Utility Pole Span Calculation

Maple2 · Aug 25, 2021

Hello Gurus,

Im working on my first powerline design. I want to design a private powerline extension (120/240V single ph, 30A) from an existing breaker panel in a residence to a proposed structure 150m away via overhead wooden poles in a rocky terrain. Whats the most comprehensive way to calculate or determine the pole span and tension? I am familiar with Ruling Span Method (see attachment). However, this method is ideal for flat terrains. For those with experience in this field, what is the most convenient method of calculating or determining cable spans in unbalanced terrains?

Another issue is the proposed routes crosses a HV transmission lines at a point, hence should I go 'Direct Buried' at the location where my proposed power line crosses the HV transmission lines? EMF effect from Tx power line is a concern for me.

While faced with this dilemma, my initial choices for design are as follows:

Cable: #2AWG (Neutral supported Cable) for the connection,
Pole Class: Class 7 Pole,
wood : Western Red Cedar
Length: 25 ft
Minimum Top Circumference: 15 inches
Min. Circumference at 6ft from butt: 24 inches

Your opinions would be appreciated. Thank you

waross · Aug 26, 2021

The last couple of similar installations that I did in your area I used direct burial cable.
The first two were done with direct burial Service Entry cable.
After a code change, the last install was with aluminum ACWU-90. It was actually a little cheaper than SE cable and is a more robust construction.
ACWU-90; Armoured Cable, Wt, Underground- 90 degree. Usually constructed with aluminum conductors but available in copper.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Maple2 · Aug 26, 2021

I am not so sure about if BC Hydro would allow that. Was the installations on flat grounds?

Due to the unbalance rocky terrain, I thought it did be best to use a Neutral self supporting (ACSR) cable for the overhead lines, and also considering that I may have to ground the neutral at first and last poles.

KillBill7 · Aug 27, 2021

In terms of convenience, you should look for power line design software like PLS-CADD. You can use simple parabolic equation to estimate the sag based on the tension, but that works only if the wire attachment points are at the same elevation.
If you are meeting the clearance requirements when crossing under the HV line, you don't need to go under the ground. CSA 22.3 No.1 has these requirements listed in one of the tables.

waross · Aug 28, 2021

From your description of the project I don't think that BC Hydro is the AHJ. You will be under the Canadian Electrical Code, and any BC amendments. It's over 40 years since I had a job inspected in BC and I don't know the present permitting procedures.
That said, some HV transmission lines have easement agreements for the land below them which limits the type of activities that may take place below the line. You should check for any restrictive easements before putting anything under a transmission line.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

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