# Why the neutral in USA wiring?

Discussion in 'Electronic Basics' started by Eric R Snow, Mar 29, 2005.

1. ### Eric R SnowGuest

I was showing a guy how to wire a lamp and explained how only one wire
was switched, and that wire was the hot. The neutral just ran straight
to the switch. He asked me why a neutral was used and I replied it was
probably something which the power company did for their own reasons.
Since 230 volts is used on appliances with only a ground and no
neutral (unless 115 volts is used in the same appliance) I'm wondering
if my answer was correct. If the neutral is used because of the power
companies what would their reasons be? I hope to find the answer soon
so I can tell the guy the real reasons.
Thanks,
Eric R Snow

2. ### John PopelishGuest

Power is supplied to residences in the US as 240 volts center tapped.
If an appliance needs a high power level (range, water heater, heat
pump) is gets the full 240 volts and both lines are switched to turn
it off. The two lines swing in opposite directions around the center
tap (picture an electrical see saw, with the seats being the lines and
the center tap being the central pivot. The center tap voltage is
grounded at the pole, and at the service entrance (fuse or breaker
box) to minimize the peak voltage on any line with respect to ground.
The grounded center tap is called the neutraled conductor or the
neutral.

Receptacles that supply lower power appliances and lighting circuits
use only half of the supply (one line and the neutral) to feed half of
the 240 volts or 120. Since the neutral conductor is at or near
ground potential (except for the voltage drop along the neutral
wiring, back to the ground point) there is little safety or functional
reason to break it with a power switch. Breaking the single hot line
turns the circuit off, while keeping the load near ground potential.

3. ### jgreimerGuest

John, while everything you wrote is true, it still doesn't answer the
question of why a neutral is needed. You say "The center tap voltage is
grounded at the pole, and at the service entrance (fuse or breaker box) to
minimize the peak voltage on any line with respect to ground." But of course
if none of the lines were grounded, none would be any closer to ground
potential than another. The real question is why is a neutral necessary? If
none of the lines were grounded, all our appliances would still work the
same. Let me explain it the same way it was explained to me.

Suppose you have a steam boiler and a pressure activated switch to prevent
the steam pressure from getting too high. The circuit would work perfectly
well if none of the lines was grounded. But suppose decades go by and the
insulation deteriorates and an unintentional ground develops at point A.
Still there's no problem and the circuit continues operating normally and
nobody even notices. More decades pass and another unintentional ground
develops at point B. Now what happens when the pressure switch opens? The
pressure switch is bypassed so the boiler keeps getting heated until it
blows up. If however there is a ground at point C, then when the first
ground develops at point A, the fuse blows and an electrician investigates
and finds the unintentional ground and fixes it.

----o~o--------x--------\--------x--------|
fuse A pressure B /
switch \
heater /
element \
-----------------------x------------------|
C

(if the circuit doesn't line up, try changing the font to courier new.)

4. ### RexGuest

Hypothetical situation...

You are standing on wet ground and randomly grab wires. Some kill you
and some don't. Switches are typically SPST. Where should you put it? By
defining one line as ground by tying it TO ground you get a reference
which will tell you the ones more likely to kill you and
(coincidentally) where to best place the circuit breakers and switches.

5. ### Roger JohanssonGuest

We humans live on the earth surface, so we are very often in contact with
the real ground potential.

If we had free-floating voltages in our mains sockets: Suppose the
wires carrying the voltages for a common light bulb would be at 2000Volt
and 2110Volt. The lamp would work just like today, because it would get
110Volt, the voltage difference between the two wires.

But the risk for us humans would be very high. If you happened to touch
the metal on the lamp while putting in a new lamp, and your feet would be
on the ground, you would die immediately, from the 2000Volt through your
body.

That is the main reason why a ground is needed in the mains wiring. It is
a reference point for the other voltages in the mains wiring, and it
should be very close to the voltage in the earth surface and concrete
floors, water pipes, kitchen sinks, etc..

It makes sure you never come into contact with mains voltages which are
further from ground potential than 120Volt in USA, or 240 Volt in other
countries.

6. ### John PopelishGuest

That is only one scenario that allows a trap to be created with an
ungrounded system. There is actually no such thing as a perfectly
isolated AC voltage supply. So if no point in the system is
referenced to ground, then the voltage at any part of the system is
almost indeterminate with respect to ground. A very high resistance
leakage to the high voltage primary side of the distribution system
may allow thousands of volts to appear on all the low voltage side
terminals, with respect to ground. This would be especially
problematical during lightning strikes on parts f the high voltage
side that cause faster than normal rates of change of voltage that
drive capacitive currents or even flash over from primary to
secondary. Without the grounded neutral or some substitute, there
would be many more dangerous over voltage situations in residences.

But for traps like you describe, the safety ground wiring in all
neutraled systems must be heavy enough to decisively trip the over
current protective devices in the system, immediately on a line to
ground short. And for that same reason, loads that have the
neutraled conductor carrying their current have all their control
devices in the hot side, so that while the fuse is blowing, the load
is shorted to ground on both sides and thus, turned off.

