Dear Mr Rahul
On your primary question whether excise duty is applicable or not, the Answer is "No"- Since you are not manufacturing the cables/transformers you are not supposed to charge excise duty.
Excise duty is charged only when you manufacture. In the present case you are not manufacturing so no question of excise duty applicability. Even if there is value addition you are not supposed to charge excise duty.
On Credit availability:- Since you are not using it for manufacture you are not supposed to avail the credit.
Additional information :- Suppose you are also manufacturing an excisable product and cable and transformers are accessories of the excisable product, and at the time of sale you are invoicing as a single product then you need to charge excise duty even on cables/transformers and also on the input side you are eligible to take the input credit.
Kindly Note :- If you are doing a trading from your manufacturing premises and your trading product is identical to manufacturing product then you need to bifurcate your premises between manufacturing and trading premises. Permission will be given by your range office.
Regards
Arupp Das