Ring Main - limits on length

[Jupiter01]

Hi All,

I was wondering if there was a hard limit on the length of cable that can be used on a single ring main? I am breaking my house down into multiple ring mains but there is a lounge downstairs which is on the large side - 9.5m x 4.7m. I am running cable in the floor joists and then dropping down to the sockets. This along with the path back to the consumer unit probably amounts to around 80 metres of 2.5mm twin and earth cable. How does that sound?

Thanks in advance.

 

[flameport]

Limit is about 100m of cable if a ring must be used.
Other wiring options are available.

 

[Jupiter01]

Thanks. I'm well within this. Preumably, a radial circuit is the alternative?

 

[bernardgreen]

With 6 kW load at the mid point the voltage drop at the midpoint will be about 10 volts (4%) so border line if using 2.5 mm² cable.

Maybe use 4 mm² for this ring.

Or use 4 mm² for the two runs from consumer unit to the first sockets and then 2.5 mm² between the sockets.

 

[oldbutnotdead]

Sounds like notifiable work to me (new circuit)- your electrician will have used experience and volt drop calculations to determine what size cable to use for your setup.

If the cable is all clipped direct (not going through insulation, not horrendously bunched in holes through joists) then 2.5mm is usually adequate. Just.

Or (if the legs from CU to first and last socket are long) consider 4mm for those 2 legs- not best practise and MUST be recorded on the installation details.

Or (if you have spare capacity in the CU) run the job as 2 radials.

Damn- too slow

 

[JohnW2]

With 6 kW load at the mid point the voltage drop at the midpoint will be about 10 volts (4%) so border line if using 2.5 mm² cable. Maybe use 4 mm² for this ring. Or use 4 mm² for the two runs from consumer unit to the first sockets and then 2.5 mm² between the sockets.

The OP will know more about the likely loading of the circuit but I would not personally get excited about getting theoretically (fairly) close to the arbitrary guidance limits for VD. I realise that, as well as 'compliance with the guidance', there is the question of 'wasted power', but, in reality, 6kW of load close to the centre of a ring is probably a very unlikely/unusual/short-lived situation.

Unless the OP's house is very large, his 80m estimate sounds very high. It would obviously reduce a lot if, instead of supplying each socket separately with a pair of cable drops, there was a fair bit of socket-to-socket wiring within the living room - but I assume that the OP must have considered that and, for some reason, rejected it.

Kind Regards, John

 

[Jupiter01]

Hi John, thanks for the response. I've done some socket-to-socket wiring but given that the room is 9.5 metres by 4.7 metres, there are sockets dotted around the room. I have preferred the drops to sockets in most of these cases. 80 metres is a liberal estimate.

 

[JohnW2]

Hi John, thanks for the response. I've done some socket-to-socket wiring but given that the room is 9.5 metres by 4.7 metres, there are sockets dotted around the room. I have preferred the drops to sockets in most of these cases. 80 metres is a liberal estimate.

Fair enough - only you know the actual situation and practicalities, but a run all the way around that room would only amount to about 28m (to which you'd have to add a couple of cable drops and the two runs back to the CU.

FWIW, I have a ring final serving two adjacent reception rooms which, taken together, are appreciably larger than your living room (about 6.0/6.5m x 15m together), and that is appreciably less than 80m in length (IIRC 50-60m).

Whatever, as has been said, 80m is well within the guidance limit in relation to voltage drop. You would also need to make sure that the earth fault loop impedance was low enough to satisfy the regulations/safety (which it ought to be).

Kind Regards, John

 

[Jupiter01]

Thanks again. Re-calculating it, I think it's more in the region of 60-65 metres. Am I on safer territory if that's the case?

 

[JohnW2]

Thanks again. Re-calculating it, I think it's more in the region of 60-65 metres. Am I on safer territory if that's the case?

Certainly fine as far is voltage drop is concerned, and probably also for EFLI - fairly similar to the circuit of mine which I mentioned.

Kind Regards, John

 

[ericmark]

I heard the comment in an IET lecture that the new limits set at 5% instead of 4% volt drop resulted in the length of a ring final being increased to 106 meters. My humble attempts to emulate these results at first failed so I raised the question at another IET lecture and it seems it is worked out by assuming 20 amp is drawn centrally and the remaining 12 amp is even distributed so when making the volt drop calculations a figure of 26 amp is used not 32 amp. Also to get to this figure you need to use all the compensating values so instead of working on 18 mV/A/m one works on 16.52 mV/A/m and at the centre point the loop impedance will be 0.59Ω lower than at the origin.

