Western Power Distribution will be closing the road through Abbey-Cwm-Hir, near Nantmel and Rhayader, in north Powys, which will lead to a very long diversion which the power company says is unavoidable.
A full road closure will take place between 7.30am and 4pm from October 11 to 25 and there will be restrictions for 24 hours.
A spokesperson said local people have been written to and offered an apology for the "unavoidable work" which they will carry out as quickly as possible.
Works are being carried out to boost power supplies in the Rhayader area and started on September 6. Its aim is to make the network more robust, to avoid outages and to make the power network more able to cope with renewable energy supplies and a growth in electric vehicles.
Paul Scrivens, WPD's team manager for the Rhayader area, said: “This investment will help to safeguard electricity supplies for our customers in the region for years to come by improving the infrastructure and security of our network."
The work is expected to take 20 weeks to complete and will affect the Abbey-Cwm-Hir road out of Rhayader Town, a small section of the industrial estate and a road crossing on East Street Rhayader.
During this time, traffic management will be in place to enable contractors M&A Doocey to excavate and install ducts for new cables. Traffic management and signed diversions will be in place as required.
Paul added: “Our aim is to keep any disruption to an absolute minimum.
"We have been working with Powys County Council to schedule our works to minimise the inconvenience in the area.
"We have kept customers and business along the route informed by letter and by visits from the civil contractors working on WPD’s behalf”
Local residents have been informed of the work by letter and are invited to contact WPD if they have any questions on 0800 096 3080.