Spur or extend the ring

[ericmark]

I always extend the ring on the basis it might need extending again at some point, e.g. in a kitchen. There's always the option of a spur if the ring is extended.

Blup

For an electrician that may be true, however DIY the plug in tester

with loop will measure down to 1.9Ω which is OK for even a 20 amp supply, but not 32 amp, so to extend a ring you need a loop impedance tester or low ohm meter with a short-circuit current of not less than 200 mA, which are not cheap, so although the electrician may extend the ring, DIY fused spur is better option as one unlikely to exceed either volt drop or earth loop impedance limits and also can get relatively cheap testers around £50 that can test it.

As to how to extend, I have found the LAP grid range good, either two independent sockets in one double socket face plate, or a socket switch and fuse in one double socket face plate, means either extending ring or adding a fuse can be done without need for crimped connections.

As to then adding more sockets, with fused spur that is not a problem.

As to radial the good point is less on each circuit, so any trip effects less items, but need three radials to replace a ring, and unless using RCBO's no advantage anyway, and if using RCBO's it gets expensive, specially if you need a new consumer unit to have so many circuits.

I do wonder if the people who sing the praises of radials actually measure or calculate the volt drop? 106 meters of 2.5 mm² or 40 meters of 4 mm² assuming Ib=26 amp. i.e. 20 amp at centre or end, and 12 amp even spread. OK ring because it returns likely longer so possibly only two radials for one ring, but drop to 2.5 mm² and 20 amp then only 32 meters. So to use 20 amp radials instead of 32 amp rings means 3 circuits so double row CU and much more expensive.

 

[JohnW2]

I don't get the obsession the younger breed of professional electricians seems to have about radial circuits.

I think that, in most cases, it's probably more a matter of their not seeing any point in installing ring finals - and that is very largely a reasonable view.

However, I'm not at all sure what that has got to do with my message to which you are replying (and quoting).

I suspect the real justification is time and money, it's simply easier in most cases to put in a radial.

As has been said, I'm not at akll sure that will often be the case.

Kind Regards, John

 

[JohnW2]

As to radial the good point is less on each circuit, so any trip effects less items, but need three radials to replace a ring, and unless using RCBO's no advantage anyway, and if using RCBO's it gets expensive, specially if you need a new consumer unit to have so many circuits.

You seem to have the fixed idea that radials have to be 2.5mm² - hence 25A at the most, and commonly 20A.

As I frequently say, I'm personally not at all keen on 20A radials (except in particular circumstances) - and, if one uses 4mm²/32A ones, then none of the issues you mention are valid.

I do wonder if the people who sing the praises of radials actually measure or calculate the volt drop? 106 meters of 2.5 mm² or 40 meters of 4 mm² ...

I suspect that most people don't bother at all about VD measurements or calculations.

As for the 106m vs 40m that you are always quoting, as I've said many times, it's rather misleading, since the "106m" (for a ring final) actually means that the furthest a socket can be from the CU is 53m, which is nothing like as much greater than 40m than your figures imply - and that difference only exists because 4mm² is less than 5mm² (2 x 2.5²mm).

Kind Regards, John

 

[EFLImpudence]

I really don't understand this silly notion that it's easier to install radials.

Well, you do it for everything else (apart from sockets)

Getting 3x 4mm² cables into sockets is a right royal PITA,

That is simply not the case.

bigger chases, bigger holes, bigger conduits, additional cost of cable, etc.

Not to mention all the times when both legs of the 'ring' are in the same place.

Invariably there seems to be more radial than rings when the calcs are done.

Not sure I understand the point.

 

[EFLImpudence]

I don't get the obsession the younger breed of professional electricians seems to have about radial circuits.

I am sure John and I are not a younger breed of anything.

The justification given is that there was a shortage of copper after the war etc etc, but the war didn't continue to the 1990's.

There was, but it allowed the doubling of capacity on the socket circuit simply by retaining the existing radial and merely extending it back to the CU making at ring.

There's a shortage now (of copper, or at least significant price inflation).

Stop wasting it, then - 25mm² tails where not necessary, still using 6mm² for 32A cooker circuits, the fashion for 1.5mm² for 6A circuits, extending the ring when a spur would do.

I suspect the real justification is time and money, it's simply easier in most cases to put in a radial.

Not sure I follow your logic. Do you mean you prefer to choose a more difficult method for doing things?

 

[Jupiter01]

If you just want one more socket, a spur is the best choice.

