The company that wired classrooms in Birmingham, Ala., for computers is billing the school system for an additional $2.1 million, claiming the system underestimated the number of data ports the schools required.

Floor plans showed fewer classrooms than actually existed, said Tim Lewis, whose T.A. Lewis and Associates has overseen the technology effort. The additional work led to a 37-percent increase over the wiring project’s $5.7 million projected cost, Lewis said.

Cable Consultants Inc., the Atlanta-based firm that installed the wiring and ports, has billed the system for the additional costs.

“My stance is that they owe Cable Consultants the money,” Lewis said.

Deputy Superintendent Abbe Boring said the school board must approve any additional payment. Neither Lewis nor Cable Consultants has offered specifics, she said.

The wiring project is part of the system’s $25 million effort to put three desktop computers for students in every classroom and to give every teacher a laptop computer. The project, funded by city, federal, and school district money, will bring 10,500 desktop computers and more than 2,300 laptops into the system.

Officials with Cable Consultants, now owned by Black Box Network Systems, said they are negotiating payment for the additional work.

Lewis said the extra costs are not uncommon in a large contract. He said school officials were aware of the problem, helped correct it, and knew additional work was being done. “This is not a surprise,” he said.