|> It is still common practice to build a three phase overhead bank from |> three single phase transformers. Its easier to balance (physically) the |> cans on a pole with a bracket that keeps the bank's CG nearer the pole |> center line. |>
|> For other installations, its all a matter of cost, although three single |> phase padmounts are rarely made into a bank because of the added clutter |> 3 boxes would create. |>
|> -- |> Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com |> note to spammers: a Washington State resident |> ------------------------------------------------------------------ |> Ask not for whom the tolls. | | Good point. I have not done many overheads in the last 20 years. Everything | I have done has been ground mount.. Gad, forgot about the loads on the | poles.
I guess the "three phase in one tank" pole pigs are not that good of an idea considering the pole balance; they'd have to be used on some cross member between 2 poles unless they are rather spamm.
I've driven by some rather large pole pigs arranged in three's serving commercial buildings. I didn't get a good look at one I saw serving a shopping plaza, but I'd guesstimate the tank volumes are about 4 to 6 times the volume of the 75 kVA single phase serving our neighborhood.
One thing I did see on a few of these is that on the two side transformers, there were 3 secondary terminals (much like a 240/120 volt transformer for home service), but the connections were made only to the center terminal and just one of the end terminals. It's like they are using 240/120 transformers to put together a 208Y/120 configuration. But wouldn't that force derating the transformer due to the current limit on the secondary? Or are these actually split secondary (e.g. 240x120 instead of 240/120) that have been rewired somewhere non-obvious to get the secondary windings in parallel with 120 volts?
BTW, the biggest "pole pig" I ever saw, which is certainly not going to be side mountable on a single pole, was about 7 feet high and 3.5 feet wide. Three of them were situated on a pad behind a building. I could not see the nameplate from outside the fence, but I could see the tap changers. In addition to the warning to de-energize before changing, there were a few voltages around 4160 volts. I don't know what the secondary voltage was, but the wires going into the building were as thick as my arm (including the insulation). The thing that struck me odd was that these units were entirely shaped as pole pigs, although with lots of large fins all around. They also looked very old, including being almost entirely a sea-green color tank that looked like copper or brass oxidation.