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Thread: Plug circuit's neutral wire is 1.5mm, cb amp rating?

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    Plug circuit's neutral wire is 1.5mm, cb amp rating?

    Hi I have recently been hired to install a new DB in a garage. There was a fire which as far as I can see may have started inside the council's prepaid meter.
    Anyhow, In a nutshell, when rewiring the DB , I noticed that the the 20amp plug circuits had 2.5mm live wires but the neutral wires are only 1.5mm.
    The house is really tiny so the length of cable run is probably at max 15 metres to the furthest plug circuit, or not even that far.
    I'm just wondering what size circuit breaker I should change to, initially I thought if I change the circuit breakers to 16amps that should be fine but when doing calculations I calculate it should be 14 amps (which you don't get as far as I'm concerned ).
    The real question here is, does anyone know if the neutral can be reduced like this? There was a COC for the house, so I'm wondering if there may be some sort of rule that allows this? ..... because as far as I know, the neutral should always be the same as the live wire, but perhaps it is something I have forgotten from my studies.
    Thanks

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    Diamond Member Justloadit's Avatar
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    I am not a qualified electrician, however, in a single phase electric circuit comprising 2 wires, a Live and a Neutral, and load connected to these two wires, will cause a current to flow of equal magnitude in each of the current carrying conductors. In other words, the Live carries the current to the load, and the Neutral returns the current to the supply. Theoretically they must be of equal size, if the current is the same in each conductor. If they are of unequal sizes, then the wire with the lower conduction area will nominate the maximum current the circuit can supply.

    In the case you have mentioned, the maximum current that the circuit can carry, is the calculated value of the 1.5mm Neutral wire, and not of the 2.5mm Live wire..
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    hartdev@hotmail.com (22-Mar-16)

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    Have a look at SANS 10142: Table 6.26 – Maximum protection rating for nominal cross-sectional area of small conductors.

    Maximum circuit breaker size for 1.5mm conductor = 16A.

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    hartdev@hotmail.com (22-Mar-16)

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    Quote Originally Posted by PaulG View Post
    Have a look at SANS 10142: Table 6.26 Maximum protection rating for nominal cross-sectional area of small conductors.

    Maximum circuit breaker size for 1.5mm conductor = 16A.
    Thanks Paul, I see 6.2(a) says 13.5 for single core PVC insulated conductors in conduit in thermally insulated wall.
    I see that table 6.26 says 16amps, bit of a contradiction but It must be safe otherwise I'm sure they wouldn't put anything like that in print.
    Thanks for that

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    Quote Originally Posted by Justloadit View Post
    I am not a qualified electrician, however, in a single phase electric circuit comprising 2 wires, a Live and a Neutral, and load connected to these two wires, will cause a current to flow of equal magnitude in each of the current carrying conductors. In other words, the Live carries the current to the load, and the Neutral returns the current to the supply. Theoretically they must be of equal size, if the current is the same in each conductor. If they are of unequal sizes, then the wire with the lower conduction area will nominate the maximum current the circuit can supply.

    In the case you have mentioned, the maximum current that the circuit can carry, is the calculated value of the 1.5mm Neutral wire, and not of the 2.5mm Live wire..
    Hi Thanks Justloadit
    Yes I understand that too but I was giving the original installer the benefit of the doubt ,thinking they may know of some rule I did not know of, but no, they had 1.5mm wire with 20amp circuit breakers.
    Thanks again

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    Quote Originally Posted by hartdev@hotmail.com View Post
    Thanks Paul, I see 6.2(a) says 13.5 for single core PVC insulated conductors in conduit in thermally insulated wall.
    I see that table 6.26 says 16amps, bit of a contradiction but It must be safe otherwise I'm sure they wouldn't put anything like that in print.
    Thanks for that
    No contradiction really - Table 6.2(a) gives the current required to raise the temperature of conductors from 30 degC to 70 degC, (14.5A for a single phase 1.5mm cable using installation method 1). So while you would not want to operate the cable at higher than 70 degC, the cable would not melt at the slightly higher temperatures caused by 16A of current flow and thus still be protected.

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    hartdev@hotmail.com (23-Mar-16)

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    makes sense now thanks

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    Diamond Member AndyD's Avatar
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    6.2.1.2 The current-carrying capacity of each conductor in a circuit that
    comes from a distribution board shall be the same at each point in the
    circuit
    , but if the conductors of lowest current-carrying capacity are
    protected against overcurrent in accordance with 6.2.1.1,
    a) the current-carrying capacity of conductors at one or more points in
    the circuit may be reduced;
    b) one or more spurs that have conductors of reduced current-carrying
    capacity may be teed into a circuit; and
    c) at one point only, a circuit may be split into two or more branches,
    each having conductors of reduced current-carrying capacity.
    I'd need to check later amendments to see if this paragraph is still valid which I don't have time to do now but the way I read this is that current carrying conductors of a circuit must be of equal CSA. Under certain circumstances the CSA can be reduced part way along the circuit if suitable protection is provided but I don't see this as an exemption to the wires have to be of equal size, it just means both wire sizes can be reduced.
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