Cable Problem

http://cr4.globalspec.com/thread/15878/POWER-CABLE-SELECTION

You can use 4cx70sqmm copper for this load.
you can refer the cable capacity details in any book

Hi,

There will be no problem in terms of current carrying capacity, However you have to make sure that the existing protection device can protect your new cable against overload and short circuit.

You obviously don't want to reduce the cable cross-sectional area so that it would be overloaded, on the other hand using the larger cable should be considered if you want to 'futureproof' the installation.

Hi,

As you can see from the Discussion, http://cr4.globalspec.com/thread/15023/Current-carrying-capacity

Cable current capacity is dependent on cable insulation type, whether single or multi-core, plus ambient temperature.

Also de-rated further if enclosed in conduit etc.

In your case, I find you can replace 1Rx4x120Sq mm Cu XLPE cable with 4Rx1x70Sq mm Cu XLPE cable.

Please refer

http://www.myelectrical.com/pages/cables/cableCurrents.aspx?id=061112125012

for more details.

Mr Jameskutty Thomas your comment...

"Cable current capacity is dependent on cable insulation type"

A serious misconception there. Your current carrying capacity is dependant on your cable size (cross section). Your insulation is dependant on your peak voltage (Vrms x 1.414= Vp). The mounting of the cable depends on local regs/codes, (open clipped, conduit, single core, etc, etc).

I hope this has cleared things up!

Sorry Mixit, Mr Thomas is quite correct. Cable ratings depend on many different variables, one of which is insulation type. Common PVC is rated to 75^oC, hi-temp PVC and XLPE rated to 90, "tri-rated" cable & most fireproof cables rated to 105 etc. As the insulation permits higher temperature rises, the cable can be rated to carry more current.

A nice free programme which demonstrates this can be downloaded here:

http://www.generalcable.co.nz/newzealand/NZDownloads/Gencalc.aspx

To the original poster, unless in the UK or New Zealand, beware using this programme for your specific location. If gives you a good idea of the parameters involved, but cabling MUST be installed in compliance with your local regulations. With respect, DO NOT make decisions with regard to cabling unless you are knowledgeable in what you are doing.

I suspect you are not knowledgeable because you cannot use SWA (steel wire armoured) with single core cabling; this must be aluminium armoured to prevent eddy current losses. Further, I would recommend the running of a copper earth cable in parallel with either SWA or AWA, as these metals have nasty corrosion and expansion habits, which can lead to bad connections.

Finally once again I am on my Guest login hobby-horse. If you are serious about what you are doing please log in as a member.

Thanks BabyGuinness...

Your expansion of the subject with regard to temperature is quite correct, that's where I made the reference to mounting the cables with local regs/codes, but you did a far better job than myself.

Using 4 run cable instead of a single run, may raise terminations issues, even if the current rating suites.

The cable size is OK. However while laying the cable, ensure that the three phase cables are arraged in triangular fashion (trofoil) to avoid heating of cable due to capacitance effect which will incidently increase loading capacity of the cable.

R.Thiyagarajan

Yes my friend you can use this cable and not only this , If you didn't purchase the cable , you can use 50mm2 for the load of 125A and you will be at the safe side if ( the distance is 20 mtrs as you said , the temperature is not very high )

go to NEC or search google in future and you will get what you need

so use 4 (1c , 70mm2 ) or 4(1c ,50 mm2) or 1(4*50mm2) cable.

regards

Thanks for all who contributed this artcle and I found many innovative ideas as well rather than conservative thinking

There are 2 criteria - what max current can a particular cable mmsq handle. There are a few tables around on the web, and I personally de-rate the values recommended down to 75%. For example look at www.energy-solutions.co.uk/cable_conductor.html

This says that if the ambient air temperature is 30C, and the cable is rated 70C, then you could even tolerate a cable as thin as 35mmsq.

BUT:

1) then the cable would be running at 70C.

2) Also that is assuming a single conductor cable, not a 4 conductor cable. In a 4 conductor cable, there's more copper generating heat, and proportionally less surface area to get rid of the heat. If my maths is right, we've 4 x conductors, but only 2 x surface area of the cable.

So the bottom of that page says for 4 to 6 bundled conductors you should derate to 70% (i.e. multiply by 0.7) of the values shown in the main table. So I calculate you are OK on 50mmsq cable, but it will run warm. 70mmsq might be a better bet. Especially because your cable run is short, so the difference in cost won't be too bad.

The other criteria is how far your cable run can be before you lose more volts in the cable than you can tolerate. In the spreadsheet below I calculate the max current rating for 4 to 6 conductor cable, and the max distance you can run before you lose 5% of the volts because of the losses in the cable.