Our yellow twinwall water services duct is used for gas supply pipes (usually yellow MDPE). It is available in 94mm and 137mm size options, in 6m lengths. The diameter measurements refer to the inside diameters (I.D.). All dimensions can be found in the table alongside the product image.Twinwall rigid duct is much stronger than flexible duct which makes it ideal for use beneath roads and pathways, or where a deeper underground installation is required. The ribbed external wall ensures that the duct can withstand high compressive forces whilst the smooth inner bore resists any snagging.You should always choose a diameter that allows plenty of room within the duct to allow the cable or cables to be drawn through smoothly. All lengths come with an integral coupler, which enables two lengths to be connected together as required.