From the distribution substation, feeders carry the power to the end customers, forming the medium-voltage or primary network, operated at a medium voltage level, typically 5–35 kV. Feeders range in length from a few kilometres to several tens of kilometres. Nearly all spot networks in North America function at a 480Y/277-V secondary voltage. High service dependability and operational flexibility are attained with a spot network supplied by two or more primary feeds via network transformers. Due to economic considerations, primary distribution is carried out by. A primary distribution substation is the connection point of a distribution system to a trans-mission or a sub-transmission network. In this article, unless otherwise specified, voltages are given as line-to-line voltages; this follows normal industry practice, but it is sometimes a source of confusion. The four major voltage classes are 5, 15, 25, and 35 kV.
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