Say one main branch 300 mm dia meter branching off to DN 200 and DN 250 with individual valves to shut off. You need 2 Nos thrust type dismantling joints of sizes DN 200 and DN 250  plus two gate valves of size DN 200 and DN 250 in the chamber. 150 mm spacing between flanges to wall. Hence you need  about 2000 x1200 chamber.

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Say one main branch 300 mm dia meter branching off to DN 200 and DN 250 with individual valves to shut off. You need 2 Nos thrust type dismantling joints of sizes DN 200 and DN 250  plus two gate valves of size DN 200 and DN 250 in the chamber. 150 mm spacing between flanges to wall. Hence you need  about 2000 x1200 chamber.