Horrible thought

[ban-all-sheds]

FFS, FFS, FFS:

Given the length of the cable - 50m and the diameter 10mm

 

[David E]

What is FFS? I hope it's not rude!!!!

OK I found FFS.

I am now a bit confused could you explain the problem. On the TLC cable size calculator. The 10mm SWA is good for 48.26A can you explain the 21A to me. Sorry to appear dim!!

David

 

[HDRW]

I am now a bit confused could you explain the problem. On the TLC cable size calculator. The 10mm SWA is good for 48.26A can you explain the 21A to me. Sorry to appear dim!!

You're confusing diameter with cross-sectional area. The diameter of a made-up cable varies with the size of the cores, the number of cores, the thickness and type of insulation.

The cross-sectional area is that of a single current-carrying conductor and is expressed in square millimetres (abbreviated mm2, sqmm, mmsq, mm^2 or whatever). As a very rough first approximation, copper wire can carry about 10 Amps per square millimeter - but don't ever rely on this because lots of other factors affect it, including how hot it's allowed to get and what contributes to keeping that heat from escaping.

The diameter (in mm) of the whole cable does not directly affect the current-carrying capability, because things like the thickness of the insulation, presence of steel wire armour, and so on, affect the diameter of the overall cable without changing the cross-sectional area of the conductors.

The problem is that people get lazy and use a "shorthand" in many walks of life, and in electrics it's common practice to omit the "square" and just talk about 2.5mm - but it's still the cross-sectional area that's being discussed. So when you suddenly start talking about 10mm diameter, people assume you mean the outside diameter of the whole SWA cable, when you probably (in my opinion!) meant 10mm2 conductors.

Just for a giggle, and in an attempt to help possible misuderstandings, I present my use of words relating to cables:

Strand = a single piece of copper, a number of which may be twisted together to form a wire
Wire = a single conductor (insulated or not) that carries a single current., consisting of one or more strands
Core = a wire within a cable
Cable = a number of cores put together in a single insulating sheath, which may have extra insulation and/or armour or other protection covering all the cores

So for example a "2.5 T&E" cable (2.5 square millimetre, Twin and Earth) will typically have two single-strand cores that are individually insulated and whose conductors are 2.5mm2 in cross-sectional area, and one uninsulated single-strand Earth core that is 1.5mm2, with an insulating sheath that covers them all.

("4mm T&E" on the other hand has two seven-strand cores, and the total cross-sectional area of the seven is 4mm2. The Earth core is still single-strand and is, I think, 2.5mm2)

Cheers,

Howard

 

[ban-all-sheds]

You said you had 10mm cable. Cable sizes refer to the cross-sectional area of the conductors, i.e. "10mm SWA" means the csa of each of the cores is 10mm².

It's very common for people to write 10mm instead of 10mm² because they can't be @rsed to copy and paste the ² into their post. So when you started talking about "10mm SWA" I assumed you meant "10mm² SWA".

But then you said that your cable had a diameter of 10mm. (Which, it turns out, it doesn't, so it baffles me as to why you said it did).

A SWA cable with a diameter of about 10mm would not be 10mm² cable, it would be 1.5mm². But your cable is about ¾" OD which is the size of 10mm² SWA, so panic over. (It's also pretty close to 6mm² cable, which is only 0.06" smaller in OD. If you know your cable is 10mm² fine, but if you're going by the OD, take a caliper gauge to one of the cores just to make sure)

BTW, not that it makes any difference, I hope, but over that distance 10mm² SWA is not good for 48.26A because of volt drop. If you use the TLC calculator and tell it 3-core SWA it does 3-phase calcs. You need to tell it you've got 2-core cable. You should also specify PVC, even if you've got XLPE, or it will use the capacities of cables running at conductor temperatures of 90°C - too hot for most accessories. Using the PVC figures will make the calculator assume 70°C temperatures.

 

[David E]

Many thanks to you both for explaining several points, I realize my terminology was lacking in clarity.

Actually the cable is probably closer to 40m than 50m, but in any case I now know that it really should have a breaker to protect the cable, so I will get that sorted.

A number of points clarified in this thread. thanks to all.

David

 

[Lectrician]

//www.diynot.com/forums/viewtopic.php?p=170018#170018

We R all 2 lazy to put the sq symbol after the size - we are all guilty!!

Have pasted it into the for-ref section more for my use than anyone elses!!

