How tall is a Power Pole?
Preschem has been receiving a few questions via email asking, “How tall is a Power Pole?” It’s an interesting question. The answer is based on many variables, so has no one specific answer. Lets explain and get to the bottom of this.
Firstly, we are only going to talk about timber poles. This discussion is not about concrete or steel/concrete “Stobie” poles used in South Australia and the Northern Territory.
How tall is a Power Pole?
Power Poles in Australia have a minimum height requirement, which happens to be 4.5 m above ground. In some situations this requirement is greater due to the need to clear terrain, or the type of pole. In such circumstances, the pole can be 5.5 m or even 6 m in height.
But of course there is also the below ground component to the pole. Usually the Pole is to be placed in a hole that’s a minimum of 2 m deep. But this can be up to 3 m depending on the height of the pole, wind loadings and the soil type.
Therefore the total butt to pole top height ranges from 6.5 m to 9 m. Though the 9 m timber poles are very rare, as it’s difficult or near impossible to source logs that long. The vast majority will be within the 6.5 m to 8 m range.
So what type of pole would require additional height?
Power poles are categorised into 3 general classes depending on the environment that they’re in. These are:
- Distribution – which supplies homes or businesses in the street with mains 240 V power
- Transmission – is the electricity backbone carrying high voltages of at least 11kVA. They are usually taller than distribution poles.
- Hybrid transmission and distribution. Carries both high voltage transmission and mains voltage distribution lines. You will note that they usually have transformers at regular intervals mounted on the poles where they need to feed power from the transmission lines into the distribution lines.
So how do you determine the height of a specific pole in the field?
If it’s a sunny day, then it’s quite easy to do safely. We measure it’s shadow. Here are the steps:
- Find a stick or straight edge 1 m long. If can be another length but as long as that length is known. 1 m is easy to do the maths.
- Place that stick vertically and measure it’s shadow. Write that value down.
- Measure the shadow of the pole.
- Calculate the shadow height to shadow ratio of the stick and then divide
Eg. Lets say the 1 m stick is throwing a shadow of 0.6 m. The height to shadow ratio is therefore:
1.0/0.6=1.67.
We measure the shadow of the pole as being 3.0 m. Now we multiply it by the ratio we calculated to find the height of the pole.
3.0 x 1.67 = 5 m
It won’t be to the nearest millimetre, but it will be surprisingly accurate provided you don’t leave it too long between measuring the stick and pole shadow. Remember the sun will shift in the sky and alter the length of the shadow.
