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Why the neutral in USA wiring?

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

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  1. Eric R Snow

    Eric R Snow Guest

    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.
    Eric R Snow
  2. 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

    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. jgreimer

    jgreimer Guest

    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.

    fuse A pressure B /
    switch \
    heater /
    element \

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

    Rex Guest

    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. 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

    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
  6. 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. Roger Johansson wrote:
    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
  8. Others have described why the neutral wire is grounded, but have not
    really answered your question.

    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 Snow

    Eric R Snow Guest

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

  10. 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 Snow

    Eric R Snow Guest

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

  13. Terry

    Terry Guest

    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
    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
    AIUI in UK practice one side of the 230 volt supply is the neutral and is
    basically grounded.
  14. 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. Jamie

    Jamie Guest

    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
  16. Barry Jones

    Barry Jones Guest

    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. jsmith

    jsmith Guest

    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 Jones

    Barry Jones Guest

    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. 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 Jones

    Barry Jones Guest

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

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