The company’s two for five issue will see 990.4m new shares at 335 pence each.
National Grid will use the money raised for a “significant increase in UK capital investment", it said in a statement.
It is also aimed at maintaining the company’s single A credit ratings for its UK operating companies.
The announcement came as National Grid reported that its full year pre-tax profit jumped to £1.4bn – from £944m last year.
A strong performance in its transmission and electricity distribution and generation operations helped to fuel the figures.
Chief Executive Steve Holliday said: "We are confident about the requirement for a step-up in UK investment. We have sized this Rights Issue at £3.2 billion to ensure we will have the financial flexibility to meet our investment needs over the coming years, whilst maintaining our current single A credit ratings."
The rights issue is fully underwritten by Morgan Stanley Securities Limited, BofA Merrill Lynch and Deutsche Bank AG, London Branch.