Do tails HAVE to be upgraded?

[guest119]

How did you kill the supply to replace the cu?
What size is the main earthing conductor with the tails?
You say ..tails.. but is it just that or an armour etc?
What is the ZE at the cu (tails length unnknown) and is it within that specified for your proposed fuse (bs88?) at incomer location. ( ZE measurement taken on the supply only, ie before connection to the cu and its associated paralell paths.)

Boydy

 

[whazza22]

Is that what it says on the label?

Yeah, I cannot remove fuse without breaking seal to check if the main fuse is different than stated.

I can ask the person installing the isolator to check.

 

[whazza22]

Also the tails are approx 5 meters from the meter to the consumer unit and run through some plastic pipe.

Is the maximum 3 meters?

 

[Robson689]

Also the tails are approx 5 meters from the meter to the consumer unit and run through some plastic pipe.

Is the maximum 3 meters?

I think it's a maximum of 3 metres without mechanical protection.

 

[Octopus]

I think it's a maximum of 3 metres without mechanical protection.

The blind leading the blind

 

[Robson689]

The blind leading the blind

I'm not blind, but maybe you could post the correct "maximum length" for metre tails. (After all, I only said I think the maximum is 3 metres as I was not sure.)

 

[Deleted member 9648]

I'm not blind, but maybe you could post the correct "maximum length" for metre tails. (After all, I only said I think the maximum is 3 metres as I was not sure.)

There is no maximum length in Bs 7671...but the DNO usually specify a maximum length without an isolator. This varies from 2.5m-5m....check with your DNO.
And it's got nothing to do with mechanical protection.

 

[Engineer54]

There is no maximum length in Bs 7671...but the DNO usually specify a maximum length without an isolator. This varies from 2.5m-5m....check with your DNO.
And it's got nothing to do with mechanical protection.

Isolator or switch fuse??

 

[londonlec]

The amount of time this has been dragging on I could have fitted the damn switch fuse myself and moved to the next job

 

[Deleted member 9648]

Isolator or switch fuse??

Yes correct, switchfuse....should have been specific.

 

[whazza22]

On further inspection, I found that the incoming tails are 16mm and the main earthing conductor is 6mm.

I am going to use the idea of installing a 60a switch fuse which will allow me to continue using the existing tails.

However, is 6mm not a bit small for the main earthing conductor, especially when I will bond gas and water with 10mm?

So my idea is to install a 60a switch fuse, upgrade the main earthing conductor to 10mm and leave existing tails. Then bond water and gas in 10mm. Thoughts?

 

[Geoffsd]

Depends on the installation and earthing method.

You will have to work out the necessary csa for earthing and bonding.

 

[electric will]

The blind leading the blind

Who said that?

 

[whazza22]

Depends on the installation and earthing method.

You will have to work out the necessary csa for earthing and bonding.

By adibiatic equation?

Those calculations always show a lot less than you would actually use.

Surely its acceptable to just upgrade main earth and bonding if 16mm tails are protected by 60a fuse

 

[Geoffsd]

Those calculations always show a lot less than you would actually use.

That could be because you actually use a lot more than you actually need.

Surely its acceptable to just upgrade main earth and bonding if 16mm tails are protected by 60a fuse

A lot would agree with you but I don't follow your logic.

 

[whazza22]

That could be because you actually use a lot more than you actually need.


A lot would agree with you but I don't follow your logic.

The reason for not wanting to replace the tails is that it will be a nightmare as the meter is in a communal hallway. Also the run is about 6 meters so will cost a fortune on cable.

 

[richy3333]

That could be because you actually use a lot more than you actually need.


A lot would agree with you but I don't follow your logic.

The reason for not wanting to replace the tails is that it will be a nightmare as the meter is in a communal hallway. Also the run is about 6 meters so will cost a fortune on cable.

But it's not you paying? I don't think cable of that length will cost a fortune, but how big is your fortune?

 

[whazza22]

But it's not you paying? I don't think cable of that length will cost a fortune, but how big is your fortune?

The person I'm doing it for does not want to fork out money if its not required. Times are tough.

It will cost approx £50 just for the cable.

25mm tails are £3 a meter and I need 6 metres of each + 16mm earth.

 

[richy3333]

The person I'm doing it for does not want to fork out money if its not required. Times are tough.

It will cost approx £50 just for the cable.

25mm tails are £3 a meter and I need 6 metres of each + 16mm earth.

I'm afraid these are the type of jobs I walk away from then. If a customer doesn't want it done properly. Pay cheap, pay twice.
At £3 per metre for tails, I'd change your wholesaler, and a 100m of 16mm earth is far less than £1 per metre

 

[telectrix]

16mm tails will happily carry 90A if ref. method C. even more if ref. F, although the osg does not quote a figure for this. if they show no signs of damage, leave 'em be. note on your cert. though that you recommend upgrading to 25mm.

