CU change - ring on end of radial

[ianmacd]

What you mean is you can;t find anything to prohibit it?BS7671 is not a definitive reference to every exact scenario and combination of circumstances ever thought to be possible. It is a guide.

433.1.1 prohibits it.

 

[initforthemoney]

What you mean is you can;t find anything to prohibit it?BS7671 is not a definitive reference to every exact scenario and combination of circumstances ever thought to be possible. It is a guide.

So, if bs7671 is just a guide, why do we have follow it so strictly with our installations?

 

[Andy™]

So, if bs7671 is just a guide, why do we have follow it so strictly with our installations?

same reason people get stuff PA tested. installation has to be safe. working to 7671 is one way of achieving this

 

[kme]

Init:

The regs (and I`m paraphrasing here) ALLOW circuits other than those demonstrated; IF they comply with 7671 OR equivalent.

Ergo; I don`t need to have a diagram of a particular circuit in order to use it.

For instance - I had 2 smoke alarms to wire in a 1 bed flat (D/LD3), surface trunking which passes the CU. So, I take 1 run of 3+E between the alarms; via the CU.

Break all conductors apart from the interlink, and connect `em to the OCPD.

It isn`t shown in 7671 (the source is in the middle of the cct, if you`re being pedantic), but there is nothing wrong with that arrangement, as seen from either 7671 OR 5839-6

In the scenario from the O/P, we have a 6mm radial, on a 30A 3036 - which complies in the basic sense.

we ALSO have a 2.5 ring final, which is effectively fed from the same OCPD - which also complies.

So the only possible issue is the joint.

Or, to look at it another way.

I started a PEL PIR (public entertainment licence periodic) today, in a fair sized hotel.

In the function rooms, they must have, by law, sound level monitoring systems; and an interlink to the fire alarms, which isolates the socket outlets in the event of excessive sound levels, or a fire alarm.

This is done with a remote contactor. The feed to the contactor is a 6mm SWA; the ring then begins at the contactor.

I won`t code that, as I see nothing wrong with it; and its essentially the same circuit we`re arguing over - but there is a contactor at the joint.

KME

 

[SteveT]

Consider the 6mm merely as an extension of the terminal at the CU MCB. The final ring is still a ring.

the capacity of the 6mm (as a radial) is 34A(enclosed, higher in free air), max length is 49M(for Zs spec)

A 2.5mm ring has a capacity of 40A

Check appropriatness of junction/connectiion

Although a 32A MCB will offer overload and fault current protection

a 20A may be more appropriate (Dependant on load on final circuit, in this case a dedicated kitchen, the demand would be less than 20A)

Zs measured from CU

Deduct R1+R2 of radial for polarity check on ring so you don't have increasing measurements

Nothing against regs, if all is sound, I think there is no problem with this set up.

 

[ianmacd]

A 2.5mm ring has a capacity of 40A

Would be interested to know which fairytale you got that from since it will not satisfy 433.1.1.

 

[Chr!s]

I would challenge you to prove it passes 433.1.1 without using 433.1.5

The Lollipop Circuit

In my design ill assume that at the far end of the circuit there is a 20 amp load and an additional 12 evenly distributed around the circuit, average current = 32 + 20/2 = 26 amps.Providing load current in any part does not exceed 26amps we may use an Iz of the below.

Iz Equal > 26 amp, 6mm, 2.5 so both are o.k

Volt drop = Radial 10 meters 6mm, Ring/Parallel circuit 60 meters

Radial = 2.336 volts Ring equals = 7.02 volts = 9.356 volts so less tan the 11.5

Fault Protection Maz Zs(32 Amp type B) = 1.44 ohms Ze = .35 ohms(TNC-S) 100 amp 1361 type II

Radial = .13 ohms Ring = .35 ohms = ZS = .83 So comply's

Fault current Protection Front end using adiabatic and energy let throughs Minimum size = . 38mm

far end minimum size .21mm so comply's

Short circuit Front end .38mm Far end = .30mm so comply's

Now Ian you tell me where my circuit Does not comply

 

[ianmacd]

try it again with 32A from a single point and use TNS not TNCS. You are also assuming lengths, none of which has ever been reviewed and assuming Ref C which also has not been confirmed.

 

[Chr!s]

try it again with 32A from a single point and use TNS not TNCS. You are also assuming lengths, none of which has ever been reviewed and assuming Ref C which also has not been confirmed.

lol

If a single load of 32 amp is place at the furthest point(mid point) the current will divide equally, 16 amps each leg, 2.5 carries 27amps.

The Iz of the cable must be equal or greater than 26 amps, so you calculate your It with correction factors

I know no parameters have been mentioned, im not saying daves circuit does comply, im saying it may.

