About 120 staff at the National Grid, including some in Warwick, are to start a ban on overtime from Friday as part of industrial action over pay.
Prospect said its members at sites in Warwick and Wokingham, Berkshire, voted to take action in a row over the way overtime pay is calculated.
National Grid said it was "disappointed" at the move, adding: "We have taken steps to ensure that this will not have any impact on electricity customers or the safe operation of the electricity transmission network.
"This action is being taken by only 119 of National Grid's 10,500 UK workforce and we are continuing to work collaboratively with all the trade unions."
