few unrelated questions ?

Discussion in 'General Electronics Chat' started by Mathematics!, Feb 19, 2009.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    I never electrically wired a house completely. But I am wondering
    when the electrical engineer installs the circuit breaker box.

    Where do they shut of the power to the main 2 (120volts) wires coming into the house from the powerlines. I am looking at the big 200Amp circuit breaker at the top of the panel but this just closes and opens the circuit to your house. What happens if you wanted to move the electric circuit breaker box to a different location you still have the main lines that will have current flow thru them if you complete the circuit. Basically when running the cables from the powerline is their a switch on the powerline to turn off power to the main lines so you can move them or extent them in some way?

    I am just wondering how the people install the boxes. Do they have to shutdown a street powerline for a few minutes while they hook the ends of the main lines from your house to the powerlines transformer etc.. ??

    Note I am not plaining on doing this I am just wondering how they go about it.

    And I am also curious about why some houses have one wire running from the powerline and why some have 2 or 3 running from the powerline.

    Is the one wire carry both the 120volts = 240 volts???
    Because I know in the case of 3 wires we have two 120 volt hot wires and one neutral ground wire coming from the powerlines.

    Maybe their are some places that use only step down transformers that step down to 240 and other places where they step it down to 120 volts and add to lines per house? Maybe that's how it is?
    Just curious how this works.

    You can assume we are in the united states.
    Last edited: Feb 19, 2009
  2. mik3

    Senior Member

    Feb 4, 2008
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    In the UK the power company has trained crews to work on the cables in the road and to your house while they are still live. They do not turn anything off to do this. They also provide the supply through a fuse whcih is sealed by them. They can disconnect this and reconnect it if they wish.

    Do NOT attempt this yourself.

    Personally I installed a fused double pole isolating switch between the company's meter and my breaker panel so that I can isolate my wiring to work on it.
    If you are proposing changes in the future I would advise having a competent electrician make this addition to your wiring.
  4. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008

    Ya, but I have seen regular houses that have three wires run from the powerline to it. And then I have seen the next house with only one
    powerline runing to it.

    I was think it had to do with the step down transformer being 240 instead of 120volts or something.

    I agree for machine shops that need the 3 phases for their machines.
    But I cann't see any machines in a regular house mattering.

    A little confused about it.

    Thanks for the clarity on the first question I was assuming it but didn't know for sure.
  5. bertus


    Apr 5, 2008

    There are houses that use an electrical cooking furnace.
    That will require 3 phase power.

  6. beenthere

    Retired Moderator

    Apr 20, 2004
    Outside the house, where the meter is located, there is a disconnect switch. It can vary with the utility, and can even include the meter proper (pulling the meter also disconnects power).

    Electricians and electrical engineers are not equivalent.
  7. thingmaker3

    Retired Moderator

    May 16, 2005
    The Kitsap Peninsula is an example where no main disconnect is used in older residences. The meter box is pulled to open the circuit. This has the disadvantage of exposing the power company's feed, so it is vital to cover the opening & keep children or other curios folk away.
  8. b.shahvir

    Active Member

    Jan 6, 2009
    The 3 wires running to an individual house, in a few cases might be ‘Phase’, ‘Neutral’ and ‘Earth’ wires resp. Even though Earth or Ground is available all around us, it is mandatory for utility companies in most cases to run an additional earth wire to adhere to safety standards and IECs.

    This is more so the case in towns and isolated bungalows where a common single phase ckt. consisting of phase wire, neutral wire and earth wire are looped and tappings taken for individual houses for supplying 1-phase power (at 120VAC or 220VAC, whatever may be the case).

    The single wire you mentioned might be only the phase wire tapped by the house owner; the earth or ground acting as the return neutral (return wire of appliance is connected to sink pipes which are grounded or a spike is driven into the ground to which the return wires of all the house appliances are connected. This practice is illegal and is discouraged by utility companies as it could lead to safety issues and interference with communication signals carried by underground cables.

    Also, voltage drop across an improperly earthed spike return can cause appliance failure due to contact resistance between earthing spike and earth. THIS METHOD SHOULD NEVER BE TRIED AT HOME, IT IS ILLEGAL & DANGEROUS. :eek:

    Dedicated ckt. breakers and isolating switches are provided upstream & downstream on the electrical feeders catering to particular localities or zones. These breaker & isolator switch panels are installed in indoor substations hidden from public view.
    I hope this would provide some clarity. :)
  9. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok so if their is 3 wires coming into the house from the powerlines then it
    is 2 hot 120 volt lines and 1 ground line.

    When it is only one wire coming from the powerlines to the house it is a 240 volt line.

    And when their is only 2 lines coming into the house from the powerlines then it is 2 hot 120 volt lines.

    If this is correct, then how do they split the 240 volt line into 2 120 volt lines in the case of one 240 volt line.

    And I have never seen it but if their where 4 lines coming to the house from the powerlines then it would be 3 hot 120 volt wires and 1 ground wire. So a 3 phase would have to have 4 wires coming in or at least 3 hots and use earth for ground.

