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GPO's per circuit, how many in latest regs?

Discussion in 'Hobby Electronics' started by Geoff C, Jan 11, 2005.

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  1. Geoff C

    Geoff C Guest

    If anybody knows with some confidence the number of GPO's per power cct in
    Aus (Vic) according to latest wiring regs, I would appreciate it. A sparky
    recently told me 14. This means 7 doubles. Is this correct? This is for
    standard 16 amp fuses per cct. Also, how is the de-rating calculted if you
    put one or more standard light fittings on a power cct. The sparky told me
    there is a de-rating but could not recall the calcs from the regs.
  2. David

    David Guest

    No limit under AS/NZS 3000:2000, (also no such thing as GPO anymore, they
    are called sockect outlets). Under the old rules there were limits to
    encourage more than one power circuit. In the old rules multiple outlets
    were counted as one less than the number, thus doubles were only counted
    as one.

    The main requirement of AS/NZS 3000:2000 is that the installation
    functions as required in a safe and expected way.

    Also, the fuse or other protection device is to protect the circuit, which
    includes the wiring to the socket outlets. Depending on the installation
    method of the wiring, and the protective device you use, you can use 16A,
    20A, or event 25A circuit breakers. You need to work this out using
    AS3008, and is mainly related to the heat loss in the wiring. You also
    need to work out the voltage drop in the conductors.

    There is no derating for connecting a light to the circuit, but the wiring
    to the light must be rated to suit the circuit breaker, (ie the same size
    as run to the socket outlets). Normaly lights are run in lighter cable
    which saves costs.

    Note however that the total number of sockets outlets in a domestic
    installation can have an influence on the maximum demand of the

  3. Phil Allison

    Phil Allison Guest


    ** It sounds very unsafe to me to have a 25A breakers on a circuit that
    supplies 10 amp rated socket outlets. The cables fitted to many appliances
    ( heaters etc) are not fused and rely on the 15 amp breaker in the supply to
    protect them.

    Or is this not in fact permitted ?

    ................ Phil
  4. David

    David Guest

    Excuse the html, hopefully it will help with formatting.

    AS/NZS 3018:2001 (Domestic electrical installations) requires the wiring for socket outlets to be a minimum of 2.5 sq mm unless more than 2300 mm above floor level (but only for appliances less than 150W or luminaires). Protective devices to be rated according to the total load and for protection of the wiring, specifically taking into account the temperature rise etc.

    You can mix final sub-circuits and contain both lighting and power points. There is a guide for that in Appendix C, Table C1 of the Standard. The mixed circuit must be RCD protected.

    Table C1 is too hard to reproduce here without knowing the conductor size you have. For a 2.5 sq mm conductor protected by a 16 A breaker the following applies:

    Installation methods ii, iii, iv, v apply (see below)
    Maximum number of points = 20
    Contibution of each point, up to rating of circuit breaker
    Lighting points = 0.5
    Socket outlets = 1.0

    So, you could have 12 lighting points (worth 6) and 10 socket outlets (worth 10) on one 16A breaker equipped 2.5 sq mm cabled mixed sub-circuit. Or, 15 socket outlets and 2 lighing points. So for general use, 7 doubles would be right with 4 or less lighting points.

    This does not apply when a specific appliance or application is intended to be connected and the load can be estimated. For example where the socket outlets are run to a dishwasher or kitchen bench and it is estimated the load will be xA. You deduct xA from the 16A available and then use the above calc for the remainder. So if you assume (cough.. estimate) 10A is needed at one particular location then with 4 lighting points you can have 4 other GPOs on that mixed circuit.

    Note that a double or other multiple outlet counts as the number of outlets it provides (so a four way is counted as four and a double as two) - see footnote (b) below.

    For non-mixed circuits, generally two final sub-circuits must be provided, and a guide to the number of points per sub-circuit appears in Table 6.1 of the above standard.

