Columbia Gas of Pennsylvania wants to boost the monthly gas bill of a typical residential customer 11.3%, but a state consumer advocate says the change could be illegal.

The gas distribution rate hike from Columbia Gas would raise the average residential monthly gas bill from $138.50 to $154.30, while small commercial customers would experience a 12% increase in their monthly bill, the company said in its filing last month.

The company serves 445,000 customers in 26 counties, among them Allegheny, Armstrong, Butler, Fayette, Indiana and Westmoreland.

Any decision by the Public Utility Commission on the proposed rate hike likely would not take effect until mid-to-late December, said Lee Gierczynski, a Columbia Gas spokesman.

The state Office of Consumer Advocate, however, said in its recent filing opposing the request, that the proposed residential rate hike would be closer to 17% and the low-usage commercial and industrial customers would see rates jump by 18.4%.

A base customer charge, regardless the amount gas used, would jump from $17.25 a month to about $32, an 85% hike, the Consumer Advocate said.

The rate request, proposed customer charges and proposed alternative rate-making mechanisms, may be unjust and unreasonable and in violation of the law, according to the Consumer Advocate’s filing.

With the rate hike on its residential, commercial and industrial customers, coupled with tariffs, Columbia Gas would generate about $110.5 million in new revenue every year, or 12% of its overall revenue, based on a 12-month projection ending in November, the Consumer Advocate said.

Gierczynski said the additional revenue will be used to invest in the gas line system, replacing aging underground pipe, such as steel, wrought iron and first-generation plastic.

Columbia Gas has invested about $3.2 billion in replacing nearly 1,500 miles of aging pipeline, the spokesman said.

The PUC has yet to conduct a public vote on formally suspending the rate hike request, according to Nils Hagen-Frederiksen, a PUC spokesman. That’s a normal process that would result in the case being assigned to a administrative law judge who will review the request and the opposition, he said.

That will, in turn, trigger public hearings. The administrative judge will make a recommendation for the PUC to consider.