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Transformer Tails Sizing

fiftyhertz
Hi All,


I am trying to nail down sizing the tails from the TX into a switchpanel.


I have reviewed the the information in GN1 and understand the methods of installing to get the best current sharing capabilities. 


The issue is getting reasonbly sized AWA singles.


Example: 1500KVA TX - FLC 2165A.


Using Table 4D3 (to run to 70C) I cannot even get Trefoil 3 sets of 4 x 630mm2 (809A) cables to work, due to a grouping factor of 0.54?


How is the dealt with practically?
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    Thanks Mike, Ive had a look at those links and still cannot find anything concrete.


    Ive had a look at a couple of old jobs where an IDNO have sized the tails and it appears they do not use a grouping factor. 


    An example is a 1500KVA TX, ACB set at 2125A supplied by 3 groups of 4 x 1C 500mm2 AWA cables in a trench between the sub and LV panel.


    If each group can carry 731A in Trefoil, which gives a total capacity of 2139A.


    I suppose the only way to get sensible sizes is to ensure that the trench has a degree of ventilation, which is possible due to being internal and also to specify a cable ladder to act as a heat sync. We could then potentially use table 4C5 and use the 2DE spacing to omit the grouping. Any kind of derating factor even at 0.8 would give ridcilous sizes it appears.


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  • 0
    Thanks Mike, Ive had a look at those links and still cannot find anything concrete.


    Ive had a look at a couple of old jobs where an IDNO have sized the tails and it appears they do not use a grouping factor. 


    An example is a 1500KVA TX, ACB set at 2125A supplied by 3 groups of 4 x 1C 500mm2 AWA cables in a trench between the sub and LV panel.


    If each group can carry 731A in Trefoil, which gives a total capacity of 2139A.


    I suppose the only way to get sensible sizes is to ensure that the trench has a degree of ventilation, which is possible due to being internal and also to specify a cable ladder to act as a heat sync. We could then potentially use table 4C5 and use the 2DE spacing to omit the grouping. Any kind of derating factor even at 0.8 would give ridcilous sizes it appears.


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