Cheers BAS

 

[bonebill]

See what you mean. Given the length of the cable - 50m and the diameter 10mm,

Cables are normally identified by the cross-sectional area of the conductor, measured in mm² (not mm). (For Twin & Earth cable, the dimension is the CSA of each of the live and neutral conductors; the earth conductor often has a smaller CSA.)

If you do mean 10mm² conductor cross-sectional area then it would best to avoid the word "diameter".

the max current would be around 48A. So 50A breaker should protect the cable before it reaches the CU and also, not that I'm going to do it again, should it br dug up it will not blow the main fuse!!!!!. Am I thinking along the right lines?

I am not an electrician, but I am confident one of the regular contributors will point out if any of the following is dangerously untrue.

In general, before any run of cable, you need to have an overcurrent protection device. This device should normally be rated no higher than the current capacity of the cable.

(This can be seen in any normal installation: meter tails are protected against overcurrent by the service fuse, premise wiring is protected by the fuses or MCBs in the CU, appliance flexes are protected by the fuses in the plugs or FCUs.)

The current capacity of the cable needs to take into account the length and type of the cable, and the environment the cable is installed into.

Assuming you are right about the rating of your cable, then your description would appear to be at odds with normal practice because your overcurrent protection device would rated higher than your cable.

It sounds like you have quite a hefty cable, which would tend to imply quite a bit of current being drawn, and the length is fairly substantial as well, it's an underground cable, it might involve water (pumps?), and it's not one of the simple stock cases like domestic ring mains or lighting circuits, and it sounds like it might not wired up properly.

Sounds like a case for getting it checked by a sparky.

 

[Adam_151]

We just won't mention rec service cables here, nor Lec's dispute with NICEIC over the hand driers...

But it is very good practice to have overcurrent protection at the start of the cable

 

[bonebill]

It's very common for people to write 10mm instead of 10mm² because they can't be @rsed to copy and paste the ² into their post. ... But your cable is about ¾".

How to get various useful symbols on a standard Wintel PC running Windows assuming ISO-8859-1 land:

Squared ² ALT+0178
Cubed ³ ALT+0179
Quarter ¼ ALT+0188
Half ½ ALT+0189
Three-quarters ¾ ALT+0190

I think I've managed to get Squared and a Half down pat. Various others I get using trial-and-error or by resorting to the Character Map program.

 

[Lectrician]

What about the deg??

 

[Adam_151]

What about the deg??

Alt + 0176

 

[bonebill]

What about the deg??

Degree Sign ° ALT+0176 (not to be confused with the Masculine Ordinal Indicator º ALT+0186)

So we have:

0°C = 32°F = 273.15 K

Try not to run your cables at 45°

Other useful ones:

Plus-Minus ± ALT+0177
Micro Sign µ ALT+0181
Multiplication × ALT+0215
Division ÷ ALT+0247

 

[David E]

Theres me starting a thread having thought I had made a cockup and finding I hadn't, then finding I had and got good advice on fixing it once we'd worked out what I meant, now the topic of conversation has got round to making posts clearer by properly expressing ones meaning ie 10mm² or 45° etc. That proves something good can come out of any situation (most of the time any way).

David

 

[Agile]

Some people think that a buried SWA cable should be fed from an RCD to protect against the onset of cable failure due to water penetration.

Others think a fuse or MCB is better.

Tony Glazier

 

[HDRW]

Some people think that a buried SWA cable should be fed from an RCD to protect against the onset of cable failure due to water penetration.

Others think a fuse or MCB is better.

A number of years ago my house had a cable that ran from the kitchen to the shed (ten feet away) then on to the garage at the bottom of the garden. It was mostly in ordinary T&E, clipped to the garden wall (I didn't do it!) and there was a plug on a flex in the kitchen to plug it in when needed. One day I got a plug-in RCD adaptor, and plugged in the garden cable. The RCD tripped immediately! I got a meter out and set up to measure the current - there was 1.5A or thereabouts leaking somewhere... the fuse didn't blow, and an MCB wouldn't have either - only the RCD stopped what could have been a fatal accident - an amp and a half is certainly enough to kill.

I never did find out where the "leak" was, I just ripped the cable out and didn't use it any more.

I know a piece of manky T&E strung outside for years is rather different from SWA properly installed, but it shows that there are cases where an RCD detects an outdoor cable problem where an MCB wouldn't.

Cheers,

Howard