Ground bars are required in all panels (main service panels include ground and neutral). To be sure you are selecting the correct ground bar, divide the number of circuits in the panel by 2 and pick the closest numbered ground bar. For example, a 20 circuit panel would require a GBK10P. The bar has 14 positions with 1-3/4 in. between mounting holes and has a total length of 6-3/4 in.