Utility Pole Transformers

Overview
  • Commercial electrical generators of any size generate what is called 3-phase AC power
  • Single-phase power is what you have in your house. Generally referred to as single-phase, 120-volt AC service.
  • The power plant produces three different phases of AC power, each offset 120 degrees from each other. There are four wires coming out of every power plant: the three phases plus a neutral or ground common to all three.
  • The three-phase power leaves the generator and enters a transmission substarion at the power plant where large transformers which converts the voltage to extremely high voltages for long-distnace transmission. Typical voltages here range from 155K to 754K volts. The transmision lines are carried by the huge steel towers you see.
  • Next is a power substation which steps the voltage down, generally less than 10K volts.
  • Typical residental has a set of poles with one phase of power (7.2 K volts) and a ground wire. Sometimes there will be two or three phases on the pole, depending on where the house is located in the distribution grid.
  • There is a grounding wire from each pole to the earth. The guy-wires are attached to the direct connection to the ground.
  • Two wires run from the utility pole transformer and three wires to the house. The two from the transformer are insulated and the bare one is the ground.
Two-Phase
CaptureTwo-phase electrical power was an early 20th century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by 90 degrees. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor.
two-phaseChris Atkins This is a 2-phase 440V service, which gives 220V phase to neutral. Since neutral is reference to ground, you don’t need it for metering, etc. Modern systems would have 2 or 3 separate transformers, but it appears in this view that this could be a 2-phase transformer, which I’ve never seen, but it theoretically possible. It’s also possible that the side view would show another one behind it. Most polyphase services supply 3 phases, but some equipment may work on just two phases, so this setup was evidently needed. I’d say this is an exception rather than the norm. Another interesting thing is that they refer to 440V and not 480V. At some time we jumped up 10/20/40V, but most devices say rated at 110/120V. The way it was explained to me 25 years ago was that it allows some voltage drop between the transformer and service. I don’t think it was always that way and at one time these voltages were referred to as 110/220/440, etc.
Three-Phase
three_phase_11000-voltThis diagram is taken from Electrical West, Volume 33 pub.1914.

I used this diagram as a guide for my own version

2trans3phsseHere we go .. side bracket transformer of the left. I will be using two of these side by side and wired as in the diagram.
doubleworking.. and .. here we go. I followed the diagram pretty much but added a platfrom, ladder and foot pegs.
2trans3phaseWiringHave all the wiring figured out except for the ground. In the wiring diagram above it shows the center lug grounded. Nothing is shown connected to the right center lug. I can *assume* that this is also grounded but .. gonna ask the experts
Notes
Cross-arms for telephone lines are usually 10 or 6 pin, the wires adjacent to the pole being 16 inches apart and others 12 inches apart. Cross-arms are mounted two feet apart. Poles are usually set to give an average span of 130 feet.1

On 40 ft. poles the transformer arms are placed 4 ft. below the 11,000 volts arm; on 45 ft. poles, 7ft; and on 50 ft. poles, 10 ft.2


  1. Electrical Engineer’s Pocket-book: A Hand-book of Useful data for Electricians and Electrical Engineers. pub. 1908 []
  2. Electrical West, Volume 33 pub. 1914 []