Upgrade tails on CU replacement?

[Mike Johnson]

Wonder how long it will be before all the Screwfix £70 Dual RCD boards will be Type A?

Not until they have sold all the in stock boards with the Type AC's in them.

 

[Ian1981]

Another think to consider - Did you upgrade the main earth conductor from the MET?
It was/is common for 16mm tails to have a 10mm earth, but for PME I believe the regs state it must be 16mm.

If it's TN-S then maybe have the calculation ready if you are asked - or say that you've done it, with a confident look!
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Wonder how long it will be before all the Screwfix £70 Dual RCD boards will be Type A?

The regulations do not state that you must have a 16 mm earthing conductor, it can be sized by calculation, regardless of the line conductor but if pme conditions apply , needs to be at least 10 mm( domestically speaking)

 

[Dustydazzler]

The regulations do not state that you must have a 16 mm earthing conductor, it can be sized by calculation, regardless of the line conductor but if pme conditions apply , needs to be at least 10 mm( domestically speaking)

I always thought it was 16mm to avoid the need for mechanical protection

 

[Ian1981]

I always thought it was 16mm to avoid the need for mechanical protection

No, it’s sized upon table 54.7 or by the adiabatic equation.
If buried etc then other considerations for CSA apply.
New installations like new build houses, the DNO stipulate 25 mm tails and a 16mm earthing conductor.

 

[HappyHippyDad]

Hi. I have my NAPIT first assessment soon and I replaced a CU in a house which has 16mm2 consumer tails in place. I did not upgrade the tails to 25mm2, so my question is should I have upgraded them or am I allowed to leave the existing tails in place?
I do not know the rating of the DNO cut-out fuse
Yes the cut-out does have 100A stamped on the fuse carrier but that means nothing
Installation is 25 years old

Thanks
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Also it was a Lewden RCBO board and I brought the cables through the knockout in the rear, am I required to use fire sealant in the rear section?

Hi Andrew,

Your assessor may or may not have a moan about the 16mm tails, each assessor seems to be different. Don't worry though, absolute worst case scenario is that you will have to upgrade to 25mm and that is unlikely.

I assess if 16mm tails need upgrading by looking at max demand and seeing if that is in excess of the CCC of 16mm cable. Also I check to see if it meets regulation 434.5.2, i.e protection of tails against fault current.

You could even show the above to the assessor as it is written in Best Practise Guide number 1 (Replacing a consumer unit) issue 4, section 5.1 note 4 as follows:

"16 mm² meter tails with a 100 A cut-out fuse could be adequate if the maximum demand of the installation (taking into account diversity and load profile) does not exceed the current-carrying capacity of the tails, and also provided that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met."

You'll have to brush up on K² S² > I²t just in case he asks you to prove it meets the regulation though

Good luck

 

[Dartlec]

The regulations do not state that you must have a 16 mm earthing conductor, it can be sized by calculation, regardless of the line conductor but if pme conditions apply , needs to be at least 10 mm( domestically speaking)

Agreed that it can be calculated. I thought there used to be a specific PME reg, but apparently not currently - that is only when applying to bonding conductors.

Table 54.7 says that for a cross sectional area of Line conductor S up to and including 16, the protective conductor must also be S (if same material), so if not calculating and using that table 10mm earth would only be allowed on 10mm tails, and 16mm required on 16mm tails (then 16mm until S > 35)

 

[Ian1981]

Agreed that it can be calculated. I thought there used to be a specific PME reg, but apparently not currently - that is only when applying to bonding conductors.

Table 54.7 says that for a cross sectional area of Line conductor S up to and including 16, the protective conductor must also be S (if same material), so if not calculating and using that table 10mm earth would only be allowed on 10mm tails, and 16mm required on 16mm tails (then 16mm until S > 35)

Yes by design however if it’s existing there’s very little chance that a 10 mm earthing conductor will not satisfy the adiabatic so does not require to be changed.

 

[andrew8902]

Thank

Hi Andrew,

Your assessor may or may not have a moan about the 16mm tails, each assessor seems to be different. Don't worry though, absolute worst case scenario is that you will have to upgrade to 25mm and that is unlikely.

I assess if 16mm tails need upgrading by looking at max demand and seeing if that is in excess of the CCC of 16mm cable. Also I check to see if it meets regulation 434.5.2, i.e protection of tails against fault current.

You could even show the above to the assessor as it is written in Best Practise Guide number 1 (Replacing a consumer unit) issue 4, section 5.1 note 4 as follows:

"16 mm² meter tails with a 100 A cut-out fuse could be adequate if the maximum demand of the installation (taking into account diversity and load profile) does not exceed the current-carrying capacity of the tails, and also provided that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met."

You'll have to brush up on K² S² > I²t just in case he asks you to prove it meets the regulation though

Good luck

Hi Andrew,

Your assessor may or may not have a moan about the 16mm tails, each assessor seems to be different. Don't worry though, absolute worst case scenario is that you will have to upgrade to 25mm and that is unlikely.

I assess if 16mm tails need upgrading by looking at max demand and seeing if that is in excess of the CCC of 16mm cable. Also I check to see if it meets regulation 434.5.2, i.e protection of tails against fault current.

You could even show the above to the assessor as it is written in Best Practise Guide number 1 (Replacing a consumer unit) issue 4, section 5.1 note 4 as follows:

"16 mm² meter tails with a 100 A cut-out fuse could be adequate if the maximum demand of the installation (taking into account diversity and load profile) does not exceed the current-carrying capacity of the tails, and also provided that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met."

You'll have to brush up on K² S² > I²t just in case he asks you to prove it meets the regulation though

Good luck

Thanks for your reply. WPD are coming out today to confirm the type/rating of the cut-out fuse, which he suggested on the phone that they never fit 100A.
The tails have been in situ since 1995 and there are no signs of thermal damage, also I have examined with my FLIR E4 camera for hot spots. Maximum demand is around 50A
Cheers