motor / fan motor start up.

https://www.automatedenvironmentalsystems.co.uk/wp-content/uploads/2019/03/AES-Hydro-Pro-P-Type-Specifications.pdf


So if its running current is 7.8A, and any induction motor surges would probably be short enough in time to not blow a 13a BS1362 fuse, Could this work off a 13a FCU spur?

A load of  a couple of kw on a ring is much more bearable, the only question is the inrush. If it has been running on the 13A fuse in a plug for a while, then a 13A spur ought to be man enough, though I'd avoid mounting it near a heater or in a poorly ventilated cupboard (13A spurs in airing cupboards seem to be prone to cooking over time, though this is a less onerous case, the risk is similar.)

I terms of surge rating, a 13A  fuse is often less sensitive than a 16A MCB.

Even so, please ask yourself how loaded the ring currently is, and will there be enough spare capacity for the other loads in the house.

I should have added that in my last post. From what I have looked at the ring feeds a lounge area, and two sockets near the CU. Lounge area has 7 Double sockets. Not much plugged in to them, TV and entertainment consoles, Lamps, etc. One feeds a pond, I took a note of as much equipment running the pond and work it out to roughly 700w. The two sockets near the drop down to the CU are just powering a wifi modem and charging one of those fancy robot vac thingys. So current occupied is probably around 1.2kw. Rooms are centrally heated and they have a log fire so I do foresee the use of filiment heaters ect (Client said they do not heat the pool during the winter months so that load would be off anyway)


I wasnt aware of spurs cooking with lack of ventilation, will note that.

Maybe I could spur through to a IP66 FCU on the outside wall, and run the SWA from there?

Given its now been removed from the latest editions, and doing so would result in none standard installation

To be accurate, while the regs no longer offer it as a 'deemed to comply' solution, that's still a long way from being a non-standard (as in doesn't comply with BS 7671) installation. I think the worst you can say is non-conventional. Much of the "rules" for ring circuits have been taken out from the normative parts BS 7671 over the years and now only exist as general guidance. It's likely a 16A MCB fed from as ring could still comply with all of BS 7671's requirements, just you'd have to do a bit of extra work to prove it.


I wouldn't want anyone to get the idea that a 16A MCB fed from a ring is an instant non-compliance with BS 7671 for instance - I know of one or two installations that have (or had) just such an arrangement.


  - Andy.

Given that you do know where to look and what to read, I think you will be best leaving this job alone.


Andy B

Starr with the OSG


So how many litres are in the pool?