 

[ianmacd]

what about just at the union between your radial semi final and your ring semi final?

 

[Chr!s]

what about just at the union between your radial semi final and your ring semi final?

In what sense

 

[SteveT]

As you know the average current carrying capacity of a 2.5 radial is around 20A in a domestic situation (insulation, grouping etc taken into account), you can double this to 40A when used in a ring, this is why you generally protect a ring with 32A MCB, not a 20A as per a radial cct. this will ensure you are well within the cables capability 9always err on the side of caution- don't design to limits!) Thats the fairytale!

 

[kme]

As you know the average current carrying capacity of a 2.5 radial is around 20A in a domestic situation (insulation, grouping etc taken into account), you can double this to 40A when used in a ring, this is why you generally protect a ring with 32A MCB, not a 20A as per a radial cct. this will ensure you are well within the cables capability 9always err on the side of caution- don't design to limits!) Thats the fairytale!

Questions:

1. Where do you find the "average current carrying capacity" of a cable?

2. If you were to be correct; why would it only be "in a domestic situation"? Can it carry less / more in a commercial situation? And is that "average" again?

With respect, your information is poorly presented (as fact, which it isn`t). We don`t deal with "averages"; we deal with maximums, and I have never seen, in ANY publication, tables of "average current-carrying capacity".

Perhaps you should have phrased your post as a question rather than statement.

KME

 

[ProDave]

The junction, and it's location remains a mystery.

Today I spent a couple of hours lifting a run of upstairs floorboards all the way from the kitchen to the landing above the CU in order to install the non existent water and gas bonding cable. No sign of the 6mm or the junction to the ring final.

So it must run downstairs under the floor or within the walls. That rather rules out the theory that it was a redundant shower feed that was used.

I lifted upstairs boards for the bonding cable as upstairs is carpeted and it doesn't matter so much if you b****r the boards lifting them. All of the downstairs is exposed varnished boards and none looked easily liftable.

 

[initforthemoney]

It'll be fine Dave. Guinness

 

[kme]

It'll be fine Dave. Guinness

Didn`t you say it wasn`t right, yesterday????

I`m confused.

Need more inputGuinnessGuinnessGuinnessthat`s better.

 

[steptoe]

So, if bs7671 is just a guide, why do we have follow it so strictly with our installations?

you dont,

its down to you to prove it is safe,

if you are unable to prove it is safe then simply do what all the 5DW do and comply with 7671 and hope for the best.,

cos they dont have a f'in clue otherwise.

 

[ianmacd]

you dont,its down to you to prove it is safe,

if you are unable to prove it is safe then simply do what all the 5DW do and comply with 7671 and hope for the best.,

cos they dont have a f'in clue otherwise.

Easy!

 

[SPECIAL LOCATION]

I have been reading through this.. and I don't think I have ever read so much tosh based around so few facts.....

As far as I read it we do NOT know;

The size of the kitchen.

The number of socket outlets this "lollipop" is feeding.

The sort of loads, (fixed/fitted appliances-vs-hand held small appliances), this circuit is supplying.

The expected maximum demand of the circuit taking into account diversity.

Therefore the "Design current" of the circuit.

etc..

etc..

Some have banded about figures of 32A....

Yet as we all know there are still many houses with a single socket circuit for the whole house, supplied by a 30amp or 32amp fuse/breaker.

Taking a reasonable guess on the LIMITED facts we have I would say the chances of this kitchen Lollipop drawing 32A is quite unlikely.??

I bet it would be pushing it to even get to 15A or 20A IMHO!

So anyway IF we are going to try to ascertain if this is electrically safe please lets get some realistic figures OR wait for more facts from ProDave?

At the moment as I read it, the elusive joint between 6.0mm & 2.5mm's is nowhere to be found..

So we can still say the circuit fails to comply based on 526.5 page 106 "accessibility of connections"..

But as for all the other reasons...

I may as well go and stand on a windy sea front and try to pi55 up the wall without it blowing back over me leg!

Pick a reg take a guess ....

may strike lucky!

:C :C:C

 

[binky]

At the moment as I read it, the elusive joint between 6.0mm & 2.5mm's is nowhere to be found..

So we can still say the circuit fails to comply based on 526.5 page 106 "accessibility of connections"..

I know where it is - JB in wall plastered over :^O

If we applied accessibility of connections to every place we work in I would end up re-wiring about 95% of the places I come across. For a new circuit it would obviously not comply, but for an existing circuit as long as it tests OK and MCB is correct for size of cables used, then that's as far as I would go unless the customer really wants it sorting out. Loads on circuit are irrelevent. Correct protection of circuit is paramount even if the circuit is non-standard.