[Boatanchors] Basic electricity question

[Gary Schafer]

A little added info:
As others have said, the power is either single phase or 3 phase. Lots 
of people incorrectly refer to 120/240 as 2 phase power.

The power that comes in from the power pole by your place comes in 
through a transformer that has a winding that provides 240 volts between 
ends. It also has a center tap which is called the NEUTRAL.

part of your house is wired so it operates (the 120 volt stuff) between 
one side of that transformer winding and the center tap (neutral). 
Another part of your house is wired so that it operates from the other 
leg of the transformer and neutral, also providing 120 volts. The loads 
are split up.

The reason the neutral has its name is because "ideally" there will be 
no current on the neutral line. That is if the load is balanced on the 
two hot legs from the transformer as the phase is opposite on each.

Example: if there is a load of 20 amps from one side of the transformer 
to neutral and another load of also 20 amps from the opposite side of 
the transformer and neutral, there will be no current flowing in the 
neutral leg. It will all flow in the two hot legs only. In this case the 
return path for each load will be back through the other load. The 2 
loads are effectively in series and both the same so the current in the 
neutral or center leg cancels.

That is where the neutral gets its name.

In a typical home the loads are never equal so there will always be 
current on the neutral line.

The neutral wire in your typical outlet connection is not necessarily 
shared with a load from the opposite side of the power, like the line 
from the transformer.
A more proper name for it in this case would be return line but it is 
still called the neutral wire.

The neutral coming from the transformer is grounded at the power pole to 
earth. It is also grounded at your breaker panel to an earth ground rod 
along with what is called the ground wire for the house (the green wire 
that runs along with the hot and neutral).
So at the panel ground and neutral are at the same potential.

The hot lead is always above ground (120 volts). So on a 120 volt 
circuit the neutral wire is the return line for the 120 volts that came 
in on the hot line at your outlet.

The ground wire is only at your outlet for safety reasons. It is to 
connect to the cabinet of your equipment. It is never supposed to carry 
any of the current. In other words it is not to be used as a return path 
as the neutral wire is, even though it is connected to the neutral line 
at the breaker panel. It is there to insure that the breaker will trip 
if the hot line develops a short to the cabinet of the equipment.  It 
also keeps the cabinet at or near ground so you don't get a shock should 
the hot line come in contact with the cabinet. Note that the neutral 
line should not be connected to the cabinet.

If you draw a little diagram of the center taped transformer as I tried 
to describe it may be clearer.

240 volt appliances only use the two hot leads from the pole 
transformer. The neutral lead is not used in this case. The return path 
for one hot lead is the other hot lead. The phase is 180 degrees from 
each other as it is coming from the full winding of the transformer.

The ground line is still connected to the equipment case for safety. If 
a short should occur from one of the hot leads to the case the ground 
wire would then be the return path back to the panel where it is tied to 
the neutral from the transformer. So you would then have a 120 volt 
circuit via the one hot lead and the ground lead that would trip the 
breaker.

Each breaker in each side of a 240 volt circuit are 120 volt breakers. 
They are mechanically hooked together so that if one trips, as described 
above, the mechanical connection to the one for the other side of the 
240 volt line is also tripped to completely disconnect both sides of the 
circuit.

3 phase power as described by others can be thought of as 3 separate hot 
lines. No neutral required. It is a similar circuit to the 240 volt 
circuit described above except that each hot line also serves as the 
return line for the other two lines in the circuit. In other words they 
are all hot leads and all are return lines. The phase is 120 degrees 
apart rather than 180 with single phase so it works.

There are different configurations of 3 phase transformers where a 
neutral is sometimes used too.

3 phase power is more efficient than single phase power. It is used in 
industry to run large motors as an example. You will not see it in any 
home in the US normally. In Europe there are places where it is in 
residential areas though.

You should be thoroughly confused by now.

73
Gary  k4FMX



J. Forster wrote:
> Simply put, the ground is a protective conductor. Akin to the case of a breaker
> box. It's typically connected to physical ground or a water pipe, depending on
> code.
> 
> In two phase circuits (as used in domestic situations) the line is like the plus
> and is hot to ground.
> 
> The neutral is the return like the minus and is usually near ground. Neutral and
> Ground are connected at some point.
> 
> If the line voltage is observed with respect to neutral on a scope, it will be a
> sine wave. If you have a multi-trace scope, and put the other channels on the
> other lines in a three phase circuit, you will see three sine waves, 120 degrees
> shifted in phase between them.
> 
> That's roughly it.
> -John
> 
> William L Howard wrote:
> 
> 
>>BA's and 3-wire cords
>>
>>For those of us who are not electricians, would somebody please explain
>>two phase versus three phase current and also what is meant by neutral
>>lead, and ground? If it were DC it would be positive + and negative -
>>but with AC it alternates.
>>
>>Simple explanations appreciated.
>>
>>Thanks,
>>
>>Bill Howard
>>
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