Cable Size Calcs..... too much !!!

sasb2003 · 6 Feb 2005

Okay, so i'm putting in a new circuit (single cable, not ring!) to the old WC at bottom of garden in order to power a ceramics kiln i have.
14M indoor cabling, 16m outdoor.
Kiln is 3.75 KW = 16.3A

So if i do a Voltage drop calc based on 2.5mm cable:
30M * 16.3A = 8.2V : Drop = 3.62%

Question is, is it okay to do one calc for the indoor cable and another for the outdoor cable ? if i split the 2.5mm indoor cable & 4mm outdoor SWA :
Drop = 2.76%

... which means I have another 7 Amps to spare and could install a light !!??

Or even (being greedy now) .. is there anything against me installing a 3 pin socket to supply <= 5 amps ? On the one hand i feel like a cheat with such a plan, it doesn't seem right to fiddle things so.... but theoreticaly i can't fault it !?!?

... lastly (now being very greedy with questions !) ... should i worry about adding in calculations such as corrections for where my indoor cabling is in a conduit alongside other cables (i.e wall chases) , or where it squeezes through a 16mm hole i drilled for it to get through a RSG (for all of 5-10 mm)??

.. or are you going to tell me to fit 4mm cable all the way through and stop being a Scrooge ?

Sorry to ask so many questions in one posting .... but hey, if you don't ask, the electrician will only tell you for £50 when he comes to 'approve' your installation, then another £50 when he comes again after you fixed it all (nothing against electricians .... i'm just skint !!)

Many many thanks
Simon

Damocles · 7 Feb 2005

Voltage drop is actual voltage drop. So if the total run is made up of ten different sized cables, then you should calculate voltage drop for each one separately and then add them up.

Design current is design current. How much current the circuit is designed to supply. If it only has a kiln hard wired into it, then that is the design current. If there is a socket on the end too, then you need to consider how much current you will really want to draw.

If you are never likely to use the kiln and socket at the same time, then I see no reason to design on the assumption that you will. Circuits are normally designed on the assumption that the owner will not use the maximum which could be taken if everything is running at once.

Having said that, you are only going to want to do this once, and you definitely will want to use quite a bit of current for the kiln. So it might be a good idea to use 4mm throughout anyway. Keep everyone happy.

You should include any corrections for cable size which apply. 5-10mm is not considered thick enough to count as needing correction. It is about heat loss from the cable, and a short enclosed section will make little difference.

paulh53 · 7 Feb 2005

sasb2003 said:
or where it squeezes through a 16mm hole i drilled for it to get through a RSG (for all of 5-10 mm)??

Oh No.....You drilled through an RSJ, That's Naughty,

sasb2003 · 7 Feb 2005

.. is that bad then ?
i saw the original cabling was going through it (several holes had been drilled togethert to get 6 existing cables through), but there was no room left to get my new cable through. To go over it would have put the cable right under the carpet surface, to go under would have been through the lower floor ceiling ...

How bad have i been ? Should i sleep safe at night ?

paulh53 · 7 Feb 2005

I take it that you drilled through an "RSJ" (Rolled Steel Joist)? ie: a Steel girder,
Or was it an ordinary wooden floor joist?

sasb2003 · 7 Feb 2005

hmm, maybe i used the wrong term .... i meant a reinforced steel joist ... a big metal "I" shaped bar running from one end of the house to the other, holding the floor up i presume !!

ban-all-sheds · 7 Feb 2005

Look in Yellow Pages under "Structural Engineers".

Get one round asap. RSJs are not supposed to have holes in. That said, they will be over-engineered - witness the fact that yours already had holes and had not failed yet.

So now you've added another hole.

And it has not failed.

Yet.

BTW - I hope you protected the cable where it passed through the hole..

plugwash · 7 Feb 2005

i know there are drilling allowances in the specification of wooden beams/joists and i presume there must be similar rules for the specification of RSJs

sasb2003 · 8 Feb 2005

thank you Plug !! I shall sleep better tonight !

Last question...

Was planning to connect all this to a new 45A 1-way Wylex consumer unit, fit a 100A DP connection block, then get the elecy board to fix tails (25mm) from main supply to connection block, and tails from block to old and new consumer unit. (old unit capacity 80A, only 65 in service).

Do i need to fit my Wylex with a RCBO or will a MCB be sufficient ?
My outdoor cabling has an earth sheath which connects to the indoor earth so does that 'do the job' or does the distance and outdoor environment mean i have to have an RCBO anyway ? Any idea what the IEE regs say ?