    To quote from Table 6.1 for socket outlets:

    Load Breaker rating Cond size Inst Method Quantity

    10A 16 2.5 ii, iii, iv, v 15
    Socket 20 2.5 iii, iv, v Unlimited
    Outlets 16 4.0 i, ii, iii, iv, v 15
    20 4.0 ii, iii, iv, v Unlimited
    25 4.0 iii, iv, v Unlimited
    32 4.0 iv, v Unlimited

    FOOTNOTES TO TABLE 6.1: (some deleted as irrelevant in this discussion)

    (a) Installation methods:
    I Completely surrounded by thermal insulation.
    II Partially surrounded by thermal insulation.
    III Enclosed in air.
    IV Unenclosed in air.
    V Buried underground in wiring enclosure.

    (b) For the purpose of determining the number of points, a multiple combination of socket-outlets
    shall be regarded as the same number of points as the number of integral
    socket-outlets in the combination.

    This info provided in good faith but is not authoratative so get the standard(s) and research further. From my experience and contact with many different electricians and/or engineers, most 'sparkies' are now unable to follow AS/NZS3000 as it no longer tells them exactly how to do things and deals with issues requiring data far beyond their reach. AS/NZS3018 is more easily followed by the average sparkie as it has some prescriptive content.

    In answer to a question from Phil elsewhere in this thread, over-current protection of devices plugged into the socket outlet is no longer considered, only protection of the installation. This places some product specific standards at odds with the wiring standards - some equipment is allowed to rely on the 'building protection' for over-current protection. So a 32A breaker equipped circuit is certainly at odds with this type of product safety requirement. As an example I haven't seen any pedestal cooling fans with fuses or circuit breakers (other than thermal fuses in motors) - a wiring fault is not supposed to happen (two independent fixings etc) according to the product standards AS/NZS60335-1 and 60335-2-80.

    Hope this helps.
  5. Geoff C

    Geoff C Guest

    Thanks for that David. I have one 16 amp 2.5 sqmm cct with 7 outlets so a
    couple more will be OK.
  6. David

    David Guest

    Do you mean 7 existing single outlets or double outlets?

    If singles then a few more will be OK. If doubles then only one more
    single is allowed (a maximum of 15 for a non-mixed circuit).

  7. Phil Allison

    Phil Allison Guest

    "David" <

    ** In the unit where I live - all the outlets and the fuse box were renewed
    6 years back when the place was fully refurbished. There are 7 double
    outlets and one single on the same circuit with a 20 amp breaker.

    No RCD is installed.

    But I have my own portable one in the workshop !

    ............... Phil
  8. Russ

    Russ Guest

    Assuming you could find the first outlet on the circuit, I understand those
    double GPOs with the inbuilt RCD will protect the downstream power points as
    well. You might want to check this though.
  9. KLR

    KLR Guest

    I believe that they do protect downstream sockets, but they would have
    to have separate in and out terminals for the active and neutral at

    Although probably a very politically incorrect test procedure -
    Putting one of these inbuilt RCD type sockets in circuit and
    momentarily shorting together the neutral to earth on the OUTPUT side
    and seeing if it trips would tend to prove if this "downstream
    protection" exists or not.
    failing that:
    If the unit was renovated only 6 years ago, then there should be a
    modern DIN rail switchboard, adding a DIN type RCD should be a very
    easy job assuming there physically is room for it on the rail

    (Im very surprised that there isnt one actually. Im told under QLD
    law that they have to be fitted on all new buildings/renovations since
    the early 1990's at least. According to a local electrician I used
    about 2 years back - the light and power circuits must now be RCD
    protected. The hot water system and stove/oven dont however

    On real estate sales contracts (as of 3 months ago when I last was a
    party to one) you must declare that there is/isnt an RCD fitted to the
    property being sold.)
  10. Phil Allison

    Phil Allison Guest

    Phil Allison wrote:

    ** Yep - there is such a beast and room for something a lot bigger than
    the present 80 amp switch.

    ** I rang the relevant authority in NSW and was told that since the place
    only had no additional power circuits after the renovation there was no
    compulsion to fit an RCD.

    The uptake of RCDs is very slow in NSW - it make take centuries to get to

    I understand that Qld has quite different regulations that mean RCDs must be
    fitted when a premises changes ownership - this should mean that nearly
    every premises has them installed before too long.

