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Topic Title: small consumer unit tail sizing?
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Created On: 23 August 2013 11:39 AM
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 23 August 2013 11:39 AM
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aatomic69

Posts: 20
Joined: 23 August 2013

Hi
I was wondering if there is any regulation on tail size to small consumer unit loads. eg.

A standalone 40amp mcb unit
is it min 16mm or can you fit 10mm tails?

The reason I ask is because I change meters and at work if there is any 10mm tails we have to fit an isolator so the customer can get them upgraded.

This is even when there is a standalone unit with with 40amp mcb and 10mm tails.

We also have to fuse the service head at 60 amp.

I can understand this if there was a consumer unit with 10mm tails and a load in excess of 60amps.

Any thoughst guys?
 23 August 2013 12:39 PM
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bartonp

Posts: 49
Joined: 28 September 2009

I guess they're just covering the likelyhood that the CU will get upgraded at some point...
 23 August 2013 12:43 PM
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jcm256

Posts: 1841
Joined: 01 April 2006

No time to look up any regulations (If any), Very good of you to fit an isolator, is your tails min 16mm or 25mm from the meter to the isolator. my thoughts are allowances should be made by the consumer for load growth, so 10mm direct in to your meter would not allow much scope. Anyhow, 10mm direct to the meter for that good sound connection you would need to use ferrules.

What about tails over 3M disappearing into the bowls of the building, would you also fit a free main switch. Only a question of no significance (nosy) is you a smart meter installer.

Sorry not much help.
jcm
 23 August 2013 01:50 PM
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aatomic69

Posts: 20
Joined: 23 August 2013

Any 10mm tails on the service provider side are upgraded to 25mm unless older style service heads where only 16mm tails will fit.

We can not however renew customer tails to consumer units.
This is dealt with by fitting an isolator and renewing all tails on service providers side and connecting customer tails to the out going side of isolator, ready for customer upgrading.

Sub mains and 3M+ switchfuses are customers responsibility.

My original question however was aimed at if there were any regs regarding small single fuseway consumer units and tails under 16mm.

Typically a customer can have a 100amp 9 way consumer unit with 25mm tails and an additional single way 40 amp mcb or fused consumer unit tailed with 10mm.

I don't see any saftey issues with the 10mm tails in this scenario.

ON an electrical safety inspection would a customer be advised to upgrade these 10mm tails??
 23 August 2013 07:51 PM
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geoffsd

Posts: 318
Joined: 15 June 2010

Originally posted by: aatomic69
I was wondering if there is any regulation on tail size to small consumer unit loads. eg.

A standalone 40amp mcb unit
is it min 16mm or can you fit 10mm tails?


Appendix 4. among others.
 25 August 2013 09:39 PM
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antric2

Posts: 1049
Joined: 20 October 2006

Originally posted by: aatomic69

Any 10mm tails on the service provider side are upgraded to 25mm unless older style service heads where only 16mm tails will fit.



We can not however renew customer tails to consumer units.

This is dealt with by fitting an isolator and renewing all tails on service providers side and connecting customer tails to the out going side of isolator, ready for customer upgrading.



Sub mains and 3M+ switchfuses are customers responsibility.



My original question however was aimed at if there were any regs regarding small single fuseway consumer units and tails under 16mm.



Typically a customer can have a 100amp 9 way consumer unit with 25mm tails and an additional single way 40 amp mcb or fused consumer unit tailed with 10mm.



I don't see any saftey issues with the 10mm tails in this scenario.



ON an electrical safety inspection would a customer be advised to upgrade these 10mm tails??

Hiya aatomic,
Have a read of my reply on 25th in your initial OP.
your 1st OP has been edited out but your question was from a regs point of view and you should look at the reg to do with the cable capacity and its protection.
10mm is fine for 60A?????.............but it is not compliant if protected by 80A or 100A fuse because it is on load side of meter.
It is about blame and traceabillity.In the real world 10mm will take alsorts but your question was from a reg point of view.
Regards
Antric.
 26 August 2013 01:11 AM
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alancapon

Posts: 5746
Joined: 27 December 2005

Originally posted by: aatomic69
Any 10mm tails on the service provider side are upgraded to 25mm unless older style service heads where only 16mm tails will fit. . .

If the DNO specifies 25mm² tails, then that is what I would expect you to do.

. . . We can not however renew customer tails to consumer units. . .

We have "Part P" registration to allow us (a DNO) to work on the customer's side of the metering.

. . . I don't see any saftey issues with the 10mm tails in this scenario. . .

Your employer probably does, as it would be against DNO policy. It would therefore be allowed at your / your employer's risk. I somehow cannot see that being acceptable.

Regards,

Alan.
 26 August 2013 04:55 PM
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jcm256

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The above advice is good. It would therefore be allowed at your / your employer's risk.

Surely, the Metered Supply Point is the point at which the Distribution Network Operator's supply cable terminates, and the customer's equipment begins. What do you do when the meter is located in an outside meter box, do you have to get a search warrant to enter the house and check any reduction branch off in meter tails?

