Cable calcs

[snowplough]

Hi Guys,

If i remember rightly, when doing cable calcs , first you work out the design current , say it was 47a, then you take it upto the nearest fuse size 50a, then look in tables to find correct cable csa.
Going on the above scenario ,say it was ref method c , page 131 O.S.G gives you a csa 0f 10mm square, if im reading correct .

At 46a it is 6mm square just outside .

Regards

Locket

 

[Geordie Spark]

Is this any help http://www.electriciansforums.net/e...tocalc-maximum-estimated-ampage-skt-wire.html ????

*hiding*

 

[malcolmsanford]

Yes basically then you would start to factor things in like CCC, VD etc, if you have an afternoon handy then really reading appendix 4 in the BRB will give you a lot of insight into design

 

[snowplough]

Cheers Malcolmsandford,

Reason i asked the question is because on a part p course im on at the momment tutor asked us to work that scenario out above for 47amp, when i gave the answer as 10mm cable with a 50 amp breaker he said i was incorrect, he said it wouold be a 45a breaker with a 6mm cable. I was shown at college to go upoto the nearest breaker size ,then choose the corraect cable size as i did above. i thought In is equal to or greater then Ib as i was shown .


Regards

Locket

 

[i=p/u]

yeah but maybe some diversity

 

[i=p/u]

on site guide is good for learning, even section that you follow and gives a distance to stay within and vd and efli all buit in think around pg 58-9

 

[snowplough]

Cheers I=P\U

Thanks for clarification, ijust wondered why the tutor who seems very experieced would have dropped it down to 45a breaker and used 6mm cable it dosent measure upto what ive been taught , am i missing something


Regards

Locket

 

[telectrix]

6mm will carry 47 if clipped direct, not buried under insulation. a 45A MCB will be happy at 47A, but thats pushing it a bit. suppose the cable were running a shower and a few meters were in the attic, then a mountain of itchy-poo was laid over it. i would be inclined to use 10mm to be on the safe side.

 

[snowplough]

Cheers Telextrix,

That may be the case through experience as nodoubtyou have but for anovice like me sticking to what ive been taught and using the tables it gives me a 50a breaker with a 10mm cable, what is the point of using the tables if it can be minipulated.


Regards

Locket

 

[telectrix]

i always say, if in doubt, go bigger option. better to under run cables and MCBs than over run. it's like your car. if you thrash it at 5-6000 revs all the time , it won't last as long as if you treat it sensibly.

 

[i=p/u]

the dude that teaches me said always go with what you only need as will be cheaper on materials and could win you the job,

 

[snowplough]

Cheers I=P/U,

Only problem ive got with that is at this point in my training at college i wouldnt no that you can actually choose a safety device that is of a lesser rating then the design current, as ive been taught to use the tabulated values

Many thanks

Locket

 

[i=p/u]

but diversity play a part which i think your tutor is on about... its kinda do the calcs, then do them with diversity, il get there

 

[snowplough]

Cheers i=p/u

I thought diversity was supposed to be applied when calculating the size for the main tails, not for individual cicuits ?

Regards

Locket

 

[Amp David]

Tell him that not every manufacturer do a 45 amp beaker!

 

[snowplough]

Cheers Amp David,

I just dan't understand how an appliance that needs 47A to run Can be down graded to 45A.
Because of my lack of knowledge i would just stick to what i have been taught ie. Ib is less then or same as In.

Regards

Locket

 

[Amp David]

Agree with you. Thats what I would have done if i were desiging the circuit

 

[Mr.Si]

Also, if you then install the "Lower" rated Cable / Protective Device and the customer then has problems and decides to take you to court over it...

It could cause you problems if you've not done it in accordance with the Regs book, as they tend to wave that around in courts don't they?

I too would err on the side of caution and use higher rated cables.