Comment by dustinmoorenet | Hacker News Reader

Comment by dustinmoorenet | Hacker News Reader

I was looking at the transmission lines one day and was curious about why there are always 4 wires. Basically, 3 lines carry equal voltage phased 120 degrees apart from one another. This kind of power is easily generated from 3 phase power generators at the power plant. The 4th wire is a common return for all three wires. It is normally smaller than any single load wire because, if the power is distributed evenly, no load is carried on it.

The second part of the system, is that, normally, no one customer will use more than one of the three load lines. That is why most houses only have one transformer on the pole out side their house. The power company makes sure they balance connections evenly within a given area. If they do this properly, the common 4th wire has no load on it.

Some buildings do use all three phases, but they would be running large industrial machines that benefit from having that type of power.

I tried to find the Wikipedia article that covered all this, but can't seem to locate it specifically.

Its not so much that you have to balance the load in a given area. You must balance the load beyond breakers as much as possible. If the load becomes too far out of balance, the much larger feeder breakers at the substations will trip to protect the grid upstream from the imbalance.

Is that an American thing?

I've never seen 4 lines on a power line in New Zealand unless they're also carrying telephone lines.

I think New Zealand is the same as Australia with 5 wires: the 3 phases, neutral (the central return wire) and earth. Grabbing a random location in Auckland off Street View shows 5 wires:

https://goo.gl/maps/wz1W73uWAiz

Depends on where you are in the country. Here's some 3 wire I found by dropping a random pin: https://goo.gl/maps/MkoiprJ1gWM2

I've always remembered it being 3 pin around where I grew up.

That's a higher voltage power line (in the low kV range) with just the three phases. They're relying on the load being approximately distributed between the three in the downstream distribution network to obviate the need for any return wire. If you move forward one step on that road you will see the next pole has a lower set of wires which will be at 230/240 volts. On these it looks like there are four wires, they're probably doing without the earth wire in this rural area and just tying to earth periodically.

That just seems odd to me to have a wire for earth... carried above the earth. I can understand having earth wires in houses made of concrete and wood since they have to get to the earth through those materials, but what makes a common earth ground better than just lots of many grounding points? All that wire has got to get expensive.

The earth is a pretty terrible conductor.

The point of an earth ground is to dissipate the accumulation of static built up. Ground is not connected to the power supply so there is nothing to return to the power company. The earth is the best place for it.

See here: https://en.wikipedia.org/wiki/Single-wire_earth_return

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