/images/buttons/TDU_thumbnail.GIF/images/southwire_100x70.jpgNew NESC Revises Overhead Conductor Tension Limitsnew-nesc-revises-overhead-conductor-tension-limits.htmThe 2012 NESC contains changes to the temperature values used to compute tension limits for overhead conductors
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New NESC Revises Overhead Conductor Tension Limits

The 2012 NESC contains changes to the temperature values used to compute tension limits for overhead conductors.

Conductor tension is a critical variable in fighting wind-induced Aeolian vibration that can lead to premature overhead conductor failure. In an effort to reduce the potential for Aeolian vibration, the National Electrical Safety Code (NESC) sets tension limits on conductors of 35 percent of Rated Breaking Strength (RBS) for initial unloaded tension and 25 percent of RBS for final unloaded tension.

"With too little tension, sag becomes unacceptable. With too much tension, the conductor is more susceptible to damaging stress fatigue from Aeolian vibration," says Mark Lancaster, Southwire Energy Division, Director Overhead Transmission Engineering.

New temperature parameters use district temperatures

In prior versions of the NESC, the unloaded tension values were computed for a conductor temperature of 60°F (15°C). The 2012 NESC, in Rule 261 H.1.b, changes the temperatures at which the tensions are to be applied. These changes result in more restrictive design limits.

The new temperature parameters vary depending on which loading zone the installation is in. Tensions for installations in heavy load zones must be computed at lower temperatures than in previous editions of the Code. Table 251-1 in the 2012 NESC lists these "loading district" temperature parameters:

NESC "Heavy" 0°F (-20°C)
NESC "Medium" 15°F (-10°C)
NESC "Light" 30°F (-1°C)

Engineers should consider each installation's requirements

A note at the end of Paragraph H.1.b in the 2012 NESC cautions that "the above limitations may not protect the conductor or facilities from damage due to Aeolian vibration." Lancaster comments, "This note means that you can't just blindly apply the limits in the table. For every installation, the engineer of record should always consider whether these limits are appropriate."

In addition, the NESC allows for exceptions to these new design limits that allow the use of the traditional design methods. These exceptions include the use of vibration dampers or installations using self-damping conductors, such as Southwire's ACSS or VR2 conductors. Lancaster concludes, "While Southwire does not make design recommendations, Southwire engineers are always available to help designers with tension and vibration questions."