He said the 150,000-square-foot plant would be about 50 percent larger than the existing plant on Progress Road.
The company manufactures outdoor enclosures for the telecommunications industry, including those for equipment at the base of cellular phone towers.
Hennessy said the company merged with several other companies in August and is now known as Hennessy Products, Chatham Technologies Company.
Hennessy said the company employs about 220 workers in Chambersburg. He said the number of employees will increase after the plant is completed, but did not know by how many.
The Chambers-5 Business Park is owned by the Chambersburg Area Development Corporation.
A $400,000 grant from the state has been approved for the corporation to relocate an electrical substation on the proposed site, according to Executive Director David G. Sciamanna.
Sciamanna said the grant is contingent upon finalizing the sale agreement between the corporation and Hennessy Products.