 

[Engineer54]

I haven't the foggiest idea why anyone would want to replace 16mm tails for 25mm on 60/80A supply fuses?? What are you actually trying to achieve?? A 100A supply fuse will need a much lower Ze/Zs to function correctly, i wonder how many of you electricians conducting EICR's and the like, ever check the Zs value against the supply fuse rating, operating requirement??

 

[Geoffsd]

I'm afraid these are the type of jobs I walk away from then. If a customer doesn't want it done properly.

There is nothing 'proper' about doing unnecessary work and charging a customer for it.

We are not talking about shoddy poor work but leaving perfectly adequate conductors in place.

Would you advocate using 4mm for ring circuits or 16mm for cookers 'to be on the safe side'?

What about a flat with a sub main - replace with 25mm?

 

[IzzyS]

I haven't the foggiest idea why anyone would want to replace 16mm tails for 25mm on 60/80A supply fuses?? What are you actually trying to achieve??

Maybe compliance with the supplier's service requirements....

• Connection tails between the consumer’s installation and the meter are supplied by
the consumer. These must be a minimum cross-sectional area of 25mm2 although
larger tails should be provided and may be required dependent on load requirements.
• Test Earth impedance Ze. The MEA will test the earth impedance of the installation
against the following values:-
• Overhead line fed circuit (TT) will require a separate earth to be supplied by
the consumer. The separate earth for this installation must not exceed 200Ω.
The consumer must provide an earth connection with a minimum crosssectional
area of 16mm2 and a suitable test point adjacent to the incoming
service cut-out.
• Underground fed circuit (TNS). The MEA will provide an earth terminal at the
point of connection. The earth fault loop impedance for this connection must
not exceed 0.8Ω for a standard domestic single-phase supply. The consumer
must provide an earth connection with a minimum cross-sectional area of
16mm2.

 

[Geoffsd]

Oh dear. The OSG is WRONG.

Although it does also refer to section 1.3 but then you have to think.

Have a look at BGB Table 4D1A.

 

[Engineer54]

Maybe compliance with the supplier's service requirements....

• Connection tails between the consumer’s installation and the meter are supplied by
the consumer. These must be a minimum cross-sectional area of 25mm2 although
larger tails should be provided and may be required dependent on load requirements.
• Test Earth impedance Ze. The MEA will test the earth impedance of the installation
against the following values:-
• Overhead line fed circuit (TT) will require a separate earth to be supplied by
the consumer. The separate earth for this installation must not exceed 200Ω.
The consumer must provide an earth connection with a minimum crosssectional
area of 16mm2 and a suitable test point adjacent to the incoming
service cut-out.
• Underground fed circuit (TNS). The MEA will provide an earth terminal at the
point of connection. The earth fault loop impedance for this connection must
not exceed 0.8Ω for a standard domestic single-phase supply. The consumer
must provide an earth connection with a minimum cross-sectional area of
16mm2.

Where did you get this numbty quote from?? I don't suppose you even noticed that it completely contradicts itself.

Perhaps you can also enlighten us, as to why a 200 ohm Ra value TT system would warrant anything like a 16mm earth connection?? lol!!

 

[MDJ]

HAHA, yes, I don't think I can ever remember a 16mm2 conductor on a TT system ever TBH, whats the point? LOL

 

[IzzyS]

I'm well aware of the relevant BS7671 regs regarding sizing of conductors.

The point, which I obviously failed to get across, is that BS7671 and the OSG are not necessarily the the only requirements when it comes to the "tails", for example this document here ->

http://www.gov.im/lib/docs/mea/customer/forms/servicerequirementsmarch2009.pdf


I can only imagine they are trying to future proof. IMHO, I don't think it contradicts itself - merely specifies min. 25mm lives and 16mm earth.


Is it reasonable to ignore additional requirements, where relevant, just because they don't fit in with accepted practice?

Reading the above would suggest that supplier will refuse connection if their requirements are not met.

The customer paying for a professional electrician would be less than impressed when they need to rip out the 16mm tails and 10mm earth and start again...

 

[Silly Sausage]

Izzy, your 'Secret Offshore Island Location' appears to be the Isle of Man where you have your own ways off doing things!
In which case, the Electricity suppliers may well have different requirements/regulations than over here.
You have your own Government as well, don't you?

 

[Geoffsd]

I'm well aware of the relevant BS7671 regs regarding sizing of conductors.

but ignore.

I can only imagine they are trying to future proof.

WHY?

IMHO, I don't think it contradicts itself - merely specifies min. 25mm lives and 16mm earth.

For which there is no need.

Is it reasonable to ignore additional requirements, where relevant, just because they don't fit in with accepted practice?

YES. If pointless.

Reading the above would suggest that supplier will refuse connection if their requirements are not met.

They could, but not because your conductors are not larger than necessary.
Did you know the DNOs use 16mm for 100A supplies?

