The GBP650m extension will allow the JV to provide engineering, management and construction services until 2017.
The AMEC JV comprises of Babcock Networks and Mott MacDonald Group.
Under the terms of the contract, the JV will be responsible for upgradation of overhead power lines and underground cables across the western half of England and all of Wales.
The JV will continue to provide engineering, management and construction services to 2017.
National Grid construction director Peter Boreham said the company expects to invest around GBP20bn in gas and electricity transmission infrastructure to support the UK’s growing energy needs up to 2021.
AMEC and Babcock International are the major partners with 47.5% share each in the JV.
