Editing Electric power transmission (section)

=== Subtransmission ===
[[File:Cavite, Batangas jf0557 11.jpg|thumb|175px|A 115&nbsp;kV subtransmission line in the [[Philippines]], along with 20&nbsp;kV [[Electric power distribution|distribution]] lines and a [[street light]], all mounted on a wood [[Utility pole|subtransmission pole]]]]
[[File:Wood Pole Structure.JPG|thumb|173px|115 kV H-frame transmission tower]]{{Unreferenced section|date=November 2022}}
Subtransmission runs at relatively lower voltages. It is uneconomical to connect all [[electrical substation|distribution substation]]s to the high main transmission voltage, because that equipment is larger and more expensive. Typically, only larger substations connect with this high voltage. Voltage is stepped down before the current is sent to smaller substations. Subtransmission circuits are usually arranged in loops so that a single line failure does not stop service to many customers for more than a short time.

Loops can be ''normally closed'', where loss of one circuit should result in no interruption, or ''normally open'' where substations can switch to a backup supply. While subtransmission circuits are usually carried on [[Overhead power line|overhead lines]], in urban areas buried cable may be used. The lower-voltage subtransmission lines use less right-of-way and simpler structures; undergrounding is less difficult.

No fixed cutoff separates subtransmission and transmission, or subtransmission and [[Electric power distribution|distribution]]. Their voltage ranges overlap. Voltages of 69&nbsp;kV, 115&nbsp;kV, and 138&nbsp;kV are often used for subtransmission in North America. As power systems evolved, voltages formerly used for transmission were used for subtransmission, and subtransmission voltages became distribution voltages. Like transmission, subtransmission moves relatively large amounts of power, and like distribution, subtransmission covers an area instead of just point-to-point.<ref>Donald G. Fink and H. Wayne Beaty. (2007), ''Standard Handbook for Electrical Engineers'' (15th&nbsp;ed.). McGraw-Hill. {{ISBN|978-0-07-144146-9}} section&nbsp;18.5</ref>