    ............... Phil
  11. Russ

    Russ Guest

    Assuming there is no load on the output of the RCD, would there actually be
    any current between neutral & earth, or are you assuming there is a load?
  12. David

    David Guest

    The overcurrent protection is to protect the fixed wiring, not the
    connected device, and is to protect against temperature rise in the
    conductors. If the fixed wiring is installed in such a way that it can
    carry 25A, there is no reason it unsafe to protect it with a 25A breaker.
    Loads connected to 10A socket outlets should not exceed 10A if they are
    fitted with a 10A plug. If they draw more then they will have a 15A or
    higher rated plug. A heater rated at 2400 Watts (the maximum for a 10A
    socket outlet), only uses 10A, and would be supplied with a 10A cord and
    10A plug, and would not pose a safety issue from overload.

    If two were connected to one circuit, the current would be 20A, and still
    nothing would be overloaded. Connect three and the breaker would trip

    Note that this is only for overload, not short circuit. A short circuit to
    either ground or to active - neutral will involve a much higher current,
    and trip the breaker within seconds, before thermal damage can be caused
    to the fixed wiring. Note that AS3000 does specify maximum earth
    resistance, to ensure that an active to earth fault will trip the breaker.

  13. Phil Allison

    Phil Allison Guest

    Phil Allison

    ** All very logical and nice - but avoids the issue I raised completely.

    " The cables fitted to many appliances ( heaters etc) are not fused and
    rely on the 15 amp breaker in the supply to
    protect them. "

    The "protect them" bit related to faults developing in the appliance which
    have the effect of increasing the current draw above 10 amps - maybe up
    to 20 or 24 amps. Enough to cause the cable and the plug to burn down but
    no fuse to blow.

    Then there are all those older multiway outlets that have no thermal
    breaker - the attached cable is safeish to maybe 20 amps at most - and
    then what - a nice domestic fire that burns babies ???

    Domestic circuits have not been permitted to have fuse wires or breakers
    with greater than 15 amp capacity since the year dot - when did the safety
    issues change ??

    ............... Phil
  14. KLR

    KLR Guest

    OOPS - Didnt think of that one.

    I would imagine there would have to be a load there as you suggest,
    however on 2 occasions (in different installations) I have experienced
    RCDs tripping under fault condition even with the main switch TURNED
    OFF and no (Active) power entering the circuit :)

    I didnt check it in detail as it wasnt in my premises, however both
    would have been pretty modern setups that were built and wired during
    the late 1980's
  15. KLR

    KLR Guest

    in the old days of the ceramic fuseholder - it was always 15 A fuse
    wire in use (8a for lights) - at least in the 70s which is as far
    back as I can remember.

    (in QLD) - It seems normal in modern times to fit 20 or 25a breakers
    onto each domestic power circuit. Had a look at the 15a circuit that
    I had installed in 1997 by licensed electrician - and it has a 25a
    breaker in there. Another I saw was in a friends 2 bed flat - where
    there was one power circuit for the GPOs (except fridge) and a
    separate power circuit that only went to the fridge socket (and didnt
    go through the ELCB), The single 10a fridge socket had a 20a breaker
    that I thought to be totallt illogical !! I would have thought a 10A
    a lot more sensible choice for a dedicated single socket like that.

    I also asked about this "higher amp ratings of modern installations"
    some years back - and was told that the breakers have to act only as
    "cable protection fuses" and values are dependant on the maximum amp
    rating of the cable. seemed that 2.5mm square called for 20/25a and
    1.0mm called for 10a.

    It didnt seem that the cable run length and other factors were taken
    into account though, and I must agree that it isnt a fully safe
    system, that would lead to certain disaster if any appliance went
    faulty and drew a current more than 10a but not high enough to trip
    the 20/25a breaker !

    The one system I do like though - for safety is the UK idea of having
    a fuse (3AG or similar I think) fitted into the power plug of EACH
    appliance, that way things with small current draw need only to have a
    relatively small fuse in there, (can be fast or slo-blow as
    appropriate) which will blow a lot earlier if the appliance goes into
    overload, without risking any overcurrent damage to the wall socket,
    plug pins or premises wiring

    also wont kill everything else on that circuit too..


