Michael Moroney said:
By this you mean that multiple transformers have secondaries paralleled
and the length between the farthest ends of a single 120/208V system can
be quite substantial?
Yes, from one end to the other the 120/208V network would be several miles.
Each street had 120/208V feeders running along it and the feeders were
cross-connected at every intersection. The cross connects were protected
by 'network protectors' which are short circuit protection only fuses.
Customers would be fed through taps at each street, also protected by
network protectors. All secondary work is done live.
Primary to secondary transformers would be placed where there was room. The
transformers had 'network relays' on the secondary that would disconnect the
transformer on reverse power flow. The network relay would reconnect power
when the transformer output was re-energized. There would be multiple
feeders to each area, and one of them could be de-energized without
disrupting a customer. This allowed work on the transformers or primary
feeders to be done without disrupting customers as long as it wasn't at peak
demand.
Once buildings started growing the voltage drop and required feeder size to
serve the building became a problem. This became even worse as electric
elevators came into use because the elevator motors were on the roof, far
from the service. The solution was to place a substation on the roof, and
interconnect it with the network. Usually at least two feeders would be
brought up and two transformers would be on the roof to allow continued
operation with one feeder out. Because the secondary conductors were not
short circuit protected, substantial fireproof ducts were required going up
to the roof. All of the feeders and transformers are owned by the utility,
and usually the transformer vaults are restricted to utility worker only
access. Services tap off the feeders at each floor, with metering through
current transformers. This allowed all of the meters to be centrally
located. It also allowed one meter to record for multiple services on
multiple floors if one tenant occupied more than one floor. As buildings
began to grow taller, mid building substations became common.
Newer buildings began to switch over to 277/480V when 277 volt fluorescent
lighting became available. This allowed the bulk power consumers of the
lighting, elevators and HVAC to all be powered from 277/480V with only the
wall outlets and other sundry devices to be powered through local stepdown
transformers. To supply the 277/480V, 'spot' networks were overlaid the
existing network. The spot networks usually fed a smaller area around the
tallest buildings in a downtown area. The spot networks usually had their
own feeders. Just like the 120/208 networks, the 277/480 networks usually
ran up a building with substations on multiple floors.