In order to comply with 533.35 installers will have to ensure that the main isolator located in the meter cabinet is rated at least IP55 and constructed of a none - conducting material (TT supply and with all supplies) (also in case the meter box lid is left open or broken). You would need to sit on your toolbox pondering various things with this simple modification. Is your isolator single pole or double pole, might open more pondering make before break or not breaking the neutral all that sort of thing.

Is this relevant to what you do.

Since 1998 there has been full competition for meter operators, allowing the meter operator for a particular supply to be contracted with the energy supplier by either the supplier's discretion or at the customer's direction. Consumption data from the installed metering is then collected by the appointed data collector to be submitted for billing.

Whom do you work for the DNO or the customer?

I suppose you are not fitting a new circuit therefore, A MWC is sufficient.

But at sometime in the future a EICR will take place and therein a check is made:

ISTRIBUTOR'S / SUPPLY INTAKE EQUIPMENT

Service cable condition

Condition of service head

Condition of tails - Distributor

Condition of tails - Consumer

Condition of metering equipment

Condition of isolator (where present)

This post is only some thoughts meant to be helpful, you should ask your Engineer supervisor for electrical advice.
Look at 433.2.1, does your modification come under 433.2.2.
 26 August 2013 09:39 PM
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aatomic69

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Originally posted by: alancapon

Originally posted by: aatomic69

Any 10mm tails on the service provider side are upgraded to 25mm unless older style service heads where only 16mm tails will fit. . .


If the DNO specifies 25mm² tails, then that is what I would expect you to do.



. . . We can not however renew customer tails to consumer units. . .


We have "Part P" registration to allow us (a DNO) to work on the customer's side of the metering.



. . . I don't see any saftey issues with the 10mm tails in this scenario. . .


Your employer probably does, as it would be against DNO policy. It would therefore be allowed at your / your employer's risk. I somehow cannot see that being acceptable.



Regards,



Alan.




How would this be against DNO policy (10mm customer consumer tails?)

The DNO's responsibility as far as I'm aware finishes at the service head (ESQCR)
MOCOPA is the regulation regarding the metering side of things.

You can't physically install 25mm tails in some older style service heads. Hence 16mm tails fitted. (these will always be fused at 80amps or below)

Where is reg 533.35 ??

My Question is still the same
I was wondering if there is any regulation on tail size to small consumer unit loads. eg.

A standalone 40amp mcb unit
is it min 16mm or can you fit 10mm tails?

The service head would have to be fused at 60amp.

But as long as appendix 4 is satisfied?

Edited: 26 August 2013 at 09:50 PM by aatomic69
 26 August 2013 11:54 PM
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alancapon

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Originally posted by: aatomic69
. . . How would this be against DNO policy (10mm customer consumer tails?)

The DNO's responsibility as far as I'm aware finishes at the service head (ESQCR) . . .

The fusible link in the cutout is the property of the DNO, as is the design of the network supplying it. As the DNO's fusible link is protecting the meter tails up to the consumer unit(s) and the metering equipment, their regulations govern the size and length of tails that it will protect. Most DNOs specify 25mm² tails with a maximum length of 3m before customer provided overcurrent protection is required.

. . . My Question is still the same

I was wondering if there is any regulation on tail size to small consumer unit loads. . .

The regulation will be in either the LV Design Guide or the LV Service Requirements Guide for the particular DNO.

Regards,

Alan.
 27 August 2013 09:59 AM
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AJJewsbury

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Joined: 13 August 2003

A standalone 40amp mcb unit
is it min 16mm or can you fit 10mm tails?

Normally you'd just apply the normal BS 7671 rules for conductor sizing - i.e. you have to consider current carrying capacity, overload and faults. Obviously 10mm2 will be fine for current carrying capacity and the downstream 40A MCB will protect it from overload. The tricky bit is fault protection - since the upstream (DNO) fuse is (probably) rated higher than the cable you can't assume that it will be protected under all circumstances - you need to do a bit of calculation to check (the adiabatic equation) - for which you'll need to know the worst-case (lowest) fault current (I), fuse opening time at that current (t)) and the heating characteristics of the type of cable (the "k" value - usually 115 for live PVC/PVC tails) and then show that sqrt(I²t)/k is no larger than 10mm².

Alternatively you can try and arrange things so that fault protection isn't required for the tails - reg 434.2.1 - but that normally requires reinforcing the protection for the tails and taking steps to reduce the fire risk. Normally it's easier just to put in a larger conductor.

But as Alan says, the DNO usually have their own requirements for customer's conductors that are protected by their fuse so that they know how they can change things (network characteristics or the cut-out itself) without having to mess about with the detailed calculations themselves.

- Andy.
 27 August 2013 11:47 PM
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weirdbeard

Posts: 1456
Joined: 26 September 2011

Originally posted by: aatomic69

Hi

I was wondering if there is any regulation on tail size to small consumer unit loads. eg.




Any thoughst guys?


There's a whole section in the regs relating to CCC. etc
Statistics

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