Size Considerations
To meet enclosure’s functional requirements, correct pit/ducting channel sizing is critical to allow acceptable working area/free space. This maintains mechanical protection of cables and uninterrupted continuity (depth & direction) of cable route.
Undersized enclosures can result in damaged cables/possible hazards to utility workers and general public.
To ensure correctly sized enclosure is selected, refer to product pages in website where clear working area within pit or ducting channel is illustrated. This is free space inside enclosure (without obstruction).
Note: Most cable pits have tapered walls.
A number of factors determine size of cable enclosure. AS/NZS 3000 and AS/NZS 3084 stipulate criteria governing the number and size of conduits in these spaces.
Other specific factors include:
For safety, cables need to be installed to minimum depth of cover and protection governed by AS/NZS 3000 and HB 29. Type of cables, communal location (e.g. footpaths as opposed to parks) and nature of overlying pavement/slab are important determinants. AS/ACIF S009 specifies cable depth requirements for larger maintenance (or access) holes. All enclosures must be deep enough to cater for these depths.
Enclosures must be sized large enough so when cable routes change direction, minimum allowable radii specified by cable manufacturer are adhered to. If information not available, consult AS/NZS 3000 & HB 29, or contact ACO. To ensure bending radius is maintained, HB 29 recommends communications cables are anchored immediately before and after bend (not at bend in pit).
AS/NZS 3000 defines a number of connection methods for joining cables. Joints are generally housed in cable pits to protect them from undue mechanical stresses. Pits must be sized to accommodate type of connector chosen for enclosure.
When drawing (pulling/hauling) cables through cable pit, care must be taken to ensure cable is not damaged. AS/NZS 2053.1 & AS/NZS 3084 specifies requirements for correct conduit preparation applicable at pit/conduit interfaces.
A correctly sized pit allows for adequate working area for cable drawing operation. Various communications carriers and HB 29 offer guidelines to assist with this practice.
Bellmouth fittings may be required to ensure no damage occurs to cable during cable drawing. Unlike other materials used in pits, polymer concrete can be ground to a smooth edge.
Some large enclosures may be divided into different sub-enclosures to preserve segregation of cables. These can comprise a mix of low voltage power cables, communications cables, gas pipes, water etc.
AS/ACIF S008 sets out conditions for segregation and clearances in enclosures specifically for communications cables with low voltage power cables, whilst AS/NZS 3000 & HB 29 gives general guidance for arrangement of cables in trenches. Larger maintenance (or access) holes and cable trunking systems may contain high voltage cables. Segregation and separation conditions for these applications are detailed in AS/ACIF S009.
Segregation in a cable pit can be achieved by installing dividing wall. A bracket (part no. 76514) can be attached to polymer concrete pit by means of nuts and bolts. Divider is cut from 15mm thick wood, plastic or other material and fitted into bracket’s angles to provide internal wall. All compartments need to be waterproofed from other compartments, therefore divider needs to be sealed with suitable sealant. Additional drainage holes may be required within each compartment of pit.
ACO does not recommend plastic pits for these applications. Wall distortion together with pit’s side ribbing profile, make sealing of dividers difficult.
For surface cable ducting applications, ACO can provide permanent cable separation with the provision of cable spacers.
Cable storage pits are enclosures used for above purposes, but are large enough to store equipment and/or additional cabling for future expansion. Cable pits must be sized accordingly.