    Of course you have the other extreme - russia - (220/380v) where
    cheaply made 2 pin sockets (that look unsuitable for 10a use) are used
    frequently - wired up by a cable I can only describe as a 0.5 MM
    square cable that was wide spaced (like the 300ohm ribbon used in tv
    aerials in the old days)

    This would run out into the hallway where it went to the electricity
    meter (exact same spinning disc types as used here in 70's and 80's).
    I couldn't find fuses anywhere in this mess

    Commercially made (and not very old) power boards and extension leads
    that had no earth and wire that was WAY too thin for anything even
    close to 10 a were in common use too and I nearly had a fit when
    seeing them.

    I could mention in very public areas lamp poles that seemed to be held
    (more or less) upright by the overhead cables, substantial size supply
    cables to buildings being draped over piles of cement blocks, or
    thrown on roofs of houses and old falling down sheds for some
    kilometres along a busy suburban rail line. but no one would believe
    me ;)
  16. Phil Allison

    Phil Allison Guest


    ** No faulty electrical item is needed to create a dangerous situation -
    merely the use of a common extension lead ( possibly 7.5 amp rated) with a
    simple double adaptor on the end supplying two 10 amp loads, ie two 2400
    watt room heaters.

    Someone is playing funny buggers with the rules here - over fusing a
    domestic circuit is inherently dangerous and one of the main causes of
    domestic fires.

    ................ Phil
  17. David

    David Guest

    RCD's will normally trip if you short the neutral to earth on an RCD
    protected circuit. This is because there is normally a voltage
    differential between the neutral and earth due to load currents in that
    circuit other circuits. When you short neutral to earth, there will often
    be sufficient current (greater than 30mA) flow in the neutral to trip the
    RCD, as there is no balancing current in the active. This will happen even
    if there is no load in that particular circuit.

  18. David

    David Guest

    This is a very unlikely case. High current devices such as heaters etc
    would typically go open circuit in case of failure, or if the element did
    short out partially, would burn out well before any wiring would overheat
    dangerously. Other devices are required to have internal protection
    against overload / fire as per AS/NZS3100 and other standards.

    I never liked these, but people need some common sense with electricity.
    Where did you get this "fact" from Phil? Please provide a reference. The
    old 1991 wiring rules clause permitted 32A circuit breakers or
    HRC fuses to protect final subcircuits having GPOs, or 25A revirable
    fuses. There is no such limiting clause in AS/NZS3000:2000.

  19. Phil Allison

    Phil Allison Guest


    ** Yawn - opinion presented as fact.

    ** Yawn - what patronising shite.

    A simple double adaptor allows a common extension lead to be overloaded by
    a factor of 3 times in current or about 15 - 20 times in heat dissipation.

    That is ** damn** unsafe.

    ** Come down in the last shower did you ???

    Domestic power circuit wire fuses were always rated at 15 amps - this goes
    back to the 1930s and right up to very recently it seems. When cheap plug
    in breakers arrived - 15 or 16 amp ones got used in lieu.

    When did the safety issues change ???

    Clearly they never have.

    .................. Phil
  20. David

    David Guest

    ** Yawn - Phils

    Clearly Phil, as you are resorting to insults, I know that you are wrong.
    You really should have finished that Uni course, then you would know that
    3 times current, would only result in a 9 times increase in power being
    lost by the cables.
    No. I am fully qualified Engineer and fully licensed Electrical
    As I asked before, can you provide a reference for your "fact". As I
    stated before which you deleted, the old 1991 wiring rules have allowed
    up to 32A breakers for 10A socket outlets. This is a FACT, in the WIRING

    While it was common practice to use 16A rewireable fuses with 2.5mm2
    cable, this was because it was normally the cheapest legal solution. Also,
    the wiring rules treated those cheap plug in breaker the same as
    rewireable fuses. If you spent more money, and installed a HRC fuse or
    proper ciruit breaker, you could use up to 25A for 2.5mm2 cables,
    depending on the installation method. If you wanted you could use 4mm2
    cable, and 32A circuit breakers, but this was not in most electrician's
    economic interests.

    Your wild assumptions are incorrect. You don't have a degree or even an
    electrical license, and obviously have no idea of the requirements of the
    standards. Buy the standards and learn.

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