I built a java script program to work out volt drop and cable length and it was tested on this forum and discussed at length. What we were considering was if even 10 years after an installation was complete if some one could return to the installer and claim they had not done their job correctly or if not raised with an EICR could in another 10 years the tester be brought to court for not spotting the excessive volt drop?

However as we continued it became apparent the problem is in the accuracy of the test equipment in the main any loop impedance tester will give readings which vary +/- 0.05Ω which does not seem much, but has a huge effect to the calculated cable length. So unless the volt drop was massively over the 5% permitted then in real terms it does not matter.

As to using 4mm² to reduce volt drop personally I think this is going OTT with most installations. Only in commercial premises is there really any need for this.

As to radial well you lose that 26 amp rating for calculations so you can use more cable in a 32A ring final than in two 16A radials you can use about 38 meters per 16A radial against 106 meters for a 32A ring with both using 2.5mm². With 4mm the radial jumps to 71 meters and the ring final to 178 meters using 16A and 32A MCB's, but in the main 4mm² would mean a lot more expense and I have not seen it used in domestic installations.

Assuming a 0.35Ω incomer you are looking for a line - neutral loop impedance of around 0.94Ω or 244 amp prospective short circuit current. As I have said meters are not that accurate so as long as you get more than 230 amp PSCC then I would expect any inspector to pass the installation.

In real terms if your supply is better than 0.35Ω then you can afford a little more volt drop within the installation. And with a living room you are unlikely to hit the 26A so I would only really be worried in a kitchen or utility room.

 

[ericmark]

Thanks again. Re-calculating it, I think it's more in the region of 60-65 metres. Am I on safer territory if that's the case?

Certainly fine as far is voltage drop is concerned, and probably also for EFLI - fairly similar to the circuit of mine which I mentioned.

Kind Regards, John

Unless there is no RCD protection then the earth loop impedance can rise to 200Ω after which it is considered as unsafe. Even without RCD protection the volt drop limit is reached before the ELI limit unless one uses type C or D MCB's. I have argued that as long as the RCD tester shows the RCD is within limits the ELI is really only to show a likely fault and there is no magic number any more. So a short ring with a 1.0Ω reading should ring alarm bells but a long ring same reading is OK.

It is down to the testers experience as to if the readings are what are expected rather than using the 1.37Ω reading and I would prefer to measure both line - neutral and line - earth in the same units as it highlights faults better. I know line - neutral is normally measured is amps and line - earth in ohms but that means one does not pick up faults in either reading as easy.

 

[ban-all-sheds]

Thanks again. Re-calculating it, I think it's more in the region of 60-65 metres. Am I on safer territory if that's the case?

Who is going to be signing this:

I being the person responsible for the design of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the design hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to 2015 except for the departures, if any, detailed as follows:

?

 

[JohnW2]

Unless there is no RCD protection then the earth loop impedance can rise to 200Ω after which it is considered as unsafe.

As I've said many times before, I really don't think that it is, in a TN installation, generally acceptable to rely upon an RCD for adequate fault protection - an RCD is meant to be providing 'additional protection'. (and, as a matter of detail, if one were allowed to rely on an RCD, then, theoretically, an EFLI up to 7666Ω, not 200Ω, would be adequate).

Even without RCD protection the volt drop limit is reached before the ELI limit unless one uses type C or D MCB's.

Indeed, which is why I said that (if VD was OK) then EFLI would almost certainly also be OK. However, that doesn't alter the fact that, as I said, one should "make sure that the EFLI is low enough" - even if the cable length is such that VD is OK, that doesn't prove that the CPC is connected to anything!

Kind Regards, John

 

[JohnW2]

Assuming a 0.35Ω incomer you are looking for a line - neutral loop impedance of around 0.94Ω or 244 amp prospective short circuit current. As I have said meters are not that accurate so as long as you get more than 230 amp PSCC then I would expect any inspector to pass the installation.

Quite apart from the fact that you talking about L-N loop impedance and PSCC when we were discussing fault protection (wouldn't L-E loop impedance (EFLI) and PFC be more appropriate?), why those figures? Assuming that we are talking about a B32 MCB, should the inspector not be satisfied with any PFC greater than 160A?

Kind Regards, John