1.5mm T&E or 1.0mm T&E?

First-time-buyer · 18 Jul 2015

17th Edition Lighting Reg's - Is it still okay to use 1.0mm?

Local wholesaler had a shortage of 1.0mm compared to 1.5mm, offered 1.5mm for the price of 1.0mm.

I remember 1.0mm was 6A MCB, 1.5mm was up to a 10A MCB (if required)/longer legs.

The house I live in has a lighting circuit in some sections without a CPC, old 1.0mm T&E with no CPC.. #oldskool

DaveHerns · 18 Jul 2015

You can use 1.5 but there's no need on a circuit fused at 6amp

AndyPRK · 18 Jul 2015

While an old large house could benefit from 1.5mm, its a pain to terminate into modern light fittings. That is the main issue.

First-time-buyer · 18 Jul 2015

Thanks all, thanks awarded.

PrenticeBoyofDerry · 18 Jul 2015

1.00mm T&E Is fine to use on a 6A circuit and on 10A circuit providing de-rating factors allow. The other thing to consider is the size of the circuit relating to permitted Zs.
Problem with 1.5mm is if you are installing a loop in at the fittings method, sometimes there are issues with terminal sizes, accommodating three conductors.
PS old T&E will still have CPC in it! If there is enough sheath inside the fitting, you can expose the CPC and lengthen it, or if enough slack on cable, maybe you are able to pull some through.

AndyPRK · 18 Jul 2015

By its name T&E will have a CPC.

Twin red/black won't have a CPC

JohnW2 · 18 Jul 2015

PrenticeBoyofDerry said:
1.00mm T&E Is fine to use on a 6A circuit and on 10A circuit providing de-rating factors allow. The other thing to consider is the size of the circuit relating to permitted Zs.

True, but Zs is virtually never going to be an issue in a domestic property, even with 1.0mm² cable - if one assumes a 'worst case' TN Ze of 0.8Ω, then one could have about 147 metres of 1.0mm² on a B6 MCB.

As eric is always saying, voltage drop is much more of a potential issue than Zs, but I personally find it very difficult to get excited about that! If one calculates on the basis of an entire full 6A load being at the end of a 1.0mm² lighting circuit, one is limited to about 26 metres if one wants to stay within the guidance VD of 3%. However, for multiple reasons, I do not personally think that is something to be concerned about.

PrenticeBoyofDerry said:
PS old T&E will still have CPC in it! If there is enough sheath inside the fitting, you can expose the CPC and lengthen it, or if enough slack on cable, maybe you are able to pull some through.

From the context of the rest of the OP's wording, I think that "T&E" was probably a misnomer - he appeared to be talking about "twin without earth". However, it sounds as if he is probably replacing it, anyway.

Kind Regards, John

PrenticeBoyofDerry · 18 Jul 2015

JohnW2 said:
True, but Zs is virtually never going to be an issue in a domestic property, even with 1.0mm² cable - if one assumes a 'worst case' TN Ze of 0.8Ω, then one could have about 147 metres of 1.0mm² on a B6 MCB.

Yes, but thought I would put my twopenneth in, as I have come across situations flying close to the permitted value.

JohnW2 said:
From the context of the rest of the OP's wording, I think that "T&E" was probably a misnomer - he appeared to be talking about "twin without earth". However, it sounds as if he is probably replacing it, anyway.

Well it is not T&E then! I'll have a bacon & egg butty, no bacon please?

JohnW2 · 18 Jul 2015

PrenticeBoyofDerry said:
JohnW2 said:

True, but Zs is virtually never going to be an issue in a domestic property, even with 1.0mm² cable - if one assumes a 'worst case' TN Ze of 0.8Ω, then one could have about 147 metres of 1.0mm² on a B6 MCB.

Click to expand...

Yes, but thought I would put my twopenneth in, as I have come across situations flying close to the permitted value.

Fair enough - but 147 metres would imply an 'unusually large' domestic building

PrenticeBoyofDerry said:
Well it is not T&E then! I'll have a bacon & egg butty, no bacon please?

Sure - but don't you think that the OP's "T&E with no CPC" was probably referring to a "baconless bacon & egg butty"?

Kind Regards, John

EFLImpudence · 18 Jul 2015

First-time-buyer said:
I remember 1.0mm was 6A MCB,

It is true that lighting circuits usually have a 6A MCB but the maximum current carrying capacity of 1mm² is 16A.

1.5mm was up to a 10A MCB (if required)/longer legs.

1.5mm² has a maximum current carrying capacity of 20A.

PrenticeBoyofDerry · 18 Jul 2015

JohnW2 said:
probably referring to a "baconless bacon & egg butty"?

If they was, how dare they?

ban-all-sheds · 18 Jul 2015

EFLImpudence said:
It is true that lighting circuits usually have a 6A MCB but the maximum current carrying capacity of 1mm² is 16A.

Sometimes cables have to be de-rated.

JohnW2 · 18 Jul 2015

ban-all-sheds said:
EFLImpudence said:

It is true that lighting circuits usually have a 6A MCB but the maximum current carrying capacity of 1mm² is 16A.

Click to expand...

Sometimes cables have to be de-rated.

Indeed - and I presume that is why EFLI (who obviously understands that as well as you do) included the word "maximum" in his statement.

KInd Regards, John

EFLImpudence · 18 Jul 2015

ban-all-sheds said:
EFLImpudence said:

It is true that lighting circuits usually have a 6A MCB but the maximum current carrying capacity of 1mm² is 16A.

Click to expand...

Sometimes cables have to be de-rated.

They do.

I would say my inclusion of 'maximum' covers that but I suppose some would say that surrounded by thermal insulation the maximum is 8A - although it would also be true that this is not the maximum CCC of the cable.

The OP implied that he thought 1mm² and 1.5mm² were related to 6A and 10A (MCBs) respectively and no one had explained otherwise.

ban-all-sheds · 18 Jul 2015

Was it a ROT which ensured that no matter what you did, installation wise, a 1mm² cable would always have Iz>In?

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1.5mm T&E or 1.0mm T&E?

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