The National Electrical Safety Code (NESC) 2023 edition introduces several updates, including revisions to Rule 253 and Table 253.1. These changes are crucial for ensuring the safety and reliability of electrical supply and communication facilities. This article explains how the O-Calc® Pro wire tension Load Factors (LF) are determined based on different structure types as outlined in the Table 253.1
NESC 2023 Rule 253 and Table 253.1 Overview
Rule 253 in the NESC 2023 focuses on the mechanical loading of structures, specifically addressing the load factors for wire tension. Table 253.1 provides detailed load factors for various structure types, ensuring that the design and maintenance of these structures meet safety standards. Table 253.1 is reprinted here:
IEEE Standards Association. (2023). National Electrical Safety Code (NESC) 2023 Edition. IEEE, Table 253.1
Load Factors (LF) in O-Calc Pro
O-Calc® Pro, a powerful tool for analyzing and designing utility poles, incorporates these load factors to ensure compliance with NESC standards. The load factors are based on the type of structure and the specific conditions under which the wires are installed. Here’s how the load factors are applied:
- Wire Tension (Guyed, Junction, Angle) Pole LF (Grade B=1.65, Grade C=1.30):
- These poles experience different tension forces due to their configurations. O-Calc® Pro automatically identifies these pole types based on the model parameters and applies the appropriate load factors from Table 253.1, namely Transverse Loads-Wire Tensions.
- Wire Tension (Unguyed Tangent) Pole LF (Grade B=1.10, Grade C=1.00):
- Unguyed tangent poles are typically subjected to less tension compared to guyed or angle poles. O-Calc® Pro determines these poles based on their straight-line configuration and applies the corresponding load factors from Table 253.1, namely Longitudinal Loads- In General
- Wire Tension (Deadend) Pole LF (Grade B=1.65, Grade C=1.30):
- Deadend poles bear the full tension of the wire, making them critical points in the structure. O-Calc® Pro identifies these poles based on their position in the network and applies the highest load factors to ensure safety. Load Factors from Table 253.1, Longitudinal Loads- At Deadends.
Automatic Determination of Pole Types in O-Calc Pro
One of the key features of O-Calc® Pro is its ability to automatically determine the type of pole based on various parameters of the model. When the Pole Type attribute is set to Auto, O-Calc® Pro will determine the pole type automatically using the following criteria:
- Guyed Tangent: If the pole has a GuyBrace and two unique span angles. And the pole line angle is small, within +- 5 degrees, i.e. span angle difference between spans is between 175 and 185 degrees.
- Unguyed Tangent: If the pole does not include a GuyBrace and there are no unique span angles. And the number of span angels is uniquely two and difference between 175 and 185 degrees.
- Deadend: If the pole has a single unique span angle. Or if there are two unique span angles and the angle difference is less than 25 degrees.
- Angle: If the pole has two unique span angles. Can be guyed or unguyed and the angle between the spans is either less than 175 degrees or more than 185 degrees.
- Junction: If there are 3 or more unique span angles.
The end-user can always override the Auto determination of the Pole Type by manually setting the Pole Type to Unguyed Tangent, Guyed Tangent, Deadend, Angle, or Junction. However, it is important to understand how the Pole Load Factors are applied when the value is manually overwritten, reducing the risk of failures and ensuring compliance with regulatory standards.