Rewiring plan

[JohnW2]

... nowadays with the narrow gap between a 27A CCC cable and a 32A MCB hardly seems worthwhile. ...

Indeed, but those very same effects mean that the 'risks' of ring finals we often hear about are more theoretical than practical. Whether one 'overloads the cable' by having very unbalanced loads or if a break arises in the L or N ring, at worst one is left with a "2.5mm² 32A radial" - and, although non-compliant, I am all-but-certain that no harm would ever result from that.

Kind Regards, John

 

[JohnW2]

Don't forget, you are perfectly entitled to wire whatever circuit you like in a ring.

Indeed - and as I just wrote, there is no reason why one can't have the CPC wired as a ring but the L and N as a radial ('HI earthing').

Kind Regards, John

 

[EFLImpudence]

Indeed, but those very same effects mean that the 'risks' of ring finals we often hear about are more theoretical than practical. Whether one 'overloads the cable' by having very unbalanced loads or if a break arises in the L or N ring, at worst one is left with a "2.5mm² 32A radial" - and, although non-compliant, I am all-but-certain that no harm would ever result from that.

Well, yes, but if that is the case, and I agree it probably is, then the standard ring, with all its conditions, is even more pointless.

You may say the same of a radial with 4mm² cable and a 40A MCB.

Why don't the CCCs and MCBs match?

 

[JohnW2]

Well, yes, but if that is the case, and I agree it probably is, then the standard ring, with all its conditions, is even more pointless.

Well, regulations aside, if the 'risk of overloading the cable' is essentially a non-issue, then we are left with the main difference being the advantage of CPC redundancy!

You may say the same of a radial with 4mm² cable and a 40A MCB.
Why don't the CCCs and MCBs match?

The ratings of MCBs etc. are in a Renard Series. Cables could obviously be made whose CCCs matched those figures (for just one installation method), but they would probably then have 'orriby untidy (non-'round') CSAs.

Kind Regards, John

 

[EFLImpudence]

Well, regulations aside, if the 'risk of overloading the cable' is essentially a non-issue, then we are left with the main difference being the advantage of CPC redundancy!

Well, yes, but we don't consider that worth doing on any other circuit.

The ratings of MCBs etc. are in a Renard Series. Cables could obviously be made whose CCCs matched those figures (for just one installation method), but they would probably then have 'orriby untidy (non-'round') CSAs.

Like 7/.029, a CSA of 0.0045in². However did they manage?

 

[JohnW2]

Like 7/.029, a CSA of 0.0045in². However did they manage?

Well, they didn't refer to cable sizes by CSA in those days, did they?

However, as I intimated before, the real problem with attempting to match cable CCCs to MCB etc. ratings is that it could only be done for one installation method and one set (e.g. 'none') of de-rating factors.

Kind Regards, John

 

[VogonPoet]

Thanks everyone, some food for thought there. Clearly there isn't a consensus on the "fridge on a separate circuit issue", or on the best wiring method out of ring final and radial.

In my house I only have space for a standalone combined Fridge/Freezer and that will be in the kitchen. Given that the fridge section will be used multiple times daily there is only a very low risk of not spotting a tripped circuit in time to stop the food defrosting. Is there any other disadvantage (apart from some additional cost) to running it on a separate radial? Also, if the regs are likely to recommend a separate freezer circuit in the near future then I'm happy to get ahead of the crowd in this area.

I'd also prefer to err on the side of over-speccing the kitchen socket circuit . With the kettle, toaster, microwave, dishwasher, washing machine plus potentially fryers or slow cookers all running from the sockets I think going bigger is probably better.

The lounge is running a large TV, other AV equipment and a hefty surround sound amp (and associated speaker package) plus a computer. The cupboard under the stairs runs a UPS and several computer servers 24/7. I think that may be a heavier load than usual so it might make sense to spec that socket circuit higher than you'd generally assume for one room + cupboard.

The upstairs sockets are only lightly used so no special considerations there.

So I need to spec the following socket circuits:

Kitchen/Utility/Outside socket - heavy load
Lounge/Under stairs - medium load
Upstairs - light load

I'm happy to take advice on the best wiring topology, cable type and RCBO rating for each but whatever gets chosen needs to be defensible to whoever comes to inspect. I don't want to put in a radial and then get told by a conservative inspector that most houses use ring finals for sockets and he'll only accept that topology.

With regards to the lighting circuits I had specced at 1.5mm because they are likely to be running through insulated spaces and I understood that a heavier cable would make them less likely to overheat. Is there a disadvantage to using 1.5mm over 1mm apart from cost?

I'll come back the subject of LABC approval later as I'd like to get in touch with them again to discuss the process in more detail first. The guy I spoke to last time didn't mention having to pay for inspections by third parties in addition to the (already fairly hefty) Building Notice fee. If I end up having to pay electricians to sign-off on the work I'm confused as to what they are actually doing in this process to justify their fee.

 

[JohnW2]

In my house I only have space for a standalone combined Fridge/Freezer and that will be in the kitchen. Given that the fridge section will be used multiple times daily there is only a very low risk of not spotting a tripped circuit in time to stop the food defrosting.

