T + E How is it measured?

[The Medway Handyman]

Am I right in assuming that when T + E cable is referred to as 2.5mm that is the diameter of the bare conductor?

Is there a chart anywhere that gives the outside dimensions including all the insulation?

[Colin Wilson]

It's the cross-sectional area of the conductor...

[Clive George]

Bugger me. You're one of the last people I'd expect to be asking that question. Surely you've seen enough 2.5 T+E over the past few years, even if you're not doing electrical bits, for the answer to be obvious?

(area, as Colin said, coz that's the relevant bit for resistance/current carrying capacity)

cheers, clive

[Bob Eager]

It's the cross sectional area of the main conductors, but probably not the CPC ('earth').

[nightjar]

No. It is the cross-sectional area. Single strand 2.5mm^2 conductors will be about 1.8mm diameter.

The cable manufacturers used to do data books giving that information, but I usually just get a bit and measure it, if I need to know.

Colin Bignell.

[The Medway Handyman]

I'd rather not, if its all the same to you :-)

Indeed I have, but I hadn't ever thought of it before.

Obvious now I think about it.

I was just after a quick rule of thumb guide so I could measure to outside of the insulation & tell if it was 4mm or 6mm without having to disconnect things & get a vernier out. 2.5mm was just used as an example.

[John]

PLEASE tell us all that you no longer do any electrical work (even making an allowance for Part P sillyness!). THIS is one of the very first things you are taught in 'electrical college' after "DO NOT put the screwdriver in there!!!"

HTH

John

[Andy Wade]

I've posted this before:

CSA (CPC) Strands Overall mm^2 No./dia. mm approx.

--------- ------- ----------

1.0 (1.0) 1/1.13 4.5 x 8.2 1.5 (1.0) 1/1.38 4.7 x 8.2 2.5 (1.5) 1/1.78* 5.3 x 9.9 4.0 (1.5) 7/0.85 6.1 x 11.4 6.0 (2.5) 7/1.04 6.8 x 13.1 10 (4.0) 7/1.35 8.4 x 16.8 16 (6.0) 7/1.71 9.6 x 19.5

• or 7/0.67

[Colin Wilson]

I never said which conductor :-p

[Colin Wilson]

You, sir, are a god :-p

[The Medway Handyman]

The reason I want to spot the difference quickly & easily is so I don't get into problem situations. I've been asked to remove an old electric cooker and install a new one. The new one is 9kw. Before I get involved I want to check that the cable is big enough. If not I will turn the job down.

Its called "if you are not sure check it out". Basic common sense.

And don't talk to me about 'qualified electricians' - I've seen more bodge ups & sloppy work from them than anything else.

[Lurch]

On Tue, 04 Sep 2007 00:23:11 GMT, "The Medway Handyman" mused:

Be aware that insulation thickness can vary wildly.

[John Rumm]

CSA of phase/neutral

[John Rumm]

In the grand scheme of things, it makes little difference. As long as you know what size cable you should be using where, whether the units are CSA in mm^2, or diameter in micro furlongs does not make much difference.

I can't believe that if you walked into an electrical wholesalers and said "give me a reel of 2.5mm twin and earth" they would have any difficulty understanding what you meant.

(in fact I can't ever remember asking for cable and going through the verbal gymnastics required to include "squared" in the request)

[Bob Eager]

For the record, in case MH finds it useful...!

Diameter of 2.5T+E is about 8.85 ufurlongs...

[Andy Wade]

A totally OTT remark, Sir, but at least you didn't use a capital G :~)

Anyway, I've previously posted a table of current ratings too. Here are the two tables combined into one, together with some new ratings from the draft 17th edition:

CSA (CPC) Ratings in amperes Strands Overall square mm A B C D+ E+ No./dia. mm approx.

---------- ---- -- -- -- ---- ------- ----------

1 (1) 11.5 12 16 13 10.5 1/1.13 4.5 x 8.2 1.5 (1) 14.5 15 20 16 13 1/1.38 4.7 x 8.2 2.5 (1.5) 20 21 27 21 17 1/1.78* 5.3 x 9.9 4 (1.5) 26 27 37 27 22 7/0.85 6.1 x 11.4 6 (2.5) 32 35 47 34 27 7/1.04 6.8 x 13.1 10 (4) 44 47 64 45 36 7/1.35 8.4 x 16.8 16 (6) 57 63 85 57 46 7/1.71 9.6 x 19.5

• 7/0.67 also available (but rare)

Key to current rating columns:

A - in conduit in insulated wall (ref. method 6) B - directly in insulated wall (ref. method 15) C - clipped direct (ref. method 1)

Ratings (BS 7671 Table 4D5A) apply for ambient temperature of 30 deg.C and conductor temperature of 70 deg.C

• New ratings from draft 17th edition:

D - above plasterboard ceiling covered by thermal insulation, insulation thickness 100 mm

(Technically these are not yet in force and should be regarded as provisional.)

All the usual derating factors apply (see appendix 4 of BS 7671 or appendix 6 of the OSG).

[Andy Hall]

It's the cross sectional area in square millimetres (not dia in millimetres).

[Adrian]

Not directly on topic - but a bit of advice, please...

We have a new heat-pump installation for hot water and central heating purposes - basically a giant fridge that sucks heat out of a ground-loop and pushes it into the circulating water.....

...anyway....

The pump is installed in my 'studio' (=big timber shed !) - and the electrical connection is via a sub-consumer unit in the studio. The power consumption of the pump is (allegedly!- can't find a rating plate on it) 15A - and it's fed from the CU with a spur of

2.5mm cable, clipped to the surface of the OSB that forms the inside wall of the studio..

All works fine - but the cable is warm to the touch - not hot - just warm - maybe 50C - will measure it later if anybody's interested.

Should I be worried that the cable's warm ? Is it worth doubling-up the cable (2 x 2.5mm cables) - or is a warm cable acceptable ??

Thanks Adrian West Cork, Ireland

[Andy Wade]

"Warm" is OK, but I wouldn't expect a clipped direct 2.5 cable running at 15 A to be particularly warm. 56% of the rated current implies about

31% of the permitted temperature rise of 40 deg. i.e. only around 12-13 deg. rise.

If you are worried, hire or borrow a clamp meter (preferably true RMS reading) and measure the actual current.

Simpler to up it to a 4 or 6 mm^2 cable if necessary.

[John Rumm]

How do you "know" its 15A?

Not wishing to be pedantic (but one needs to be in this case, for reasons I will cover), 15A is a current loading and not a power consumption.

It would be interesting to know what the actual temperature is.

Depends on how warm, and probably not for the reasons you expect! [1]

There is a popular misconception that something must be wrong when a cable gets warm. The reality is that the way we deduce the cables current carrying capacity, is based on how much current it will carry at a maximum conductor temperature.

With PVC clad cables the maximum operating conductor temperature is 70 deg C. Cables running at up to this temperature should still enjoy a reasonable and safe productive life. For a 2.5mm^2 cable clipped direct to a surface as you describe, the current that would cause a rise to this temperature is 27A. (it is this notion of a temperature "budget" that means when you install cables in ways that prevent them losing heat as quickly, you need to "derate" the current carrying capacity to take account of the faster rate of temperature rise that will result).

[1] Unless the cable is dangerously hot then there is no direct problem with the cable being warm, however it may suggest that your equipment is drawing far more current than you previously thought.

Given that we are talking about a heat pump which probably has a compressor driven by an induction motor, you may also have to consider the implications of a poor power factor here. i.e. the cable may have to carry a higher current than the power consumption alone would suggest - but that will not necessarily correspond to a higher energy consumption.