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Thread: Maximum overcurrent protection on a plug circuit

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    Site Caretaker Dave A's Avatar
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    Maximum overcurrent protection on a plug circuit

    What is the maximum overcurrent protection allowed in terms of the regulations for a plug circuit on a typical house or flat installation - single phase, 2.5mm wiring, 16A socket outlets?
    The trouble with opportunity is it normally comes dressed up as work.

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    20A C/B

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    Diamond Member AndyD's Avatar
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    Hmmm, am I smelling a trick question? OCPD's protect the circuit cabling that's connected to them. They would usually be sized according to the cable so for example if you have a 4mm radial socket circuit you would specify a 25A mcb regardless that the socket outlets are only rated at 15A or 16A. A 20A MCB would be used for a 2.5mm cable.
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    Site Caretaker Dave A's Avatar
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    Quote Originally Posted by AndyD View Post
    Hmmm, am I smelling a trick question?
    Not really. I was wondering why so many original installations (dated about 1960's / 70's) in Durban have 30A circuit breakers on their plug circuits. It's so widespread, there must be a story.

    BTW - the regs say you can only have overcurrent protection exceeding 20A (on a single phase circuit that contains only 16A socket outlets) if there is overcurrent protection on the socket that doesn't exceed the socket's current rating.

    6.15.3
    (I better check and edit that in the morning if my memory has let me down).
    The trouble with opportunity is it normally comes dressed up as work.

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    the regs say the overcurrent protection may not be more than 25% of the rating of the socket outlet.
    16 amp plug + 4 amps = 20 amps 2,5mm
    32 amp plug +8 = 40 amps 6mm
    63 amp plug +15,75 = 78.75 amps ( 80 amp breaker ?) 16mm-25mm depending

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    Look up on page 194 6.15.5

    You my not use a bigger than 125% rated CB
    So
    16 x 1.25 = 20 A

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    Site Caretaker Dave A's Avatar
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    When it comes to 6.15.3 to 6.15.5 , I suggest you need to pick the section that applies to the actual circuit configuration.

    Ultimately it's the 6.15.3 situation that has my interest piqued. I suppose what I need to do is hunt down one of our tests where this has come up and look at the wire size on record against the "offending" CB.
    The trouble with opportunity is it normally comes dressed up as work.

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    Dave back in the day all plugs circuits where wired with 4 mm house wire and a 30 amp circuit breaker was installed as was the geyser, 4mm wire with a 30 amp circuit breaker. The HOB and ELO was wired with 6mm, things have changed.

    The rules has since changed and not plugs nor geysers are wired using 4 mm wire anymore (speaking in general). however you need to be very careful of these older installations because the bright sparks of today don't know about 4 mm wire and 30 amp breakers. I have come across many houses which still have 30 amp breaker, the catch comes in with additions or alterations to an existing installation many "electricians" just use 2.5 mm twin+ earth and don't derate the breaker in the DB or even add an earth leakage when adding sockets. Kitchens are a bad place for this type of problem. A customer will get a kitchen company in to replace the kitchen and the shop fitters are jacks of all trades, before you can say refurbish kitchen the 2.5 mm twin +e is in, the DB is hidden away nicely right at the point where the top part of the cupboard meets the wall so you cant open the DB, the plugs are connected to an existing socket in the kitchen and bingo 1 circuit does it all, well until a real electrician comes along to do an inspection report, the 30 am is replaced with a 20 amp and for some unknown reason when ever you run the kettle, the dishwasher, the washing machine and the tumble drier together it just seems to trip and never did that before. I am sure there are a few of you smiling as you read this...been there done it.

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    Site Caretaker Dave A's Avatar
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    Thanks Ian. I just knew there was a story there... and the hassles that go with just downgrading the breaker are exactly as you say.

    Now to massage the problem for possible alternative retrofit solutions.
    The trouble with opportunity is it normally comes dressed up as work.

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    Gold Member Sparks's Avatar
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    And to crown it off, the sod got his tiler to do the plugs so all the earth wires of the T&E were cut off and they installed the cheapest plastic framed plugs they could get. Just before the place was sold!

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