How many wires to feed a single-phase xfmr?

Reply

  #1  
Old 01-16-06, 03:17 PM
Member
Thread Starter
Join Date: Oct 2005
Location: Milwaukee WI
Posts: 1,338
How many wires to feed a single-phase xfmr?

I was just rereading _Wiring Simplified_ this afternoon (holiday relaxation). In the farm section they mention that only two wires are needed from the utility to serve a single-phase transformer, while three wires are needed for three-phase. I have lived in other places that have two wires on the poles for single-phase.

However here on my block there is only one high-voltage aerial wire feeding the transformers. (All of the underground high-voltage distribution is three-phase, from which backyard or alley poles are fed on each linear block. In other words, underground distribution runs north to south every four blocks or so and overhead distribution runs west & east. Some blocks with apartment buildings & commercial customers have three aerial wires.)

Why aren't there two wires stretched on top the poles?

Furthermore, about eight feet below the high-voltage wires, the 120/240-volt distribution wires run the length of the block and appear to be connected in common to the secondaries of all three transformers. Is this for redundancy? Economy? To reduce voltage drop? Are my eyes deceiving me?
 
Sponsored Links
  #2  
Old 01-16-06, 06:53 PM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
The distribution system is using 'wye' connected primary coils, where the transformer is connected between the primary 'hot' leg and the grounded neutral. The grounded neutral conductors is shared between the primary distribution system and the 120/240V system. That is why you see only a single high voltage wire; the return conductor is the low voltage neutral.

-Jon
 
  #3  
Old 01-16-06, 07:02 PM
WFO
WFO is offline
Member
Join Date: Feb 2004
Location: usa
Posts: 247
Originally Posted by ArgMeMatey
Furthermore, about eight feet below the high-voltage wires, the 120/240-volt distribution wires run the length of the block and appear to be connected in common to the secondaries of all three transformers. Is this for redundancy? Economy? To reduce voltage drop? Are my eyes deceiving me?
We call this underbuilt. They are paralleling the three distribution transformers so that they each share the load. So redundancy, economy (maybe), and reducing voltage drop are all legitimate.
It is also a bonafide death trap to a lineman who doesn't pay attention. If one tranformer fuse blows, the other transformers connected backfeed it, causing the primary bushing to be hot.
We've gotten rid of most of ours.
 
  #4  
Old 01-17-06, 03:02 PM
Member
Join Date: Apr 2004
Location: Ontario Canada
Posts: 1,767
Typically on residential distribution in the USA and Canada, the neutral is shared between the utility primary and secondary.

If you look, the LV wires are likely not connected between transformers.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes