Despite a two-day blackout in the Back Bay that shuttered businesses and cost some wage-earners their pay, NStar officials said Friday the utility will not pay damages for lost business, income, or food.

“If it’s a normal situation, we do not typically, when there is an outage, provide for losses,’’ Thomas May, top official of the utility, said in an interview. “We do not typically do it for an event that lasts for a day, a day and a half… . People normally have their own insurance.’’

A normal outage, according to NStar spokeswoman Caroline Pretyman, includes those caused by mechanical failure, such as this week’s disruption, and even severe weather such as snowstorms and hurricanes.

The absence of human error or negligence in a transformer fire that cut power to 21,000 residential and business customers on Tuesday night means that the utility is not liable for business or income losses, Pretyman said.

Meanwhile, NStar said it planned to temporarily cut power to 8,000 customers beginning at 3 a.m. Saturday for further repair work. The utility said the outage, expected to last up to three hours, was needed so workers could change customers “from a temporary connection to the company’s permanent underground network.’’

NStar’s stance on damages is likely to cause friction with Mayor Thomas M. Menino, who has demanded that the utility reimburse restaurants for spoiled food, pay restaurant workers for lost wages, and cover more than $85,000 in overtime costs for police and other city employees.

The effect of the outage was felt far beyond the 21,000 customers who lost power. The sprawling Prudential Center, for example, counts 8,000 workers in its hundreds of offices and stores. Power at the Pru was not fully restored until 11 p.m. Thursday.