If 1/2 mile, 20 amp requires 500 kcmil, why do utility poles of many miles have such small wires?

S

Stevenfyeager

Senior Member
Location
United States, Indiana
Occupation
electrical contractor
I was explaining voltage drop to a customer who wanted me to install a 20 amp circuit for 1/2 of a mile. When I told him it would require 500kcmil AL, he pointed to powerlines on the utility poles and asked ‘why can they transfer electricity for many miles on those small wires? ‘
I did not have a good answer. I told him one line man told me it is because those wires are in open air.
Your thoughts, please.
 
Short answer: the powerlines on the utility poles that run for miles are at a higher voltage than typical utilization voltage.

Power loss depends on current (the square of the current, I2*R), and a for a fixed power delivered, current varies inversely with voltage (P = I * V). So using a high voltage on transmission lines reduces power loss for a given power delivered.

Cheers, Wayne
 
Is it a 2400W load (20A @ 120V) ?
Yeah even with 8awg AL you'd be hard pressed to get over 200' and not hit 5% voltage drop or whatever the energy code limit is in your area.

If we took two old utility transformers a 2400V : 120/240 single phase to step up to 2400V then back down to 120/240 the
we could probably go about 20 miles on 8AWG and not hit 5% voltage drop?
(Though we'd probably see a 4800VA load at the source to account for the transformer & wire losses)
 
High voltage is the answer, others have stated. The problem is clearly explaining that to a non technical customer.

"The power delivered to the load is voltage time current. But it is current that causes voltage drop.

The utility company uses high voltage for power distribution and long distance transmission. The higher the voltage the lower the current to deliver the same power, so higher voltage means less voltage dropped. On top of the, the higher voltage means losing a few volts is less important, so you get a double benefit.

But higher voltage is more dangerous. You wouldn't want 2400V wiring in your home, and have to glove up just to toast some bread. So the power company uses transformers to convert the high voltage low current distribution power to low voltage high current for use in your home."
 
Most of our distribution around here is 19,900V to ground. That's 165 times the usual 120V your customer is used to. Given that VD is linear with distance and current, your half mile is equivalent to 16 feet at the higher voltage. So, higher voltage is the answer for the VD part. As far as conductor heating, the rest of the answer is "free air" and no insulation (excluding Hendrix cable).

Mark
 
If the OP can use a 15 KV supply he can use #12 ;)
 
Stevenfyeager said:
I was explaining voltage drop to a customer who wanted me to install a 20 amp circuit for 1/2 of a mile. When I told him it would require 500kcmil AL, he pointed to powerlines on the utility poles and asked ‘why can they transfer electricity for many miles on those small wires? ‘
I did not have a good answer. I told him one line man told me it is because those wires are in open air.
Your thoughts, please.
Click to expand...
A couple things. Voltage drop doesn't change merely because the wire is insulated or open air. Technically voltage drop will change some because a hotter conductor has higher resistance. Others have answered your question. The explanation behind it is that volts= current times resistance. you will get the same voltage drop for a given distance of wire (which is due to the resistance). So as a percentage the value is higher for the lower voltage.
 
2400 VA @ 120 volts is 20 amps

2400 VA @ 2400 volts (some smaller villages do use that for distribution voltage) is only 1 amp.

7200 volts is pretty common distribution voltage so 2400VA is only a third of an amp on that system.

Many those overhead medium voltage POCO lines could be even smaller and still carry the load imposed on them but still need to be a certain size just for physical strength.
 
I am honestly surprised in 2025 its still typical to see large services 2000 - 4000A range.
Since the code bumped low voltage up to 1000V moving to 1000/577 would seem more logical, 2X smaller wire, less 'fault current'.
 
tortuga said:
I am honestly surprised in 2025 its still typical to see large services 2000 - 4000A range.
Since the code bumped low voltage up to 1000V moving to 1000/577 would seem more logical, 2X smaller wire, less 'fault current'.
Click to expand...

It seems like I have been seeing more 208 volt, thousand amp services nowadays than before, when they were usually 480. Maybe they don't want transformers. I worked in a building last year that was 10 stories and all 208 volts.
 
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