The Iowa Utilities Board (IUB) has issued the final order for the $3.6 billion project, unveiled in April. MidAmerican asked the IUB to approve its request by mid-September so that it could take full advantage of the extended Production Tax Credit support.
MidAmerican expects to complete the project by the end of 2019 but some of Wind XI's multiple sites could be completed as early as 2017.
The Berkshire Hathaway-owned utility is seeking to provide its customers in Iowa with 100% renewable energy.
It has invested $10 billion in building 3.45GW of wind energy since 2004. It currently generates 85% of its retail load from wind.
In June, Vestas announced it had received a conditional order for up to 1,000 V110-2MW turbines for Wind XI.
Delivery and commissioning of the turbines is expected to be from 2016 to 2019.
The turbine order includes a five-year Active Output Management 4000 service agreement, with an option to extend for up to ten years.