It is the second phase of the National Gas Transmission System reinforcement required to transport natural gas from two liquefied natural gas terminals under construction at Milford Haven in west Wales.
The planned route starts at a new compressor site in Felindre moving north west crossing the River Loughor northwards before following the A483 from Ammanford to the north west of Llandeilo.
It will then cross the River Towy, traverse the north-west of the town and follow the A40 north east, crossing the River Towy again to the north of Llangadog.
From there it will turn east towards Brecon, before running north to follow the A470 and A438 to Hay-on-WyeIt will then move south east in
the direction of the B4348 where it crosses the A465, continuing south east towards a new valve compound at Peterstow, to the west of Ross-on-Wye, close to National Grid's existing compressor station.
The route then moves east, north of the M50, passing under the motorway near Junction 3, ending in the Tirley area.
National Grid will work will landowners to agree the best route possible and it plans a full Environmental Impact Assessment before applying for consent to build the pipeline.
It expects more than 1,000 people to be working on the job at the peak of construction .