kid_stevens said:
http://en.wikipedia.org/wiki/History_of_electricity
To save on the cost of copper conductors, a three-wire distribution system was used. The three wires were at +110 volts, 0 volts and −110 volts relative potential. 100-volt lamps could be operated between either the +110 or −110 volt legs of the system and the 0-volt "neutral" conductor, which only carried the unbalanced current between the + and − sources. The resulting three-wire system used less copper wire for a given quantity of electric power transmitted, while still maintaining (relatively) low voltages. ___________________________________________
I'm finding this quote hard to digest, especially the 'less copper wire for a given quantity of electric power transmitted.'
If all loads were 110 V, wouldn't the neutral need to be the same size as the non-neutral conductors ?
Now, if they were talking 220 V loads, perhaps.
There is a very substantial saving in copper by useing a three wire DC system versus 2 wire.
The neutral was typicly half or less the size of the outer conductors, even with most loads being 110 volt, the neutral current would be very small and a reduced wire size suitable.
Smaller premises would be connected between the neutral and the postive OR the negative, with alternate premises being connected to positive or negative thus ensuring a good balance.
A full size neutral would be required within small premises, the saving was in the street mains, not in house wiring.
In practice cable sizes for street mains had to be selected for voltage drop and not current carrying capacity (voltage drop would often be a problem long before overheating of cables)
In this case the saving in copper by useing a three wire system was even greater.
As an example consider a system with a maximum voltage drop of 10 volts (120 volts at generating station, 110 volts at the load)
With a 2 wire system this would allow 5 volts drop in the positive and another 5 volts in the negative.
With a three wire system 10 volts could be dropped in the postive and 10 volts in the negative (120-0-120 at generating station and 110-0-110 at load)
This would allow a given cable to carry twice the current, since the voltage has also doubled the power transmitted is four times as great.
Therefore to summarise, if the load is limited by cable ampacity, then changing from 2 wire to three wire doubles the load that can be supplied for only 25% more copper
If (as is more likely) the load is limited by voltage drop then changing from 2 wire to 3 wire quadruples the load that can be supplied for only a 25% increase in copper.
