Peel Energy and United Utilities want to add a further 16 turbines at Scout Moor in Greater Manchester, on land between Rochdale and Rossendale.
Planning permission was granted by both Rochdale and Rossendale councils but the government called in the decision.
The hearing is expected to last about a week but a final decision is not expected until next year.
There was strong opposition to Scout Moor wind farm being built in the first place with environmental campaigner David Bellamy among the high profile figures who claimed it would damage the countryside and create a blot on the landscape.
But the development went ahead in 2008, with 26 wind turbines which, it is claimed, generate renewable electricity for 40,000 homes.
Developers Peel Energy had wanted to expand the site for a number of years - and after scaling down their original plans to double the site - they settled on an extra 16 wind turbines.
A planning inspector will hear all the arguments for and against then recommend a decision to the Communities Secretary Sajid Javid. He will then make the final decision on the proposals but we are not expecting his decision until next year.
Opponents fear the expansion of the wind farm will impact negatively on the natural beauty of the area.
Local resident John Batchelor told the BBC it would "create an industrial landscape... an unaesthetic view which isn't acceptable".
Developers Peel Energy said it had amended the plans to lower the height of three of the turbines.
Their lawyers told the inquiry the expansion would help to "combat climate change".
In the planning application for expansion on Scout Moor and the neighbouring Rooley Moor the document states "if all the proposals... are consented they will result in the largest wind farm in England".
A public inquiry into plans to create England's largest onshore wind farm has opened.