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I live in Minnesota, and a few years back, Excel Energy constructed a massive power line stretching from Monticello, MN to Fargo, ND.

This is not the exact photo of the power lines, but it's very similar in design:

enter image description here

The power line is wired on only one side of the tower for the entire trip of the line. The only exception is when the power line crosses Highway 94, both sides are wired. Once the line has crossed over the highway, it continues on only one side. The line crosses several times for the length of the trip (approximately 200 mi,) each time, both sides coupled.

My question is, what is the purpose of having both sides of the tower completely wired when crossing the interstate when only one side is clearly being used? Are these "extra lines" underground cables that need to be crossed overhead?

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    \$\begingroup\$ Just a guess: They designed in the ability to expand the capacity of the grid. For most of the length, it's cheaper to leave off the extra wires until they're needed. For the highway crossings, it's cheaper to wire them up now and avoid the cost of closing the highway when they want to expand. \$\endgroup\$ – The Photon Aug 19 '15 at 16:41
  • \$\begingroup\$ @ThePhoton Nice guess. \$\endgroup\$ – Eugene Sh. Aug 19 '15 at 17:30
  • \$\begingroup\$ not that massive either - where I live we have a couple of local ones with larger capacity. \$\endgroup\$ – Oleg Mazurov Aug 19 '15 at 18:10
  • \$\begingroup\$ Is it cheaper to cable everything on the same side ? The cable weight is not well balanced, it may stress the structure. \$\endgroup\$ – TEMLIB Aug 19 '15 at 23:37
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    \$\begingroup\$ @TEMLIB: all conductors on one side for electrical reasons, safe working practice reasons (can turn one side off and work on it, without being near the other side which is live), and ease of taking off the circuit at the end of the line (easier to bring the wires down to the ground if the wires don't have to criss-cross on the way down.) I'm not a mechanical/structural engineer but I imagine the weight of the conductor is not so significant as the wind loading. \$\endgroup\$ – Li-aung Yip Aug 20 '15 at 2:14
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I posted this initially as a guess in comments, but the community seems to agree with me, so I'll make it an answer:

They designed in the ability to expand the capacity of the grid. For most of the length, it's cheaper to leave off the extra wires until they're needed. For the highway crossings, it's cheaper to wire them up now and avoid the cost of closing the highway when they want to expand.

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  • \$\begingroup\$ +1 to add that the wires shown are 3 phases with one pair per phase and form one complete distribution circuit. The extra wires are for one complete additional circuit \$\endgroup\$ – crasic Aug 19 '15 at 20:43
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    \$\begingroup\$ Absolutely correct. Road closures are a pain in the ass. \$\endgroup\$ – Li-aung Yip Aug 20 '15 at 0:43
  • \$\begingroup\$ @Li-aungYip, thanks for confirming. I was hoping for someone who actually knows power transmission systems to drop in. \$\endgroup\$ – The Photon Aug 20 '15 at 1:08
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    \$\begingroup\$ Another similar practice is to run spare ducts/conduits under roadways, in anticipation of future need. If you are installing one conduit, it doesn't cost much to install some extras at the same time. Far cheaper than installing new conduits after the road has been paved over - which would involve closing the road, ripping up the asphalt, and fixing it when you're done. (It's also possible to install conduits using directional boring, where you "drill in from the side", but i's more expensive and more risky.) \$\endgroup\$ – Li-aung Yip Aug 20 '15 at 2:11

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