Marne and Associates NESC and OSHA Newsletter addresses issues relevant to power and communication utilities.

In This Issue:

• NESC Live Web Seminar Series
• Reader Comment 1: Joint Use Clearance to a Neutral
• Reader Comment 2: Additional Joint Use Clearance at Pole Required by Midspan Clearance Rules
• NESC Rules for Joint-Use Construction

Applying the NESC to Day-to-Day Utility Work
a four-part live web seminar

Applying the National Electrical Safety Code® (NESC®) to Day-to-Day Utility Work provides a general overview of each part of the NESC and focuses on practical examples and applications. The class includes time for questions and is rich with graphics. Understanding the NESC® rules is a must for personnel responsible for operating a safe utility system.

The first session of this four part series is scheduled on September 10 or September 24, 2008.
More info.

In Question 2 of your last newsletter [August 20, 2008] you did not mention that the neutral is normally the lowest conductor. The clearance would not have to be more than 40" in that case, and in fact could be as little as 30".

Our answer to Question 2: Greater Than 40" for Joint Use Clearance in our August 20, 2008, newsletter focused on the increased clearance required by higher voltages and assumed the neutral was in an upper position next to the phase wires on a crossarm, see Fig. 1. All applicable NESC clearances must also be met.

By permission of McGraw-Hill
Fig. 1. Example of vertical clearance between power and communications (neutral on crossarm)

Footnote 5 of NESC Table 235-5 allows a clearance of 30" between a neutral meeting Rule 230E1 and a communication cable in the communication space, see Fig. 2. Other items to consider include a review of the joint use agreement (contract) on file between the power and communication utilities. Some contracts do not allow the reduction to 30". Also Rule 238 must be referenced for the neutral and communication hardware. Rules 235C and 238 require 40" for secondary conductors; therefore, if a secondary conductor drip loop is hanging near the neutral, 40" is required to the secondary. No exception for 30" exists for a 230C3 secondary cable (i.e. a 120/240V triplex).

By permission of McGraw-Hill
Fig. 2. Example of vertical clearance between neutral and communications (neutral low)

The 40" is a basic dimension and your response to Question #2 with regard to the voltage over 8.7 kV is correct, but it doesn't address the differential ice condition where the mid span separation must be a minimum of 75% of that required at the structure. Thus, sag information must be taken into account with the upper conductor iced and the lower conductor bare. Rule 235.

Our answer to Question 2: Greater Than 40" for Joint Use Clearance in our August 20, 2008, newsletter focused on the increased clearance required by higher voltages and assumed the neutral was in an upper position next to the phase wires on a crossarm, see Fig. 1. All applicable NESC clearances must also be met.

NESC Rule 235C2b(1)(a) requires a minimum clearance in the span of not less than 75% of that required at the supports. For a 24.9/14.4kV line the value would be 0.75 x 43" = 33". Rule 235C2b(1)(c) outlines the required temperature and loading conditions. One condition is with the upper conductor at final sag at 120°F (or maximum operating temperature if greater) and the lower conductor at final sag at the same ambient condition with no electrical loading. The other condition is with the upper conductor at final sag at 32°F with ice specified in Table 230-1 and the lower conductor at final sag at the same ambient condition with no ice and no electrical loading. There is an exception for neutral conductors meeting Rule 230E1 that allows a clearance of not less than 12" out in the span (rather than 75%).

Meeting the clearance requirements out in the span will often require more clearance at the pole, especially on longer spans. See Fig. 3. and Fig. 4.

By permission of McGraw-Hill
Fig. 3. Example of midspan joint-use clearance to 24.9/14.4 kV phase conductor

By permission of McGraw-Hill
Fig. 4. Example of midspan joint-use clearance to neutral conductor

NESC Rules for Joint Use Construction
More info.

Coming to Dallas, TX (Sept 16-17); Atlanta, GA (Nov 12-13); and Missoula, MT (Dec 3-4).

This NESC® seminar is a two-day class focusing on the joint use construction rules in the National Electrical Safety Code® (NESC®). The class will provide an overview of each part of the NESC® that deals with joint use construction (Day 1). Applying the Code to day-to-day work will be stressed by focusing on practical applications and class attendee questions (Day 2). The class includes ample time for questions and attendees are encouraged to share their NESC® applications with the class. The presentation is rich in graphics. More info.