A thought on this...
I believe that a spurred socket (single or double) should not have a combined load exceeding 13 amps?

If it's on the Ring, can it not draw up to 32amp (assuming nothing else was pulling ampage) on a 32MCB with a 2.5mm T&E cable?

Also, what happens if the spurred socket is suddenly drawing more than 17 amp? This is above the 16 amp rating of the 2.5mm cable going to the spurred socket but still below the MCB rating. Would this not cause this section of the cable to overheat?

Look forward to your thoughts on this. Thanks again.

 

[JohnW2]

A thought on this... I believe that a spurred socket (single or double) should not have a combined load exceeding 13 amps?

You believe incorrectly (in the case of a double socket - 13A is obviously the biggest load one can feed from a single one). Don't forget that the cable feeding the socket must have a CCC of at least 20A, and commonly is 27A.

Also, what happens if the spurred socket is suddenly drawing more than 17 amp? This is above the 16 amp rating of the 2.5mm cable going to the spurred socket but still below the MCB rating. Would this not cause this section of the cable to overheat?

Where does that "16A rating" come from? For ring finals themselves (hence most people believe also any spurs therefrom) the cable is required to have a CCC of at least 20A - and, as above, will commonly be 27A.

Kind Regards, John

 

[Jupiter01]

You believe incorrectly (in the case of a double socket - 13A is obviously the biggest load one can feed from a single one). Don't forget that the cable feeding the socket must have a CCC of at least 20A, and commonly is 27A.

Where does that "16A rating" come from? For ring finals themselves (hence most people believe also any spurs therefrom) the cable is required to have a CCC of at least 20A - and, as above, will commonly be 27A.

Kind Regards, John

Excuse me but what is a CCA?
Looking into this again, the 2.5mm T&E has a capacity of 24 amps according to this:

6242Y 2.5mm² Grey Twin & Earth Cable - Cut to length - (Max 100Mtrs) | Basec_Approved (2.5TE)

Appreciate that this will be influenced by the installation method.

I want to understand what would happen if more than 24 amps (but less than 32amps) was being drawn from the double socket on the spur? In my mind this would be exceeding the capacity of the cable (single 2.5mm feeding the spur) but not the MCB of the ring.

 

[JohnW2]

Excuse me but what is a CCA?

CCC - Current-Carrying-Capacity.

Looking into this again, the 2.5mm T&E has a capacity of 24 amps according to this:

6242Y 2.5mm² Grey Twin & Earth Cable - Cut to length - (Max 100Mtrs) | Basec_Approved (2.5TE)

Appreciate that this will be influenced by the installation method.

'We' normally use the current carrying capacities tabulated in BS7671 (Table 4D5 for T+E) which gives a figure of 27A for Method C ('clipped direct'/buried in plaster/masonry etc.) and 21A for Mthods 100 and 102.

I want to understand what would happen if more than 24 amps (but less than 32amps) was being drawn from the double socket on the spur? In my mind this would be exceeding the capacity of the cable (single 2.5mm feeding the spur) but not the MCB of the ring.

It would, if the current-carrying capacity were 24A but, as above, the CCC of the cable will commonly be 27A, and the maximum load that could be presented by a double socket is 26A (and some would day that a double socket should not be loaded bwyond 20A total).

In any event, even if one assumed your current-carrying-capacity of 24A, even if it were loaded to 26A, then "what would happen" would essentially be 'nothing', given the safety margins that are undoubtedly built into the tabulated CCCs.

Don't forget that a cable with a CCC of, say, 24A would be deemed to be safe if protected by a (hypothetical) 24A MCB - which would allow 34.8A (24A x 1.45) to flow for an hour.

Kind Regards, John

 

[EFLImpudence]

The TLC link is wrong.

Excuse me but what is a CCA?

Misspelt CCC ?

Looking into this again, the 2.5mm T&E has a capacity of 24 amps according to this:

6242Y 2.5mm² Grey Twin & Earth Cable - Cut to length - (Max 100Mtrs) | Basec_Approved (2.5TE)

Appreciate that this will be influenced by the installation method.

2.5mm² T&E clipped direct or buried in masonry has a Current Carrying Capacity of 27A.

I want to understand what would happen if more than 24 amps (but less than 32amps) was being drawn from the double socket on the spur? In my mind this would be exceeding the capacity of the cable (single 2.5mm feeding the spur) but not the MCB of the ring.

The double socket current is limited by the two 13A fuses in the plugs.