CONTINUOUS SUPPORT OF CABLES For installed cables, certain mechanical and physical restrictions must be taken into consideration. Formulas for calculating the maximum weights under various installation conditions follow. Continuous Vertical Support Support by Conductor The maximum weight in lbs. to be supported by the conductor is: Wmax = K x CM where:   K = .004 for soft copper    K = .003 for aluminum (1350 H19) CM = Circular mil area of cable involved The above is based on limiting the working load of soft copper to about 5,000 lb./sq. in. and the aluminum to about 4,000 lbs/sq. in., thus giving a safety factor of about seven. Support by Bend at Top (or Bottom) Maximum weight of cable in pounds to be supported when cable hangs on a bend. (Note: Support by conductor maximums may be more limiting.) Conduit Bends • For a single conductor cable in a conduit of diameter no greater than 2 times the cable   OD:  Wmax of 1/C cable = 25 x R x D1 • For 3 1/C cables in parallel (not twisted), with 2 cables on bend surface and third   above: Wmax of 3 cables = 37.5 x R x D1 • For 3 1/C cables twisted (triplexed):   Wmax of 3 cables = 10 x R x D1 • For 3/C cables with overall cover:   Wmax of 3/C cable = 10 x R x D3 where: Wmax = Maximum weight of cable in pounds R = Radius of bend, conduit or support surface in inches D1 = Single conductor O.D. in inches D3 = Three conductor O.D. in inches Tray or other flat surface bends Use one-half of the values for the appropriate conduit bends, except for 3 1/C cables in parallel. For 3 1/C cables in parallel use one-half the value of a single conductor cable in a conduit. This assumes that all 3 1/C cables are lying on bend surface, not stacked on other cables. Care must be taken that the cable is secured in the horizontal portion immediately before the supporting bend to prevent ratcheting and build-up of tensions in the cable on load cycling. The above formulas are based on limiting pressure on the cable surface to 50 lbs./sq. in., assuming contact width of 1/2 cable diameter when in conduit and 1/4 cable diameter when on flat bend. Assumed factor of 0.4 to account for intermittent contact on three conductor twisted cables. The pressure in lbs./sq. in. on a single conductor cable is: P = Tension      R x W where: P = Pressure on single conductor cable in lbs/sq. ft. R = Radius of bend in inches W = Contact width in inches Cable in Grips Continuous cable support by grips requires transfer of lifting forces to the load bearing components of the cable (conductors, concentric wires or armor wires) without physical damage to the insulation, shielding tapes, outer jacket, etc. Adequate force transfer is depending on many variables, the most important of which is a sufficiently high coefficient of friction at all interfaces between the grip mesh and the load bearing component(s) of the cable. If any such interface has a coefficient of friction less than 0.25, the weight of the cable supported by a conventional Kellems grip alone shall be limited to 50 pounds. If the cable support is partially horizontal (as in a cable tray at the upper end of a vertical run), the limit is determined by the use of equations under "Support by Bend (Top or Bottom)," plus the 50 pounds limit for intervals between grips. Typical slippery interfaces include: Permashield and conducting fabric tape. Metal tapes and conducting fabric tape Metal tapes and bedding tapes. Any interface involving a polyester tape. For cables with acceptable coefficients of friction, the allowable loading per grip must be less than the maximum load based on the strength of the conductors or other load bearing components, and also less than the maximum load based on the ability of the cable to resist compressive forces developed by the grip.  When more than one grip is used in a vertical support arrangement, it is imperative that each grip carry only its own share of the load. Usually the lowermost grip is applied first, and the cable lowered until the grip "grabs". The next higher grip is then applied, the cable lowered into it, and so on until the uppermost grip is applied last of all. Maximum Load Based on the Strength of Load Bearing Components Wmax = K x CM where: Wmax = Maximum allowable total weight per grip K = .004 for soft copper K = .003 for aluminum (1350 H19) CM = The total circular mil area of all load bearing components to which there is adequate force transfer. For more than one cable/grip, total the CM of all cables. Maximum Load Based on Ability to Resist Compressive Forces Compressive forces caused by the grip result in pressure under the grip wires which tend to make them penetrate into the cable. The magnitude of this pressure is related to many variables including the load on the grip, size and geometry of the grip, the number of cables per grip, and the diameter of each cable. For cables with concentric wires the effects of "bridging" of the grip over the concentrics must also be considered. The following formulas allow a solution for Wmax: Wmax for cables with an outer finish on concentric wires and In the above formulas: d = Diameter of concentric wires or width of flat concentrics (inches) D = The diameter of one cable (inches) DList = The smallest rated cable diameter for which the grip is designed (inches). (See manufacturer’s recommen-dations) LLay = Lay length of concentric wires (inches) n = Number of concentric wires on one cable Pmax = The maximum pressure under the grip wires or con-centric wires = 200 lbs/in.2 for most finishes excluding those previously referenced Wmax = Maximum allowable total weight per grip (lbs.) *If is > 0.75, treat as cable without concentric wires. Cable in Block Clamps Maximum support to be expected per block clamp is 50 lbs. Bearing surface must equal or be greater than the cable diameter but not less than 1 1 /2". Support by Messenger We do not recommend that self-supporting cables be hung vertically by the messenger. However, there are messenger grip assemblies available which may be used. Spacing of grips to be governed by previous paragraph on "Cables in Grips". Continuous Horizontal Support Cable on Porcelain or Block Cup Supports The maximum distance between supports (in feet) is given by the following: L = 12.5 x s x D   W where:  S = Length of support in inches               D = Diameter of cable in inches              W = Weight of cable in lbs./ft. The above is based on limiting the cable pressure to 25 lbs./ sq. in. (to account for the cable resting on the edges of the support) and assuming a contact width of 1/2 the cable diameter. More frequent supports may be desirable, depending on the installation from an appearance viewpoint. Cables in Trays The maximum weight (per foot of cable tray) that can be installed on top of a bottom cable resting against cable tray rungs is calculated as follows: (1) Weight (per foot of cable tray) = 12.5 x s x D for a 12 inch rung spacing (2) Weight (per foot of cable tray) = 16.5 x s x D for a 9 inch rung spacing (3) Weight (per foot of cable tray) = 150 x D for a solid bottom tray where: s = Rung width in inches             D = Diameter of cable in inches             Weight is in pounds The above is based on limiting the cable pressure to 50 lbs./ sq. in. and assuming a contact width of 1 /4 cable diameter.   Home | Index | Terms | www.kerite.com Copyright © 2000 Kerite. All rights reserved The Kerite Company Phone: 203.888.2591 / 800.777.7483 Fax: 203.888.1987