The customer paying for a professional electrician would be less than impressed when they need to rip out the 16mm tails and 10mm earth and start again...

Why would they have to do that?


Question -

You have a supply with an 80A (or even 60A) fuse.
If the existing conductors have a current carrying capacity of more than 80A please state as many reasons as you can think of for increasing the size of these conductors.
Please note: future proofing does not count as if needed it can be done in the future.

Please learn to think logically and not keep trying to justify something which is unnecessary

 

[imago]

There's a lot of talk about DNO requirements regarding tail sizes. I don't know about other people's experiences, but from my own DNOs rarely know what their supply to a house consists of. As an example, a recent customer asked the DNO about their supply (fuse size, earthing, etc) as they wanted a new meter along with a rewire. They didn't need a rewire, it turned out to be BS from a 'quote from the van' merchant. Anyway, the customer was told "there is no record of the earthing arrangements on the computer. Your electrician should be able to tell you." The fuse size was given as 60 amp.

So, long one short. The DNO gave an unacceptably long delay and high price to fit an isolator. As luck would have it, the fuse fell out due to a lack of seals on the morning of the CU change. In that condition it was easy to spot the text "100 amp" just under the "English Electric" name.

16mm original tails fitted with no sign of damage or deterioration. I replaced them with 25mm tails for several reasons, anyone without full knowledge of the checks/calcs involved would think it was compulsory to do so (and I'm too lazy/busy to argue the ----), new tails look better, the DNO will be changing the meter soon (good job too as the seals had fallen off of the tail connector cover too!) and the cost to me was small and thus is priced in with a CU change.

 

[Octopus]

Maybe compliance with the supplier's service requirements....

• Connection tails between the consumer’s installation and the meter are supplied by
the consumer. These must be a minimum cross-sectional area of 25mm2 although
larger tails should be provided and may be required dependent on load requirements.
• Test Earth impedance Ze. The MEA will test the earth impedance of the installation
against the following values:-
• Overhead line fed circuit (TT) will require a separate earth to be supplied by
the consumer. The separate earth for this installation must not exceed 200Ω.
The consumer must provide an earth connection with a minimum crosssectional
area of 16mm2 and a suitable test point adjacent to the incoming
service cut-out.
• Underground fed circuit (TNS). The MEA will provide an earth terminal at the
point of connection. The earth fault loop impedance for this connection must
not exceed 0.8Ω for a standard domestic single-phase supply. The consumer
must provide an earth connection with a minimum cross-sectional area of
16mm2.

This reads like the rules for new builds not upgrades.

 

[Guest55]

This reads like the rules for new builds not upgrades.

exactly.
the dno cant deny a new connection to an already existing installation so theres no reason to automatically fit 25mm tails when doing your CU change on a 60a / 80a
supply.
;-)

 

[soulman]

Interesting thread, i am registered with elecsa, and on an assessment i was showing a partial rewire with a new ccu and the assessor said i hope the tails have been upgraded to 25mm2 and the earthing conductor to 16mm2, (tncs) which i had done, the assessor said they expect all domestic installations to have upgraded tails to 25mm2 & tn installs must have 16mm2 earthing conductor when changing the ccu. On a different occassion, i installed a ccu on tt installation, & used 6mm mpb & 10mm earthing conductor, i was engaged in a argument with another electrician stating that i should have installed 10mm2 bonds & 16mm2 earthing conductor. I phoned the tech helpline when he was there and they said, 6mm to be used for bonds & earthing conductor. I have always been taught tails over 3m fit switched fuse. The OP, if it was going to be a pain installing new tails no matter how close they were to the ccu, i would keep the existing 16mm tails provided they are adaquate for the installation & install a switched fuse.

Cheers

 

[Lilyrose08]

What I tend to do now if the main fuse is 100A and the tails are 16mm and its a nightmare to replace the tails I put an 80A fuse in replace of the 100A which makes the tails acceptable as 16mm can carry about 87A. Job done

 

[Octopus]

What I tend to do now if the main fuse is 100A and the tails are 16mm and its a nightmare to replace the tails I put an 80A fuse in replace of the 100A which makes the tails acceptable as 16mm can carry about 87A. Job done

Hum, but you shouldn't be messing with DNO equipment should you!!

 

[amlu]

wait for the dno to fit isolator, see if they are happy with existing tails, if they are leave it alone.

 

[Engineer54]

The customer paying for a professional electrician would be less than impressed when they need to rip out the 16mm tails and 10mm earth and start again...​

​

You could also make the argument, that the customer is paying for a professional electrician to KNOW the difference between what is and what isn't required. And not be billed for something they don't need or require!! lol!!

Bunging in 25mm tails on a 60A supply is lunacy, and what i'd expect from a electrical trainee that doesn't know any different, and that has to get he's clues from a guidance based book, that expects/assumes an understanding at least, of electrical theory!!!!