Does adding a 16A MCB to a spur meet regs, or do I need a 13A fuse?

[ericmark]

2.5 mm² will not take 32A so one has to ensure the two cables of the ring final will not be overloaded should you draw 16A for garage plus things in the house off the same ring final, so if the spur is central to the ring no problem, and if the spur is spurred at the consumer unit no problem. But if the spur is 1/8th around the ring then there could be over loading.

As to working it out, the Prospective short circuit current at the point where the spur is taken from, the PSCC at the CU, and the lowest PSCC at any socket can be used to approx work out how far around the ring the spur is taken. Plus if the volt drop is exceeded.

However you need the meters, and the skill to work it all out, although I have written programs to help me work things out, in the main you have a good idea how close one is to the limit, and unless close to limit don't bother working it out. But you still need either PSCC or impedance readings to even have an idea how close you are.

I have questioned the idea of a garage off the ring final, it is an appliance which could have a load over 2000W so it should be arranged so it can't result in an overload, however a garage is a place to store a car, and really the only load on a socket should be a battery charger, fact that it is rarely used to store a car does not mean designers have to design it for anything else.

 

[JohnW2]

2.5 mm² will not take 32A so one has to ensure the two cables of the ring final will not be overloaded should you draw 16A for garage plus things in the house off the same ring final, so if the spur is central to the ring no problem, and if the spur is spurred at the consumer unit no problem. But if the spur is 1/8th around the ring then there could be over loading.

I've already been through that.

As I said, and the OP agrees, if one attaches a 2.5mm² spur which is protected by a 16A or 20A (or possibly even 25A) OPD to a ring , then the situation as far as the the ring is concerned (i.e. potential overloading of cables) is, at worst, no worse than having a double socket at the same point on the ring - and no worse than having an unfused spur supplying one double socket originating at the same point on the ring.

People seem to get confused by the fact that the examples shown in the ('informative') Appendix of the regs are limited, and far from exhaustive They only show a (potentially 20A/26A) 'unfused spur' and a (13A) 'fused spur', but there is no electrical reason why one can't have a 16A or 20A 'fused' (or 'MCB-protected') spur - which, as above, is no worse than having an unfused spur supplying one double socket.

With any ring, the designer has to satisfy him/herself that "it is unlikely" that any cable will be "overloaded for long periods of time". As you imply, one has to think about that when decided where things are connected to a ring - but the consideration is exactly the same whether one is talking about a double socket on the ring or the origination on the ring of a spur.

Kind Regards, John

 

[winston1]

Great point that the spur load for a double socket could actually be 26A anyway

It shouldn't be. a double socket is rated at a max of 20 amps, not 26.

 

[JohnW2]

It shouldn't be. a double socket is rated at a max of 20 amps, not 26.

As always, tht's a matter of uncertainty/debate - but in this case it's irrelevant, since the OP was not considering protecting the spur with greater than a 20A MCB. (I know people have been talking about 25A ones, but that would be pushing things a bit)

Kind Regards, John

 

[EFLImpudence]

I am a bit confused.

The spur cable (2.5mm²) will be quite happy with 26A flowing through it, so is someone saying that there should be a 20A maximum OPD to protect the socket?



Can anyone see a (can't think of the word) contradiction?

 

[rsgaz]

single 2.5mm T&E cable through the wall to the main house.

I'm confused, too.
Just pull this cable out, drill the hole bigger to fit two T&E through and extend the ring into the garage.

It's challenging to get any more cable from the main house into the garage.

How so? Or did you mean 'all the way from the consumer unit' with that comment?

 

[JohnW2]

I am a bit confused. The spur cable (2.5mm²) will be quite happy with 26A flowing through it, so is someone saying that there should be a 20A maximum OPD to protect the socket?

I can't speak for others, but I've been a little cautious since I don't think that we are certain that the cable will be installed entirely Method C - that's why I've been a bit guarded in my few comments about 'possibly a B25' (and have even added a "probably" to mentions of a B20, just in case the cable is buried in thermal insulation somewhere!).

Kind Regards, John

 

[JohnW2]

I'm confused, too. Just pull this cable out, drill the hole bigger to fit two T&E through and extend the ring into the garage.

Certainly a possible approach, but why use two cables when one would be sufficient?

Kind Regards, John

 

[AndyPRK]

I recall the op didn’t want to draw too much current in the garage.

A B20 would be a good compromise all round.

The op is probably not confident to break the ring, and it adds no advantage.

Don’t thibk I would want my main ring running around the garage. Not easy to isolate etc.

A B25 could allow a large current at one end of the ring if the garage were to be used more heavily in the future.

 

[Justin0]

I am a bit confused.

The spur cable (2.5mm²) will be quite happy with 26A flowing through it, so is someone saying that there should be a 20A maximum OPD to protect the socket?

Can anyone see a (can't think of the word) contradiction?

I'm confused, too.
Just pull this cable out, drill the hole bigger to fit two T&E through and extend the ring into the garage.

How so? Or did you mean 'all the way from the consumer unit' with that comment?

Well this is partly an issue because the other side of the cable (that is, in the house) is no longer accessible. The builders did the cabling before adding the large glued-down floor panels, plasterboard on the walls etc. You're right though that if I was determined I could probably get to it and potentially extend the ring, it is maybe worth considering.

But if I extend the ring into the garage don't I then need to be able to do the requisite measurements and calculations to make sure that the existing ring is suitable to be extended?

