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Underground foul drainage should have enough capacity to carry the anticipated flow at any point along its length. The flow depends on the appliances connected, whereas the capacity depends on the size and gradient of the pipes used.

Flow rates for underground foul drains

Sanitary appliances are seldom in use simultaneously and the minimum drain sizes in normal use are capable of carrying the flow from quite large numbers of appliances. The flow depends on the type, number and grouping of appliances.