Philadelphia Gas Works said its residential natural gas rate increased on Dec. 1 by 3.5 percent, from $1.41 per hundred cubic feet to $1.46.

PGW, which serves city customers, and Peco Energy Co., which last week announced a rate increase for suburban natural gas customers, say the increases reflect the growing commodity cost of natural gas since it hit historic lows in April. Both utilities said that, despite the increases, their rates are still lower than they were a year ago.

The utilities are required to adjust their rates quarterly to reflect changes in market costs of energy. The costs are passed along to customers without a markup.