Mountain Fuel Supply Co. has won approval for a plan to extend natural gas service into southwest Utah. But the company is warning potential customers to expect higher rates than are currently charged to customers already receiving natural gas service.

The Utah Public Service Commission approved the company's request for a new certificate to service Millard, Beaver and Juab counties plus portions of Iron and Washington counties not presently served. Mountain Fuel plans to serve the area by tapping into a new high-pressure pipeline being built by Kern River Distribution Co. The Kern River pipeline, which is now under construction, will run from southwest Wyoming to Kern County in Southern California.Mountain Fuel officials say rates will be set about 20 percent higher than those charged elsewhere to help recover the cost of installing the new distribution lines. Officials estimate it will take about 20 years to recover the costs.

A similar rate structure is being used to recover costs on a company expansion in northern Utah and southern Idaho.

The company's original certificate for the area ran out in 1989. The company sought last year to have it renewed after another company, Central Utah Gas, tried to win approval for a certificate to serve the area. Central was unable to get the necessary franchise commitments from potential customers and closed its operations.

The commission denied Mountain Fuel's request last year because there was no clear-cut plan for providing the service. With the Kern River project now under way and a potential source of gas in the offing, the commission agreed to rehear the company's request.

Mountain Fuel plans to build a secondary supply line from its existing line that runs from Sanpete County to St. George. The secondary line will tie into the new distribution system when it is complete, allowing the company a two-way supply capability.