Over the next five years the power business expects to plough £22bn into upgrading transmission and gas pipelines in UK and US. Around three quarters is earmarked for the UK alone.
Steve Holliday, chief executive, 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.”
Last year National Grid spent a record of £2.2bn renewing its gas and electricity assets in the UK.
On top of the continuing replacement programme, the firm plans to spend an extra £1bn on power line replacement and £3.6bn extending the grid to cope with wind farm development.
Over the next five years, National Grid said it also plans to spend an extra £1bn expanding its gas import facilities and developing carbon dioxide capture and storage.
Last year the firm spent around £1.5bn on electricity transmission. This included the Thames Estuary reinforcement, and the London cable tunnels project as well as projects to hook up renewable energy schemes.
On the gas side contractors, replaced over 2,000km of gas mains in the UK at a cost of £465m, up 9% on last year. Spending on new gas related projects rose 18% to £205m.