Hi and thank you for using Just Answer
You have me at a bit of a loss here ... is this new service? Are you replacing the current one? or is this something added for future building like to a garage or shed?
You must be busy ... So I'm going to switch this to Q&A so we can have more time to address your question. However as for the last part of your question ... yes it has to be 3 cords. All we need now is the size.
there is an existing overhead cable which is due to be replaced withib the next few years, I would like to lay an underground cable now so that I dont have to dig up my garden when the overhead one needs replaced
it is a domestice supply
The kind of wire you need is called *Armoured cable*
Armoured cable is normally used to transfer power either underground or overhead. The construction is normally the inner cores (which are generally multi strand) being individually sheathed, then covered by an initial overall plastic sheath, followed by the protective wire armour with a final outer sheath to hold it all together. The inner cores may number two, three or four depending upon the application.
The wire protective armour should always be earthed at at the supply end to ensure that it cannot become live if the the cable is severed or a fault should develop within the cable. Installation of armour cable should be left to professional contractors, not so much because it is difficult but because of the safety implications if overhead or underground installations should fail to meet the exacting installation requirements.
With all of this being said there is some work that needs to be done on your part so here you go:
Cable size depends on the maximum demand, the length and the current-carrying capacity of the cable, and any supply requirement by the company for a pilot cable to control a hot water control relay. (This is becoming less common as most supply companies use frequency-controlled or ripple relays.)
To size a mains cable for a domestic property:
In accordance with the electrical code, you need to do the above first. Once we get that answer, then we can decide what size of wire you need. Otherwise, if you want to have an idea, stop at a place that is doing new home construction with underground mains and see what size they use. Just remember to "add" for additional demand down the road. Please let me know I wish you the best.