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Wire Size/Load Question

Discussion in 'Electrical Engineering' started by Ron Reaugh, Feb 23, 2004.

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  1. Ron Reaugh

    Ron Reaugh Guest

    In the standard 60cycle 240V 3 wire feed from the pole to a house or small
    business I've noticed that the neutral wire is sometimes smaller than the
    two hot wires. Why is that? Has the assumption been made that 1/2 or more
    of the load will be from 240V devices that don't use the neutral wire? Is
    that always a reasonable assumption?
     
  2. Ron Reaugh

    Ron Reaugh Guest

    So it's a power dissipation issue and not just circular mils/voltage drop
    per foot issue? The neutral wire should be twice the circular mils(area)
    from a voltage drop standpoint if the load is all 120 gadgets, right?
     
  3. The neutral only has to carry the unbalanced difference between the other
    phases. So if each hot wire carries 40 amps, the neutral has nothing.

    In a more realistic situation of 50 on one and 30 on the other, the
    neutral only (if you're talking a single phase 120/240 set of circuits)
    carries 20 amps.

    (If you're talking about a neutral as part of a three phase circuit, the
    math is a bit trickier, but the concept is the same).

    Note that this use of smaller neutrals is frowned upon by many current
    local codes, and may have been dumped in the latest National ones. (I
    don't have a copy at hand to double check).
     
  4. Bob Weiss

    Bob Weiss Guest

    Nope. The neutral only has to carry the DIFFERENCE between the currents
    drawn off the 2 hot wires. When the branch circuits are connected to the
    panel, the electrician SHOULD distribute the loads equally between the 2
    sides of the panel, minimizing neutral current.

    Bob Weiss N2IXK
     
  5. Ron Reaugh

    Ron Reaugh Guest

    DUH, thanks.
     
  6. Mark or Sue

    Mark or Sue Guest

    The increased harmonic current can only happen in 3 phase line to neutral loads and not split phase
    120/240 line to neutral loads. What happens is that the current is increasing as the voltage is
    decreasing along the waveform. At some point, the current really drops. If they all drew current
    proportional to their voltage, then the neutral currents would cancel. But the definition of
    non-linear load is that the current drawn is not proportional to the voltage at any given time.
    Therefore, you can get two harmonic waveforms adding current and the third waveform that normally
    subtracts current happens to be 0 at that time.

    From what I read, the worst case possible current overdraw was 1.717 times the rated load current of
    one phase.

    Search for "3 phase harmonic current" and you'll get a better explanation than the one I just gave.
     
  7. I read in sci.engr.electrical.compliance that

    Yes. You have done well to work out these scenarios independently. But
    the situation isn't quite as bad a pure arithmetic addition of the
    current pulses in the neutral. The upper limit is about 200 %, not 300%.
    But there is a balancing effect; these short pulses are associated with
    less than maximum loads on the power supplies. In other words, a 300 W
    supply would do this if loaded to say 100 W, but the currents involved
    would be related to 100 W, not 300 W.
    Yes, it does. Smoke from the neutral cable.
     
  8. I read in sci.engr.electrical.compliance that Paul Hovnanian P.E.
    The same effect occurs to some extent when increased demands are made
    for 'hold-up' when the supply voltage decreases or is interrupted, even
    though there is no increase in system capacity (as opposed to
    'capacitance'!); the conduction angle of the rectifiers is still
    decreased.
    Yes. AFAIK, there is no condition in which the ratio of the r.m.s.
    triplen components to the **full load** fundamental current increases
    with decreasing conduction angle, at least down to 36 degrees, below
    which the inrush current becomes a big problem and it is, in any case,
    difficult to get a low enough total resistance in the rectifier circuit
    economically.
     
  9. I read in sci.engr.electrical.compliance that
    The matter has been thoroughly investigated many times, largely in
    connection with the provisions of IEC/EN 61000-3-2 on limitation of
    harmonic current emissions into the public low-voltage supply. Practical
    ranges of rectifier conduction angle are approximately 36 degrees (below
    which inrush current becomes a problem) and 72 degrees (above which
    efficiency is much too poor).
    Instead of derating, new neutral cable can be installed. Still not
    cheap, but at least the outcome of the expense is that the system has
    been uprated rather than downrated.
     
  10. Guest

    Actually, you don't need that kind of load (hundreds
    of computers) before it becomes a concern. They
    manufacture cable (called super neutral?) for branch
    circuit wiring in offices with 12 gauge phase and 10
    gauge neutral conductors. So the excess neutral
    current is a concern even on 20 amp circuits.
    Certainly the problem would be magnified at the
    service, which is what you may have in mind.

    By the way, your approach to showing the problem
    with a diagram is great. You can see where the
    pulses on each phase would create neutral currents
    that would not cancel each other out.
     
  11. [ snip ]
    I'm finding this fascinating as I have just enough background in this
    stuff to be dangerous without being helpful...

    so please forgive what may be a dumb question to you both:

    In the case of the 300 watt supply provviding 100 watts
    of power, the diagram, as I read it, demonstrated some
    hefty power peaks. BUT they only seemed to be for
    one third the time, with the other 2/3rds showing nothing.

    Did I interpret it correctly? If so, shouldn't the wiring
    capacity be rated for the averaged out load rather than
    the short term (fractions of seconds) peaks?

    Or am I completely off here... (wouldn't be the first time)

    Thanks
     
  12. Robert Casey

    Robert Casey Guest

    Also offices full of electronic florescent ballasts also do this.

    As for the computers, most are configurable to operate on 208-250VAC and
    100-125V. So all the computer power supplies could be set for 208-250V
    and connected to 208v sources. Thus the neutral never sees any of these
    loads. But this would require new power cords on all the computers to be
    fitted with 6-15 plugs. And don't forget to switch all those switches
    on the
    supplies (some are hidden behind plastic bezels on the back). Probably
    cheaper to
    just install a super neutral.
     
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