| Figured that this would be a good site to post my question.
| Was looking up at all the poles in my neighborhood that carry power, cable,
| etc., and was wondering a bit on how the power lines are configured.
| Is this a 3 phase system on these residential street lines ?
| From what I see, there are "usually" 3 lines on the upper part of the poles
| which I'm reasonably sure are the power lines.
Sounds like three phase there. Some places have fewer wires and either
1 or 2 of the three phases, depending on what is economical for the power
| The upper, middle one, I'm guessing is a "high voltage" line.
It depends. There may be a ground wire running. There may be 1 or 2 or 3
of the 3 phases running.
| What voltage do they usually carry (residential area, street poles) ?
In the USA, voltages before transformers generally range from 2300 to 34500
volts. Quite a number of different voltages might be in use depending on
the particular power company and the age of that section of distribution.
New construction going out in residential areas will typically have one of:
12470/7200 - 12470 volts line-to-line, 7200 volts line-to-ground
13200/7620 - 13200 volts line-to-line, 7620 volts line-to-ground
13800/7970 - 13800 volts line-to-line, 7970 volts line-to-ground
Transformers could have either of the voltages as input. Note whether
there is one bushing, or two bushings, on top.
| If so, where is the ground for it ?
Several reasons. It's the return circuit on single phase connections
(the lower of the voltage pairs shown above). It serves other purposes
as well. It is often attached to a grounding electrode every few to
| What are the other 2 ?
| Are they different phases (115 V each relative to some ground ?) after the
| voltage is reduced via the periodic pole transformers ?
The wires from the output of transformers are kept lower on the pole. In
the USA, the typical voltage is 240/120 split single phase using 3 wires
(one is neutral). In some places it can be three phase with 208/120 volts
(the high end being slightly lower due to the 120 degree phase angle of
sine waves and can be calculated using the square root of 3 (about 1.732)
times the line-to-ground voltage).
In other parts of the world, voltages differ, frequencies differ, and three
phase may be more readily available, or even required, for residential use.
And three phase can come in two basic configurations called "delta" or "wye"
(most countries use "star" instead of "wye", but the USA uses "wye").
The USA and some other countries, due to the legacy of single split-phase
dual voltage, has ended up with several voltage choices (under 600 volts)
available to end users. Europe and some other countries, due to higher
voltages and general lack of split-phase, have fewer voltages available
because what they do have satisfies the needs better.
In the USA single phase generally is just the 240/120 volt split-phase.
But 480 volts, or 480/240 split-phase, is often used for major roadway
or tunnel street lighting. Three phase is more complex due to trying to
also provide voltages suitable for single phase and/or attempting to use
common transformer voltages. The result is voltages like 208/120, 240/139,
416/240, 480/277, or 600/347 (in the "wye" configuration), and 240, 480,
or 600 (in the "delta" configuration). The 208/120 wye and 240 delta are
intended to supply common single phase 120 volt circuits, too (using a
split-phase on one of the three phases for the 240 delta).
Motors designed for 240 delta can usually be connected to 240/139 wye.
Likewise, motors designed for 480 delta can usually be connected to
480/277 wye. But the wye configurations have a lower line to ground
voltage, which allows using lower cost wiring systems, and is safer for
| Rather than continue with these questions, might someone please explain how
| this is all put together, please ?
The specifics vary, but basically high voltage is transformed to low voltage
for delivery to the home.
Power distribution (getting power around to customers) and transmission
(getting power between power company facilities) uses the term "low
voltage" for anything under 600 volts, "medium voltage" up to about 40000
or 50000 volts, and "high voltage" for anything above that.
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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