7. ### John PopelishGuest

Roger Johansson wrote:
(snip)
Pedantic quibble for the beginners:

You would die immediately from the high current driven through your
body by the 2000 volts this situation would place across two points on
it.

8. ### Peter BennettGuest

Others have described why the neutral wire is grounded, but have not

You need two wires to complete an electric circuit - the neutral wire
is that second wire in our 120V wiring. If you just connect the hot
wire to a lamp, with no neutral, no current will flow, and the lamp
will not light.

9. ### Eric R SnowGuest

I wasn't clear in my post. I wanted to know why one wire was grounded
and the other not.
ERS

10. ### Michael A. TerrellGuest

One is grounded for safety. You can't ground bother of them or you
would short out the power transformer. The center tap of the power
transformer is grounded and connected to the neutral at the main breaker
or fuse box. This is done to keep the maximum voltage to ground at 120
volts instead of 240 volts, again for safety reasons.

Neutral = little or no voltage different from safety ground.

11. ### Eric R SnowGuest

I understand why the neutral is used for safety reasons now. It makes
tons of sense. Especially having controls all on the hot side so
failures to ground can't circumvent the controls. Thanks to all who
responded to my question. This is what makes usenet great.
Eric R Snow

12. ### Rich The Newsgropup WackoGuest

Because if you grounded them both, it would short out the power plant.

;-)

13. ### TerryGuest

Simply; the neutral is required to carry the current back to the centre tap
of the 230/115 volt supply. All circuits must be complete for current to
flow.
The neutral conductor is close to ground voltage (except for the slight
voltage drop due resistance of the wire and the flow of current through it).
The neutral is not switched as are the live 115 volt wires.
In North American practice the centre tap of the distribution transformer is
grounded.
AIUI in UK practice one side of the 230 volt supply is the neutral and is
basically grounded.

14. ### Roger JohanssonGuest

In Europe we usually have 3-phase 240Volt.
I have that in my summer house.

It means I can use any one of the 240V lines and the ground wire for most
household uses. And I try to divide my use fairly equally between the
three phases to keep the system balanced.

Or I can use any 2 phases and get 380-400Volt between them.

For heavier appliances and machinery I use 3-phase power.

15. ### JamieGuest

the CT here in the USA is also grounded(Neutral).
the advantage we have his is that we have 2 separate
120Vac lines out of phase and non load sharing or, 1 Single Phase of 240Vac
using only the outer taps..
so we have a choice.
we have a real ground also on site to insure safety.

i am sure you understand this, this is for every one else that is
interested.

16. ### Barry JonesGuest

Here's my understanding of the need for a ground and a neutral wire on
the same circuit. No guarantees. . .

The ground (green, uninsulated) and the neutral (white) wires are both
connected to the same place in the breaker box, which is connected to
the incoming neutral line. I know this because I wired my house when I
built it, and the electrical inspector passed it.

110 volt outlet circuits connect to one hot side in the breaker box, or
the other, corresponding to the two incoming hot lines, which are 240
volts apart. The white or neutral wires are connected in the breaker box
to the neutral bar, which is connected to the incoming neutral line,
being center tapped between the two hot lines which are 180 degrees out
of phase. The incoming neutral line is also attached to 'earth', with a
heavy wire going to a long heavy copper bar buried in the earth.

When you wire an outlet box, you attach the uninsulated green wire to
the box if it is metallic, and to the third 'ground' connection of the
outlets. A three prong plug (presumably attached to an electrical
device) picks up the hot, neutral, and ground connections.

So . . .

When a device is running, there is current in the hot-neutral loop. If
there is a long run between the breaker box and the outlet (or device),
there will be some small voltage drop across the neutral wire due to
resistance or impedence in the line. It is no longer necessarily at
'earth' level. If someone were attached to the earth and grabbed the
neutral line, and were a Very Good Conductor, (s)he could get a shock.
But the green ground wire never carries current, so it should be at
'earth' level. It's nice to have that reference around. Especially when
you're in a puddle, or a shower.

Well, that's my story and I'm stickin' to it! :^)

17. ### jsmithGuest

For crissakes don't any of you realize that the neutral is the return path
for the current and completes the circuit!!
The ground (green wire) makes sure the hardware/enclosures the current
carrying circuits are housed in are grounded for safety reasons. . . .DUH!!

18. ### Barry JonesGuest

Fer crissakes, I thought that's exactly what I said. And I described the
system for those not familiar with US wiring.

And fer crissakes, learn to bottom post.

19. ### Rheilly PhoullGuest

Also the MEN system helps out adjoining premises if they have a high
resistance earth and seems a good way to have a complete earth 'mat'.

20. ### Barry JonesGuest

That's an Australian term, right? Multiple Earthed Neutral? How is that
implemented?

I've seen earth potentials range over a few volts within a few tens of
meters. What does that do to the neutral line?