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Breaker tripping?

Discussion in 'Electrical Engineering' started by daz, Nov 21, 2003.

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  1. daz

    daz Guest

    I have a question that won't tax too many people, but it's got me scratching
    my head.

    Recently, I installed a set of lights on the landing of my home. These use
    3x40 watt bulbs as opposed to
    1x60 watt bulb. Ever since then, every time a bulb blows, (regardless of
    what room it's in), the breaker trips. Now am I right in thinking that
    because of the increased load on the circuit, the breaker trips more easily?
    If so, do I buy myself a rechargable torch and live with it, or do I replace
    the existing breaker with one that has an increased rating?

    Help, comments and thoughts would be smashing.


  2. I'm assuming you are UK based with an email, but
    you should say when asking such questions.

    It's unlikely this has anything to do with changing the light,
    but you should add up the power rating of all the lights on
    the circuit and make sure you aren't exceeding it.

    It would be useful to know the exact rating of the MCB -- it's
    going to be something like 5A or 6A, and marked Type B or C or
    Type 1 to 4. One problem when filaments break is that they can
    start an arc in the bulb, and the arc very quickly shorts out
    the lead-in wire creating the classic pop and flash which happens
    sometimes when a lamp dies. This is pretty much a dead short
    across the mains. Lamps mostly have integral fuses in the lead-in
    wires in the base, and this failure mode causes the fuse in the
    lamp base to go. However, UK/European MCB's contain a magnetic
    fault current trip which trips in half a mains cycle on a short
    circuit, and this can operate faster than the lamp base fuse.
    This is why such a lamp failure can trip an MCB. The 'Type'
    rating of the MCB is a measure of how much fault current it
    will allow to pass before triggering the magnetic fault current
    trip part. In most homes, you could go to a 6A Type C MCB on
    the lighting circuit which will allow 5-10 times the MCB current
    rating to pass before the magnetic fault current detection will
    trip, whereas a Type B will only allow 3-5 times. However, a
    more reliable way to remove the problem is to swap out the
    lighting MCB for a BS1361 5A cartridge fuse carrier, which is
    slower acting in the face of fault (short-circuit) currents, and
    will allow time for the lamp base fuse to blow and remove the
    fault current.
  3. Guest

    No, he's absolutely correct. As he stated, breakers are there to
    protect the wiring. Think about it - when a house is built
    there is no way in hell to size a wire to a load that does not
    exist! Therefore it must be sized to an expected load, with
    whatever safety factor mandated by code added in. When the
    expected load is countertop kitchen appliances, the circuit
    must be sized for 20 amps.
    You are talking "wire" which refers to the conductor(s) itself
    and is a completely different thing than the term "wiring"
    which refers to branch circuits installed in a house. You
    also use the term "amperage" - but branch circuit conductors
    are sized in terms of "ampacity" which is a different thing.
    Here's the definition from the National Electrical Code:
    "Ampacity The current, in amperes, that a conductor can carry
    continuously under the conditions of use without exceeding its
    temperature rating." When you look up "amperage" in Webster,
    you will find that it refers only to the strength of the electrical

    In addition wiring is sized based on voltage drop, of which
    there is no mention in your post. You appear to think that
    "amperage" is all one needs to know in addition to the
    load. And as already mentioned, one cannot know what the load
    will be at a receptacle that has not yet been installed.

    You do not understand what sizing means. First, all branch
    circuits in the US must be sized at a minimum of 15 amperes.
    #18 wire is never allowed to be used in branch circuit wiring.
    If the expected load is a lighting circuit, you size it for
    at least 15 amps. If it is countertop kitchen appliances, you
    size it for 20 amps. Same is true for bathrooms - hairdryers
    are an expected load, so circuits to bathroom receptacles
    must be 20 amps.
    It has nothing to do with measured!!!!!!! You can't measure
    the load when you are installing the circuit. Hell, you don't
    even know who's going to buy the house, let alone what they
    are going to plug in.
  4. Guest

    If the *only* change was from 60 watts to 120 watts, and
    now the breaker trips as you describe due to that extra
    60 watts, that circuit was already overloaded.

    I would suspect something other than an overload caused
    *only* by the additional 60 watts. Check out the wiring
    to the new lights very carefully. You may have a resistive
    short circuit - a frayed wire or loose connection are likely
    culprits. And since your breaker has tripped numerous times,
    replace it with one of the same rating.
  5. Jb

    Jb Guest

    Seems strange that changing the fitting produces this effect, unless the
    'breaker' is in fact an RCD which means there may be some earth leakage in
    the new fitting.
  6. daestrom

    daestrom Guest

    Wiring for specific loads, such as industrial settings or a dedicated
    circuit for a furnace or baseboard heater, *is* sized based on the 'expected

    But residential branch circuit wiring, it is not so 'cut and dried'. The
    total number of circuits is determined by the VA per square foot. One isn't
    allowed to use smaller wire just because the outlet is in a location where
    the only 'expected load' is one 40W lamp. There is a minimum wire size
    allowed (14AWG IIRC). Given the ampacity of 14AWG, the circuit-breaker
    rating rewquired is 15A.

    When you get down to this 'lower' end of circuit wiring (as opposed to
    dedicated circuits), the 'minimums' start to come into play more than the
    total current draw. In an 'average' room, you might not 'expect' more than
    two, 100W bulbs and a 150W TV. This 350W/120V load *could* be supplied by
    wiring smaller than 14AWG and a 15A breaker. But anything less than 14AWG
    and 15A isn't allowed. After all, what you 'expect' for the typical load,
    and what another one 'expects', is not the same thing.

    Homeowners aren't required to get a PE to evaluate every change in appliance
    usage/arrangement because the NEC sets these minimums.

    On the other side of the coin, if the *minimum* circuit breaker rating
    *available* is 15A, then all components in the circuit must be rated for
    15A. The NEC requires that if any component in a circuit is rated for a
    lower current than the other components, the circuit breaker must be rated
    for this lowest current device. But a sort of a backward result of this is
    that you can't use wiring rated for an expected load of only 1/2A when the
    circuit breaker is rated for 15A Even if you are only feeding a 1/2A load
    (one single 60W bulb), all the components in the circuit (other than the
    plug-in appliance itself or the lighting fixture) must be rated for the 15A
    because the minimum breaker size you can find for a service panel is 15A.
    IIRC, there *is* an exception for this rule (gotta love the NEC), in that
    multiple 15A outlets can be used on a 20A branch circuit (with 12AWG wire of

    And of course, voltage drop is another consideration. But that is another

  7. In the u.k. the lights are taken as 100w even if they are actually lower.
    Check BS7671 or your on site guide
  8. Try a type C MCB

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