Fair enough.

Is there any other disadvantage (apart from some additional cost) to running it on a separate radial?

Not that I can think of.

Also, if the regs are likely to recommend a separate freezer circuit in the near future then I'm happy to get ahead of the crowd in this area.

If you're saying that because of my comment, you misunderstood me. There is no expected change in regs to require separate freezer circuits. My comment related to the fact that there is an upcoming regulation which will essentially require all sockets to be RCD/RCBO protected. Currently, some people install dedicated freezer circuits without RCD protection (to avoid the risk of 'nuisance trips' whilst they are on holiday. When the new reg appears, unless one hard-wires the freezer (so that there is no plug/socket involved) even a dedicated ('one socket') circuit will have to be RCD protected.

I don't want to put in a radial and then get told by a conservative inspector that most houses use ring finals for sockets and he'll only accept that topology.

That won't happen. Radial, rather than ring final circuits are increasingly being installed and, in any event, both (if properly designed) are fully reg-compliant, so no inspector could refuse to accept either of them.

With regards to the lighting circuits I had specced at 1.5mm because they are likely to be running through insulated spaces and I understood that a heavier cable would make them less likely to overheat. Is there a disadvantage to using 1.5mm over 1mm apart from cost?

1.5mm² cable for a 6A lighting circuit is way OTT, particularly given today's low lighting loads - were it available/allowed 0.5mm² would usually be quite adequate. However, I suppose the only disadvantage of 1.5mm² is that you might have difficulty getting 1.5mm² conductors into the terminals of some lighting fittings.

Kind Regards, John

 

[winston1]

There is also saving the planets dwindling source of copper.

 

[EFLImpudence]

Just to say:

I'd also prefer to err on the side of over-speccing the kitchen socket circuit .

If you decide to have a Ring circuit, then 2.5mm² with 32A MCB is what you must have.
Over-speccing would mean having TWO or more.

Similarly with Radials of 4mm² and 32A MCB.

I think you are overestimating the current needed for your appliances.

 

[John D v2.0]

I think you are overestimating the current needed for your appliances.

Agreed, to overload a 20a circuit in one room with 24/7 loads, it would be unbearable to stay in the room for more than a short time in summer. Unless one of those loads is an a/c of course.

 

[securespark]

Seeing the figures from the 14th prompted me to post the equivalent figures from the 13th. They are lower in the smaller sizes and higher in the larger CSA conductors.

 

[ericmark]

With a ring final you can have 106 meters of 2.5 mm cable and be within the volt drop, with a radial you can have 32 meters of 2.5 mm cable and be within the volt drop using a 20 amp MCB using a 16 amp MCB it goes up to 42 meters. So what you can find is a single final can need 3 or 4 radials to replace it not just 2 so some careful planning is required. With a 100 meter roles of cable measuring is easy, one role = one ring final. But as you move to radials one has to be careful not to exceed the limit.

Up to now I have not seen many record the line - neutral loop impedance, well normally measured as prospective short circuit current, but the limit for a radial of 16A is 0.97Ω on top of the supply reading, or 0.59Ω on top of the incoming reading with a ring, one of the DIY problems is to measure these limits, but using a loop impedance meter easy for the electrician. So in the future an electrician could highlight being over the volt drop limit on an EICR. I think it is unlikely as so hard to prove the meter is accurate enough, it would depend if we get equipment which does not like volt drop.

 

[VogonPoet]

I like the idea of using radials and running some extra earth cable to make the earth into a ring for added safety but I find 4mm twin and earth cable a bit harder to work with than 2.5mm so would prefer to limit how much of it I use.

Would this be acceptable:

Kitchen/Utility/Outside socket - 4mm radial. 32amp RCBO. 4mm earth run from final socket back to CU to make a ring.
All other downstairs sockets - 2.5mm radial. 20amp RCBO. 4mm earth run from final socket back to CU to make a ring.
Upstairs sockets - 2.5mm radial. 20amp RCBO. 4mm earth run from final socket back to CU to make a ring.

As I said it is a small house, meaning the cable distances are so short that running three extra lengths of earth cable wouldn't be much additional work (especially if it has a safety benefit) or take much extra copper from the worlds supply. I suspect I'll be at least "copper neutral" after this renovation anyway, once I recycle all the old wiring together with the plumbing pipework and old hot water tank that is coming out!

Edited to add: All three circuits would use less than 30 metres of cable, I'd be surprised if any used more than 20 metres.

 

[JohnW2]

Seeing the figures from the 14th prompted me to post the equivalent figures from the 13th. They are lower in the smaller sizes and higher in the larger CSA conductors.

Thanks. To facilitate comparisons, I've put 13th & 14th ed. Imperial CCCs and 17th ed. Metric ones together in a graph. Since the only common installation method is 'in conduit', I have assumed Method B for current regs.

The 13th and 14th are very similar, other than the differences at the very lowest cable sizes which you note (and a small 'glitch with 7/0.044), but the 17th ed. CCCs are consistently higher for the same CCC.

Kind Regards, John