Feb. 16--For the second time in two months, natural-gas bills are going up for Tucson residents.

State regulators on Wednesday approved a rate increase for residential customers of Southwest Gas Corp.

The average January bill will rise by as much as $7.69, while the average August bill will increase by as much as $2.26, according to estimates from the Arizona Corporation Commission and Southwest Gas.

The rate increase takes effect March 1, said Libby Howell, a Southwest Gas spokeswoman.

The rate increase follows a surcharge implemented Feb. 1 that raised the average residential bill about $21 a year, or about $1.75 a month.

Some renters will not experience the same rate increase as homeowners.

The commission voted to allow Southwest Gas to create a separate set of rates for people who live in apartments or duplexes. Those rates will be less than rates charged for people who live in single- family houses, Howell said.

Estimates were not available Wednesday for the annual costs of the rate increases for residents' homes or apartments.

Southwest Gas and the Corporation Commission did the math, but discrepancies between their estimates remained at the close of business on Wednesday, and neither offered an official estimate, spokeswomen for the gas company and the commission said.

Wednesday's decision marks the first rate increase for Southwest Gas customers since 2001.

Bills have changed for other reasons, including the surcharge that began this month. That surcharge helped cover the company's high costs of buying gas that customers already used. Southwest did not profit from that surcharge.

Southwest Gas requested Wednesday's rate increase to cover its increased costs of wages, taxes, pipeline safety inspections and billing.

At current rates, the average residential customer's January bill totals $96.14. That bill will rise 7 to 8 percent after Wednesday's rate increase, Howell said.

The average August bill -- $20.62 at current rates -- will go up 10 to 11 percent.

The commission raised rates more in summer than in winter to help people stay warm during cold months, said Heather Murphy, spokeswoman for the commission.

"That's so people don't get walloped with gigantic, unmanageable bills when they need the heat," Murphy said.