Promise that's the last Q ! (and promise no more drilling through RSJs !!)

Cheers

ban-all-sheds · 9 Feb 2005

plugwash said:
i know there are drilling allowances in the specification of wooden beams/joists and i presume there must be similar rules for the specification of RSJs

Presumably. And presumably if you're right the RSJ will presumably not fail.

But it can't be easy to drill big holes in RSJs, so maybe another presumably is that the makers presume nobody will.

Might be worth asking on the Building forum....

ban-all-sheds · 9 Feb 2005

sasb2003 said:
thank you Plug !! I shall sleep better tonight !

Last question...

Was planning to connect all this to a new 45A 1-way Wylex consumer unit, fit a 100A DP connection block, then get the elecy board to fix tails (25mm) from main supply to connection block, and tails from block to old and new consumer unit. (old unit capacity 80A, only 65 in service).

Good luck. Given that everything after the meter is "yours", be prepared for them to say no. And if they do have an engineering/services organisation that will do it, be prepared for them to charge more than a regular electrician will....

Do i need to fit my Wylex with a RCBO or will a MCB be sufficient ?

You really should have RCD protection.

My outdoor cabling has an earth sheath which connects to the indoor earth so does that 'do the job' or does the distance and outdoor environment mean i have to have an RCBO anyway ?

I'm starting to get the feeling that you don't really know enough. Firstly, of course the outdoor environmnt means that you should have RCD protection. Secondly, fitting an RCD to get round a high earth fault loop impedance is really tacky, IMHO. Thirdly, please don't say that you've installed two-core SWA and are using the armour as the cpc. Have you terminated it correctly? Do you know what R1+R2 is for the circuit? Do you know what your Ze is? Do you know if you will be able to meet the disconnection time? Do you know what sort of earthing your supply has, and if it's OK to just extend it to the bottom of the garden?

Any idea what the IEE regs say ?

All sorts of things....

Promise that's the last Q !

I fear not....

Big_Spark · 9 Feb 2005

A couple of questions I have not seen mentioned here, and certainly not seen answered, is this;

What cable are you planning on running this in?

Will the Cable be buried or surface mounted in the house and in the garden?
If buried in the garden, are you planning on ducting it in, or simply burying it?

Have you given any thought on how best to protect this cable from damage, whether it be physical or electrical, and what protective device(s) are you planning to install?

Taking many factors into account, like ambient temperature, installation method, design current rating of the Circuit, characteristics of the protective device etc, then the smallest cable to be installed should be a 4mm XLPE SWA if you assume that your design current is 16A, the breaker rating is 20A, it's thermal rating is 20A and the ambient temperature the cable is exposed to is 15C.

However, if you intend to increase the load on the circuit, I would recommend that you give serious consideration to 6mm XLPE SWA and a 32A 100mA RCBO at the House end of the cable. The Kiln should be protected locally with a suitable rated (16A 30mA) RCBO/RCD, and any lighting from a seperate 3A/6A Type B MCB.

Albert · 9 Feb 2005

So if i do a Voltage drop calc based on 2.5mm cable:
30M * 16.3A = 8.2V : Drop = 3.62%

could be that i'm wrong but your Volt drop calculation has something missing; the "volt drop per A per metre", depending on your method and cable size, for example a T&E 2.5 mm2 the value will be 18 mV/A/m (OSG table 6D2).
Vd = 18/1000 * 30 * 16.3 = 8.82A, This makes 3.83%

paulh53 · 9 Feb 2005

sasb2003 said:
or where it squeezes through a 16mm hole i drilled for it to get through a RSG (for all of 5-10 mm)??

Another point is, Any steel girder which has cables running through it, Or very close to it, Should be bonded to the main earth terminal using a minimum 10mm2 earth cable,

Damocles · 11 Feb 2005

Should it? why?

Am not in the habit of drilling through RSJ as they are difficult. But the principle of drilling through the centre should be exactly the same as with a timber beam. The centre of the beam is under neither compresion or tension forces, so nothing terrible will happen if you remove it. it just serves to hold the top and bottom apart. The tope of the beam is being force together by the weight above, the bottom is being pulled apart. In the middle all should be calm. Have you seen those exciting cut away beams they put on bridges or fancy victorian railway stations?

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Cable Size Calcs..... too much !!!

Big_Spark

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