June 8 (Reuters) - Italian energy utility Enel SpA raised 3.25 billion euros ($3.96 billion) on Tuesday via a triple tranche sustainability-linked bond sale, a record amount in the format the company pioneered two years ago.

Enel's deal will be the biggest sustainability-linked bond sale to date, dwarfing a 1.85 billion-euro issue outstanding from drugmaker Novartis AG, according to data from Refinitiv.

It reflects the growing popularity of sustainability-linked bonds, which force issuers to increase coupon payments should they fail to meet pre-set company-wide targets.

This format is fast emerging from the shadow of green bonds - debt that specifically funds climate-friendly projects - as investors seek new opportunities in the environmental, social and governance (ESG) space.

Enel's latest bond raised 1 billion euros from a six-year tranche, 1.25 billion euros from a nine-year issue and another 1 billion euros from a 15-year deal, according to a lead manager memo seen by Reuters. Demand was 10.4 billion euros, it said.

Should Enel's greenhouse gas emissions exceed certain limits by end-2023, coupons on the six- and nine-year bonds will step up by 25 basis points each, according to an earlier memo.

There will be an equivalent step-up in the 15-year bond if an even stricter pre-set target is exceeded by the end of 2030.

Since Enel sold the first bond in this format in 2019, "the market for sustainability-linked bonds seems to be maturing and could therefore attract a larger crowd of issuers," said Shanawaz Bhimji, senior fixed income strategist at ABN AMRO.

"As long as there's a credible story behind the sustainability (key performance indicators), investors will scoop up this type of debt."

Refinitiv data shows companies sold $10 billion of sustainability-linked debt in the first quarter of this year, surpassing the entire issuance across 2019 and 2020.

Sustainability-linked bonds are considered a useful tool for supporting traditionally "brown" companies' transition efforts, and also for smaller companies that cannot identify individual sustainability projects.

The bonds are linked to a tender exercise and will replace conventional bonds as part of Enel's commitment to promote sustainability-linked issuance in their capital structure, a lead manager said. ($1 = 0.8212 euro) (Reporting by Yoruk Bahceli in Amsterdam Editing by Matthew Lewis)