Slowly, the large conventions and trade shows that can shower money on a city are returning to Philadelphia.

The Philadelphia Convention and Visitors Bureau has booked more than 20 big groups - called "citywides" and defined as those with 10,000 or more people attending - for conferences and trade shows in the soon-to-be-expanded Convention Center, starting in 2010, bureau officials say.

The bookings are a sharp contrast to the decline in citywides at the Convention Center since the peak year of 2002, when there were 27. A typical citywide convention needs at least 6,000 hotel rooms on its busiest night.

Only 12 big groups are scheduled for this year, a direct result of the reputation the Convention Center had from the late 1990s through 2003 for high labor costs and jurisdictional disputes among its six unions, said Jack Ferguson, the bureau's executive vice president for conventions. The Convention Center hosted 14 citywide conventions a year in 2004, 2005 and 2006.

Now, within the fraternity of people who plan big meetings, word has spread that the customer-service agreement the unions and the Pennsylvania Convention Center Authority signed in 2003 has made costs more predictable and the working atmosphere better, Ferguson said.

The bureau is especially pleased that the International Association of Chiefs of Police will bring its convention back to the city in 2013 and 2014. It was the first large group to use the building, in October 2003, after the customer-service agreement was signed.

Dan Rosenblatt, executive director of the group, said its members had such a good experience that Philadelphia now is one of four cities where annual meetings will rotate for the foreseeable future.

But for many convention managers, the need for a larger center matters even more than labor costs. The expansion plan calls for the main part of the building, which now covers the area between 11th and 13th, and Arch and Race Streets, to be extended from 13th to Broad Street. The city Redevelopment Authority is in the midst of acquiring land for the $700 million expansion project, which is scheduled for completion in late 2009.

The enlarged building will have 541,000 square feet of contiguous exhibit space, the most of any convention center on the East Coast, and that was key for the International Association of Fire Chiefs, said Mark Light, the organization's deputy executive director.

The group, based in Fairfax, Va., knew about the center's labor troubles but expects costs will be reasonable enough to use it for its annual trade shows in 2010 and 2013, he said.

"The expansion makes it big enough to hold us. It was not on the list before," Light said. "Our show has 180 pieces of equipment and requires 500,000 square feet of exhibit space."

Also helping the cause, Light said, was the active involvement of the Philadelphia Fire Department when the city was making its sales pitch.

Ferguson said to look later this year for stepped-up efforts, similar to the way the fire department was involved with the fire chiefs' group, to get local members of national organizations to help persuade their groups to hold meetings here. The bureau will work with Select Greater Philadelphia, an affiliate of the Greater Philadelphia Chamber of Commerce, on that program, he said.

The bureau is working as well with the Greater Philadelphia Tourism Marketing Corp., giving brochures and other marketing material to everyone attending a convention or trade show that, it's hoped, will entice them to return with family or friends for a vacation, he said.

Officials at both the bureau and the tourism marketing agency have data that show why visitors who stay overnight are so sought-after.

Last year, the tourism agency surveyed those who bought hotel packages through its Web site and found that the average pair of visitors spent about $900 during a two-night stay. Leisure travelers make up about one-quarter of Center City's overnight visitors, but are the majority of all visitors throughout the region.

Conventioneers and others who come in groups make up about 40 percent of the Center City overnight visitors, and they tend to be even bigger spenders, according to national surveys by the Destination Marketing Association International, the trade group for convention bureaus. Each person attending a convention stays in a city 31/2 days and spends an average of $436 per day, in 2005 dollars, for food, lodging and other expenses, the surveys show.

Contact staff writer Tom Belden at 215-854-2454 or tbelden@phillynews.com.