    I am also assuming all houses in US need 240 volts even though must circuits in the house work well with just 120 volts. Because their are some circuits that need the full 240 like water heater , or something etc.
  10. SgtWookie


    Jul 17, 2007
    Not quite.
    In the USA, the two "hot" lines are called L1 and L2. They are the opposite ends of a transformer secondary winding.
    The third line is called "Neutral". It is a transformer center tap between L1 and L2. If you measured voltage between the Neutral line and either L1 or L2, you would measure (nominally) 120VAC, but if you measured from L1 to L2 you would measure 240VAC.

    In that case, I think you're looking at a telephone line - or perhaps the three of them (L1, L2, Neutral) twisted together.

    More like it would be 240VAC.

    There would have to be a large transformer on the property with a center tap.

    L1, L2, L3, and Neutral.

    Electric ranges, electric dryers, water heaters and heating/air conditioning systems most frequently use 240VAC, as they are high wattage devices. Increasing the voltage available to them reduces the losses in the wiring up to the appliance.
  11. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok, but if the neutral (white wire ) is the center taped wire on the transformer won't it have a voltage different then ground voltage?

    Because when I touch me digital multimeter to the white wire and the other prong to a water pipe (earth) . I get zero volt's???

    I would think if it is connected to the middle of the transformer you would get about half the 120 volts and the neutral would also be a hot wire???

    Are all transformers for a house step down to 240 volts with a center tap of 120volts?
    Even still if this is the case how is the white wire have no voltage if touched to earth?
    Last edited: Feb 19, 2009
  12. SgtWookie


    Jul 17, 2007
    The neutral wire is connected to earth ground at the electrical service panel (aka breaker box). This keeps the Neutral line from "floating" to an arbitrarily high (and dangerous) voltage level, and thus keeps L1 and L2 pretty much centered around ground. The electrical service panel is the ONLY place that Neutral and Ground should ever be connected together. Outside of that box, they are very different animals; Neutral is the power return line, and Ground is for human safety. The ground wire should never carry current; if it does, there is a fault somewhere that must be corrected.

    In the "old days", it was accepted practice to ground household electrical panels via copper plumbing that was connected to either a well or city/county water. This practice has long ago fallen out of favor. Nowadays, it's more like two or more 8' long copper-clad rods that are pounded into the ground several feet apart, with AWG 4 or larger solid copper wire connected to the ground buss.

    Also, nowadays most plumbing is performed with PVC (CPVC) pipe, which is non-conductive. Copper is still occasionally used, but it's gotten ridiculously expensive, and doesn't stand up well when exposed to water that has high sulphur content.

    Neutral is the center tap. Since Neutral is connected to earth ground at the service panel, it doesn't stray very far from that, unless there is a severe load somewhere.

    In the US, it's 120-0-120. The ends of the transformer are equal in voltage, but opposite in phase; ie they are 180° apart. A single phase is split in two by the neutral wire, which is grounded.

    It's grounded at the service panel.
  13. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    I am still a little confused.
    Ok at the transformer drum on the powerline T post
    The primary coil has something like 2400 volts and the secondary coil has a center tap neutral white wire and 2 black hot wires which are steped down to 120 volts.

    AC Transformer
    induced voltage starts flowing back and forth <--->

    When you complete the circuit in the wall outlet
    (i.e connecting black/white together)
    w b

    How is the earth even involved in the transformer circuit.
    Basically how do you complete the circuit if you hold only the black hot wire or only the white wire? Won't you need to hold both white and one of the blacks or both blacks?

    How does earth come into this circuit?
    Because if earth is not in this circuit in some way their will never be a saftey green wire that will be a short in case of an accident.
  14. SgtWookie


    Jul 17, 2007
    Actually, I think the primary is usually somewhere around 11,000vac - but your mileage may vary.
    The circuit isn't actually completed in the wall outlet/receptacle. It's just a simple, efficient and relatively safe way to have power available in a convenient form. Sure beats having to twist wires together.

    Because the neutral (white) wire is tied to earth ground at the service panel. In case there is a fault in the transformer (a short between the primary and the secondary, or some other severe fault) the ground path of the neutral on the secondary will cause the fuses on the primary side of the transformer to blow.

    You wouldn't want to hold ANY of those wires. Only the ground wire should be considered safe.

    For appliances that are so equipped, the ground wire is connected to the chassis. This insures that in the event of an electrical fault within the appliance, the human user of the appliance will not be subjected to electric currents passing through their body.

    In particularly sensitive areas such as kitchens, bathrooms, garages and outside areas where the human user may have direct contact to earth ground via conductive water, GFCI (Ground Fault Circuit Interrupt) breakers are used; if the current in the ground wire exceeds around 15mA, the power is disconnected.
  15. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    So when the power plant generates their ac current they must have a circuit.

    power plant generator ---high voltage lines--- substation ----- powerlines

    ignore the ....

    where is the completion of the circuit. Do they use the earth for one side of their circuit and the high voltage lines for the other side?

    So is it like the generator is a battery that alternates in polarity and earth is one side of the battery and high voltage lines are on the other side.

    Because if they don't use ground at the main generator then this isn't a complete circuit either. Unless the 3 black wires touch some where?
    Last edited: Feb 20, 2009