cable calc and reduction

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Lee321

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an existing 120mm swa is 100m long feeding a 100a 3ph db fed by a 100a mccb

a roller shutter door is being installed adjacent the db meaning the swa will not reach once it is lifted up and over the door

the cable size is large due to the length of run? and not load? so i intend to make the swa into a switch disconnector then from there a swa to reach from the new SD to the existing db, the cable length will be around 19m and so the 120 will reduce by similar amount.

what (smaller) size cable can i install and how do i calculate it?

what other info is needed

thx in advance

 
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Don't see how a switch disconnector helps, unless you are fusing down to 80a or 60A. Think I would be more inclined to calculate voltage drop for cable size over new distance and join in new length of same size cable. Load should be based on 100A (could this be changed) as this is fuse protecting cable - try voltage drop calculator on TLC to double check your results after doing sums on paper.

To help some additional info would be nice:-

total load

total lengtth of run

conductor sizes ( or is that 120mm)

 

benji

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if you go down your route not only will you need a switch you will probally need spreader boxes to fit to switch which is time consuming, best option new piece off 120mm and a through joint

yours benji

 
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120mm armoured is a nightmare to work with. Recently done a farm job 120 mm armoured connected into contactum 250 amp switchfuse. there is quite a bit of room inside but still a struggle, managed to do it without spreader box though. If i where to do job again i think i would terminate cable in box first with copper ring terminal and then take smaller cable with ring terminal on end into switchfuse the two rings could be bolted together and then covered with heat shrink sleeving.

batty

 

Lee321

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i did this job already!

was it right?

yeah 120 is a hand full, i did use a spreader but as it happened the cores came out perfectly ;)

so is my off the head theory of "the volt drop was taken care of" by the majority of the run (est 95%) in 120mm then the short cable needed from the SD to the db could be much smaller a riska one?

 

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