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# Minimum earth for tncs

Discuss Minimum earth for tncs in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

#### Ampo

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Hi all, need some help as I'm having a blank. Replacing a consumer unit on a tncs that currently has a run of 2.5 metre tails
16mm tails and 10mm earth. I don't want to waste money for no reason replacing them if not necessary, I know a lot of people would fit 25mm and 16mm earth but the supply is a 60a carrier soni cant see a problem. The issue in having is whether the 10mm earth is ok? I'm looking at table 54.7 and to be honest I'm getting confused with it. Looks to me like its saying it can be less than 16mm but should surely give a size?

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#### telectrix

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perfectlyOK. the main earth must be equal,or greater than 50% of the N tail, so 16mm with 1mm is fine, lomuited by ther supply fuse. in this case 60A.

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Arms
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You not feeling the romantic filter I put on there Tel?

#### westward10

Mod
Using Table 54.7, up to and including 16mm will require a conductor of the same csa as that of the line conductor. Alternatively you can use the adiabatic equation in 543.1.3

#### Julie.

Arms
It's a bit of an unknown actually, Main protective bonding dictates in table 54.8 that a 10mm^2 conductor is OK for a PEN/CNE up to 35mm^2; however table 54.7 dictates the use of the same size protective conductor up to 16mm^2 (unless you use the adiabatic equation).

This contradiction in the standard leads to on-site guide etc sticking with 16mm^2 minimum to make sure!

BTW the concept of 50% CSA is only: "except where PME apply.."

#### Wilko

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My read of it- happy to hear if I’ve misunderstood

If selected - Table 54.7 says to use 16mm

If calculated from Adiabatic Eqn 543.1.3 then because it’s also TNCS/PME then 544.1.1 refers us to Table 54.8 which says it can’t be less than 10mm.

So if the equation says 10mm is ok, then I think it’s ok.

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Put a 35mm just to be safe.....your welcome

#### telectrix

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It's a bit of an unknown actually, Main protective bonding dictates in table 54.8 that a 10mm^2 conductor is OK for a PEN/CNE up to 35mm^2; however table 54.7 dictates the use of the same size protective conductor up to 16mm^2 (unless you use the adiabatic equation).

This contradiction in the standard leads to on-site guide etc sticking with 16mm^2 minimum to make sure!

BTW the concept of 50% CSA is only: "except where PME apply.."
i stand corrected. get your whip out.

#### Ian1981

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Table 54.7 is so you do not need to apply any calculations for your protective conductor size. As the earthing arrangement is TNCS, then table 54.8 should be applied for the protective bonding conductor CSA in relation to the PEN conductor (absolutely nothing to do with the Neutral tails csa).
as your existing earthing conductor is 10mm then this will most likely be adequate for the size required as an earthing conductor (adiabatic equation) and meets the size required as a bonding conductor

#### Julie.

Arms
i stand corrected. get your whip out.
That's a different hobby!

Oops, might have said too much!

#### Ampo

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Thanks guys, I completely missed the
adiabatic equation bit so will dust off the calculator and try and work that formula out

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