Advice - First Fix Ring main or Radial?

[ericmark]

You always quote these lengths but how high is a tree compared with the combined length of its branches?

It comes from a IET seminar when the percent volt drop changed from 4% to 5% and 3%, and I had tried to work out where the figures had come from, seem to remember 86 meters to 106 meters, but not re-worked out figures. What I was told was it is assumed a load of 20 amps at centre (ring) or end (radial) of the cable run, with the rest considered as even spread, so the "Design current for circuit Ib" for a ring is 26 amps, and for a 20 amp radial 20 amps. This is why we liked the ring final, as it covers such a large area.

But only you can work out best options.

 

[EFLImpudence]

But only you can work out best options.

Exactly.

 

[ebee]

You can of course install whatever you want, but the point is to actually consider what is appropriate for the installation, rather than just slinging in 'standard' circuits because some guidance book that hasn't been updated in multiple decades said it was the thing to do.


Either. The only modern adjustment is to use 3C&E between the light and the switch so that the neutral is available at the switch, and the light has switched and permanent line available at it.

Although Flameport and I have opposing views for Ring Final Circuits in general I think we can both agree with this part of his statement.
My own opinion is that it a "Horses for Courses" sort of thing and depending upon all circumstances either a Ring or a Radial might be the most appropriate approach. Each has their own advantages/disadvatages over the other too.

What do you want to achieve and how do you achieve it, and also what might you want to achieve at some forseeable future date and how might that affect your decision and how likely is that to change.
If you consider all round it greatly exceeds 50/50 all told then that might nudge you in that decision.
You might consider that both have similar merits.
There is no absolute One Way Suits All, very often.

 

[Harry Bloomfield]

Upstairs ring main.
Downstairs ring main.
Kitchen ring maim.
Plus separate feed for boiler.

Upstairs lighting
Downstairs lighting.

That makes it x6 RCBOs.

Good choice, that allows you to make sensible use of electric heaters, should the need arise. I would suggest at least doubling the number of RCBO's, to allow for additional circuits, such as boiler, burgler alarm, fire alarms, door-bell, immersion etc.. Redundancy in circuits, means faults will be easier to identify. Consider putting fridges and freezers, on their own, separate circuit, to reduce chances of tripping.

My original CU was a 10way, with an additional 10way fitted in the garage for outdoor circuits. I ended up swapping the 10way main CU, to a 22way.

 

[JohnW2]

A 20A radial is only enough for one full-sized load (e.g. kettle or electric heater) plus smalls. As such if I was wring my house with 20A radials. I would want one circuit per habitable room, plus one for kitchen countertop and fridge/freezer one each for washer/drier/dishwasher. That adds up quickly, I reckon if having a house wired with 20A radials I'd want ten or so socket circuits, compared to the typical three with 32A circuits.

Quite so, but it is essentially all down to personal opinions/preferences.

As I often say, whilst I'm very happy with 32A radials (or rings), I'm much less happy with 20A ones, particularly in kitchens and workshops. In a kitchen, I would not be very happy with a 20A circuit for the 'countertop', given that many kitchens these days have multiple 'plug-in' items that each may consume 2-3 kW. For the same reason, I would not be very happy with 2 (or more) 'countertop' 20A circuits, since, even if they were 'labelled' most domestic users would not understand about spreading loads appropriately between circuits.

2.5mm² 25A radials are sometimes possible (depending on availability of MCB/RCBO and cable installation method), and they are a bit better than 20A, being more-or-less able to cope with 2 x 3kW or 3 x 2kW loads.

32A radials require at least 4mm² and depending on installation method can easily require 6mm². The latter in particular as well as being expensive is likely to make termination very annoying.

As above, I'm very happy with 32A radials but, for the reasons you give, I would probably steer away from installing them if installation method was such that 4mm² T+E was not adequate. 4mm² is fine - in fact I personally prefer working with it than 2.5mm² T+E.

However, again as above, so much of this is down to personal views/opinions and preferences - and I certainly agree with you (hence disagree with {predictable!} others ) in saying that those who say that ring finals "are no longer a thing" are merely expressing their own opinions, rather than 'asserting facts' (which many readers might think they are doing)!

 

[JohnW2]

These days, not running/having a neutral at a light switch is silly imo in the age of smart light swicthes and IoT stuff in general, hence I would allways loop at the switch and not the rose myself.

If one wants to maximise 'versatility' ('future proofing'?), there's no reason why one can't do 'both' (and I often do these days) - i.e. 'loop at rose' AND ALSO run a neutral from rose to switches.

 

[Harry Bloomfield]

As I often say, whilst I'm very happy with 32A radials (or rings), I'm much less happy with 20A ones, particularly in kitchens and workshops.

I installed 3x 16amp radial circuits in my garage/workshop, The main reason for which was to allow circuits to be easily switched off, as I exited, but still able to leave one on if necessary - maybe charging batteries.

 

[JohnW2]

Technically you can have 106 meters of 2.5 mm² with a ring final, and 32 meters of 2.5 mm² with a 20 amp radial, so to replace one ring final may need 3 radials, so with the cost of RCBO's it is often cheaper to use ring finals. But it is all down to the amount of cable you will be using.

You are always quoting those figures, which derive from guidance regarding voltage drop, but, as I always say, I think that readers need to understand:

• The voltage drop figures on which that is based are merely 'guidance'. The only regulatory requirement is that the VD should not be so great as to "impair the safe functioning of loads" - and I struggle to think of any load for which that would be the case, with any amount of VD.

• The VD guidance figures relate to the 'worst possible situation'. In the case of a 20A radial, that means having an entire 20A load all at the furthest point of the circuit - something which is almost never going to happen.

• The difference between 106m and 32m sounds dramatic, but one has to remember that 'a ring is a ring' - so the furthest a socket can possibly be on a ring final is 53m from the CU, which is much closer to the situation with a radial than is 106m!

• In the average domestic property, sockets circuits are often not all that long, probably often all less than 32m. The longest ones probably exist in bungalows in which all sockets are fed by drops from above.​

One obviously also has to consider Zs (which IS a regulatory requirement) and that will be appreciably higher for a radial, but that is a different matter and results in different 'maximum lengths' from the VD-guideline-related ones you always quote. However, in that case, one cannot offer any 'general' figures, since the Zs of a circuit will depend upon Ze as well as on the circuit itself.

 

[JohnW2]

I installed 3x 16amp radial circuits in my garage/workshop, The main reason for which was to allow circuits to be easily switched off, as I exited, but still able to leave one on if necessary - maybe charging batteries.

Fair enough. I would personally be totally happy with a 16A circuit which supplied just one single socket, and pretty comfortable with a 16A circuit supplying just one double socket (which options would facilitate your 'switching off') However, as said, I would be less comfortable with more sockets, particularly in a kitchen.

 

[ericmark]

I struggle to think of any load for which that would be the case

Fluorescent lights, and refrigeration when not using HF ballast or inverter drives.

 

[JohnW2]

Fluorescent lights, and refrigeration when not using HF ballast or inverter drives.

Most people don't seem to have had the same problems as you in relation to either of those things but, even if we had had, I don't see what is 'unsafe' (I presume that BS7671 means "electrically unsafe") about a fluorescent light or refrigeration appliance not working (or not working properly)?