Introduction: Why is cable “sag” critical?
When you look at a newly constructed power distribution network, you might think that perfectly straight and taut cables between two poles are the best and most aesthetically pleasing installation. However, in overhead network engineering, “absolute straightness” is a disastrous mistake!
Low-voltage self-supporting cables (especially standard 6-core types), like any metal, expand and contract with temperature changes. In summer heat, conductor length increases and the “sag” increases. Conversely, in freezing winter cold, the cable contracts and shortens, dramatically increasing tensile forces.
If you tighten the cable too much on a hot summer day, what happens? With the first winter cold snap, the cable contracts and the tension becomes so high that either the messenger wire breaks, hardware fractures, or the pole is pulled out of its foundation.
If you leave the cable too loose, what happens? In strong winds, cables oscillate violently and gallop, potentially violating standard safety clearances to ground and buildings.
Therefore, sagging (tension adjustment) is the art of engineering balance—ensuring that under the worst temperature conditions, the cable neither breaks nor hangs excessively. In this operational guide from the technical unit of Simrod Sama, we detail the exact steps of this process.
Prerequisites: Tools and knowledge required for sagging
Before climbing the pole, you must have the appropriate equipment.
Essential tool list:
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Tirfor or come-along hoist: The main tool for applying tensile force.
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Cable grip: Specifically for gripping the messenger wire.
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Dynamometer: The most critical tool for accurately measuring tension (in kgf).
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Accurate ambient thermometer: To measure the air temperature at the installation site at the time of work.
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Sag-tension table: The heart of the operation, pre-calculated by the project designer based on the 6-core cable weight, span length (distance between poles), and climatic zone.
Heart of the operation: Understanding the sag-tension table
No two projects have identical sag tables. A table for a desert region differs from one for a mountainous region.
Simplified example of a sag table (for educational purposes only):
*Assumption: For Simrod Sama 6-core low-voltage self-supporting cable (size 70+120×3), span length 40 m*
| Ambient Temperature (°C) | Target Sag (cm) | Approx. Tension (kgf) |
|---|---|---|
| -5°C (Cold) | 45 cm | 680 kgf |
| +15°C (Moderate) | 60 cm | 520 kgf |
| +35°C (Hot) | 85 cm | 410 kgf |
Key point: The relationship between temperature and tension is inverse. If you are working on a spring day at 15°C, you must adjust the cable so that the dynamometer reads 520 kgf.
Step-by-step procedure for sagging a 6-core cable
Step 1: Initial preparation and stringing
Before applying tension, the cable must be properly positioned on the poles.
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Intermediate (suspension) poles: The cable must be placed in specially installed running blocks so it can slide easily. Never tighten the cable into a suspension clamp before sagging is set.
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Dead-end pole (starting pole): The cable must be fully fixed and anchored at the start of the section using a dead-end clamp.
Step 2: Measure temperature and determine target value from table
This step is critical:
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Using a thermometer, measure the ambient temperature in the shade. Assume the thermometer reads 20°C.
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Refer to the project’s sag-tension table.
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Example: According to our project table, at 20°C and 40 m span, the target tension should be 480 kgf.
Step 3: Apply initial pull
In Simrod Sama’s 6-core low-voltage cables, an independent, very strong “messenger” wire (made of AAAC or ACSR alloy) is incorporated to bear the entire weight of the network.
⚠️ Critical safety and technical note: The pulling tool (grip) must be attached only to the independent messenger wire. The phase, neutral, and lighting strands, which are pure aluminum, must not be subjected to tension!
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Securely attach one side of the tirfor to the cross-arm or pole body.
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Install the cable grip onto the alloy messenger wire of the 6-core cable.
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Start pulling with the tirfor to take up slack and allow the cable to move smoothly over the running blocks on intermediate poles.
Step 4: Fine adjustment with dynamometer
This method is recommended for long spans and projects requiring high precision (e.g., main urban feeders).
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Place the dynamometer between the tirfor and the cable grip.
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Continue pulling (or partially releasing) with the tirfor until the dynamometer needle points exactly to the target value (e.g., 480 kgf).
Step 5: Final fixing and clamping
After confirming correct tension:
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While the tirfor is still under tension, install and tighten the dead-end wedge clamp onto the messenger wire.
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Slowly release the tirfor so that the tensile load is fully transferred from the tirfor to the dead-end clamp.
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At intermediate poles, remove the cable from the running blocks, place it into suspension clamps, and tighten the bolts.
Why does Simrod Sama’s 6-core cable perform better in sagging?
As you have seen in this guide, all mechanical installation stress is applied to the “messenger” wire. The biggest problem for contractors is that if they buy non‑standard cable, the messenger wire creeps after a few months, and the cable sags again.
Simrod Sama Company is one of the main and specialized producers of aluminum alloy wires (AAAC) in the country. We use the same super‑hard, engineered alloys for the messenger wire in our own 6‑core low‑voltage self‑supporting cable.
What is the result? A cable that is installed and sagged once according to the principles of the temperature table will maintain its stability against storms, ice loads, and extreme heat for many years, requiring no re‑adjustment.