Bore and Duct Spacers Information
Bore and duct spacers are used to keep pipes, conduit, and cable racking supported and evenly spaced for underground encasement. They can be designed to provide uniform separation between the rows and columns of the conduits, providing a stable structure for concrete or grout and eliminating voids.
Since bore and duct spacers help maintain accurate duct position, they allow for reduced installation time, isolation boosting efficiency, and they ensure proper installation.
When selecting bore and duct spacers several factors need to be considered, including conduit size(s), number of conduits, type of conduit, casing type, casing OD, casing wall thickness, length of run, min. conduit-to-conduit separation, and grout: filled or not.
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When designing a duct system, consideration should be given to the duct stiffness (DS) requirements to withstand the specific application loadings. Duct stiffness is a useful test value for evaluating the load bearing and deflection characteristics of the duct. Stiffness depends on the modulus of elasticity of the duct material and the moment of inertia of the duct (which is a function of the duct diameter and the geometry of the duct wall). The value of stiffness is determined at the specified ID (inner diameter) deflection of 5%. Values are expressed in pounds of force per inch of deflection.
DS = duct stiffness in lbs/inch
F = load in pounds per inch of duct length that will defect the duct ID 5%
Δy = change in ID in inches due to the applied load F
Duct separation is determined by whether the conduits are going to have an encased burial or direct burial. Commercially available spacers can be used for encased burial. For direct burial, spacers must meet certain conditions as specified by the design engineer. The use of duct spacers for direct burial, without regard to proper compaction of backfill material, may result in excessive point deflections unless proper design engineering is applied.
- In power duct banks, individual ducts should be separated from one another to provide adequate dissipation of the normal buildup of heat from cables within the duct and to physically separate ducts in the event of a fault.
- In communication duct banks, separation is intended to allow encasement material to fully surround each duct since there is no buildup of heat.
- In combined power and communication ducts, specifications require three or more inches of separation between power duct groups and communication duct groups. Spacer manufacturers generally do not stock spacers for more than three-inch separations so dummy spacers must be used to produce the acceptable separation.
There are several standards to which bore and duct spacers must adhere to ensure proper design and functionality. ADS AS4821 dicusses spacers for cable duct banks, while NEMA TC 14 and UL 2515A discuss thermosetting resin conduit and fittings.