Just to recap, my assumption here is that when the house was built (last year) they have installed the electrics properly to the regs (probably a whole other debate, but lets just assume). I have a 2.5mm cable coming into the garage with a double socket on the end, which is clearly a spur off the downstairs sockets circuit in the house. Likewise for the single light bulb and 1mm cable connected to downstairs lights. I want to add lots of extra sockets for convenience, but I'm happy to live within the limits of 13/16/20A/whatever total within the garage, so I'm adding a breaker where the cable enters the garage so I can add as many sockets as I like without worrying about it.

If you want "a switch for the whole garage lighting circuit" (which isn't a bad idea, so that you could isolate the garage part if it developed a fault), then I would advise you to use just a switch - and would advise a double pole one, to provide total isolation if/when necessary.

So you're saying that no matter how many lights I add, I'm not at risk of overloading the spur along the 1mm cable, based on the fact that the circuit is 6A in the main house CU?

Thanks everyone for the advice/discussion.

 

[JohnW2]

So you're saying that no matter how many lights I add, I'm not at risk of overloading the spur along the 1mm cable, based on the fact that the circuit is 6A in the main house CU?

Indeed I am.

In fact, if (as is probable) you're talking primarily (or exclusively) about LEDs or CFLs ('energy-saving') then you're house+garage would probably be fairly hard-pressed to overload even a 1A or 2A lighting circuit, let alone a 6A one - 230-460 W worth of LEDs/CFLs would produce an awful lot of light!

Kind Regards, John

 

[ericmark]

If you measure the loop impedance or resistance from each leg of the ring final to the garage and leg to leg on the ring final then you have the info required to work out how close to centre of ring final the garage is, as said not different to having double socket except that the chances of loading a double socket to 26 amp over an extended time is a lot less than loading many sockets in a garage. And the ring final will not only have the garage load it will also have some load from the house.

At over 32A load the MCB on the ring final will in time trip, the question is will the legs of the ring share that load enough to ensure neither leg is loading over 22 amp for an extended time, from the point where the spur is taken from to the garage is not a problem, the problem is from the consumer unit to the point where the spur is taken from.

Well this is partly an issue because the other side of the cable (that is, in the house) is no longer accessible.

Although it is possible that maintenance free junction boxes have been used, normally it is taken from a socket or FCU so under normal conditions the cable is assessable at end even if it can't be easy up graded.

Extending the ring final will increase the loop impedance of the ring final so it will reduce the chances of over loading either end, however it may be completely unnecessary, until the loop impedances have been measured no one knows if within limits or not.

To be frank to work out volt drop where you have part of the circuit protected to 32A and other parts to 20A is some thing I have only done as an exercise in University and I could not remember now how it was calculated. I would say around 0.48Ω higher then the incoming impedance for 20A and 0.72Ω higher then the incoming impedance for a 13A supply.

So until a figure is produced no one knows if it is within limits, we can debate until cows come home, but until given some figures we don't know. Be it loop impedance, resistance, or prospective short circuit current step one is measure and the consumer unit both in house and in garage and see how close it is to the limits, so a reading of 0.82Ω or better then chances are it's OK, I am allowing 0.35Ω at the CU. But need to know the reading before anyone can say OK or not.

 

[JohnW2]

.... as said not different to having double socket except that the chances of loading a double socket to 26 amp over an extended time is a lot less than loading many sockets in a garage. .... At over 32A load the MCB on the ring final will in time trip ....

... and the 20A MCB protecting the feed to the garage will eventually trip if the total load on garage sockets is markedly above 20A.

... the question is will the legs of the ring share that load enough to ensure neither leg is loading over 22 amp for an extended time ...

Did you mean 27A (ie. 2.5 mm² Method C), rather than 22A?

Quite honestly, in the case of a 2.5 mm² Method C ring, I think that 27A is so close to 32A that, except in exceptional circumstances (knowledge about likely very large long-term loads) that my feeling is that all this concern about overloading the short leg of a ring with a load applied close to one end of the ring is probably not really necessary.

Kind Regards, John

 

[Justin0]

If you measure the loop impedance or resistance from each leg of the ring final to the garage and leg to leg on the ring final then you have the info required to work out how close to centre of ring final the garage is, as said not different to having double socket except that the chances of loading a double socket to 26 amp over an extended time is a lot less than loading many sockets in a garage. And the ring final will not only have the garage load it will also have some load from the house.

At over 32A load the MCB on the ring final will in time trip, the question is will the legs of the ring share that load enough to ensure neither leg is loading over 22 amp for an extended time, from the point where the spur is taken from to the garage is not a problem, the problem is from the consumer unit to the point where the spur is taken from.

The op is probably not confident to break the ring, and it adds no advantage.

Don’t thibk I would want my main ring running around the garage. Not easy to isolate etc.

It does seem far more sensible to get an electrician to do work like this.

I'm not sure it would give me enough advantage over treating the whole garage as a 20A-limited spur, provided that is a valid setup?

Although it is possible that maintenance free junction boxes have been used, normally it is taken from a socket or FCU so under normal conditions the cable is assessable at end even if it can't be easy up graded.

That is interesting to know and it may be the case that the spur is connected in one of the kitchen sockets.

However if it is, it is probably running up inside the wall of the kitchen behind kitchen cabinets, because it enters the garage at ceiling height. So it wouldn't be possible to simply use the existing cable to "pull in" a second in order to create a ring.

 

[JohnW2]

I'm not sure it would give me enough advantage over treating the whole garage as a 20A-limited spur, provided that is a valid setup?

If you're happy that 20A is enough for your garage (you were initially talking about 16A, or even possibly 13A), then extending the house ring into the garage would not really offer any significant advantage.

Kind Regards, John