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OSG Standard Circuits - 3A lighting

Former Community Member
Former Community Member
As we are all using led lamps / fittings now it seems this should be a standard circuit.

3A MCB with 1.0/1.0 cable


Lighting circuits used to be based on 100w incandescent lamp / room at 1800 lumens which is equiv to 20w led.

What are the figures for max distance for a 3A type B using 1.0/1.0 as an addition in table 7.1 page 66 in OSG?
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    Current Bun:

    As we are all using led lamps / fittings now it seems this should be a standard circuit.

    3A MCB with 1.0/1.0 cable


    Lighting circuits used to be based on 100w incandescent lamp / room at 1800 lumens which is equiv to 20w led.

    What are the figures for max distance for a 3A type B using 1.0/1.0 as an addition in table 7.1 page 66 in OSG?


    I wonder if many new L.E.D. lamps on one circuit may create a high initial inrush current when first turned on, that may trip a B3 breaker.


    Z.


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    I agree - we have enough trouble with B6 MCBs and switch-on surges (a few years ago there was a drift towards going 10A for lighting circuits). When power returns after a power cut, every light that had been in use when the power went off will start simultaneously and very likely trip a B3.

       - Andy.
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    Agree, in general avoid 3 amp circuits due to concerns about tripping on inrush currents.

    I would only consider a 3 amp circuit in special circumstances, such as extreme length to lamps in a distant outbuilding or at end of a large garden. Or possibly when it is desired to limit misuse such as connecting appliances to lamp holders, or to limit loading on a small generator or inverter.

    To a very good apoximattion, the maximum circuit length would be twice that for a 6 amp MCB.
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    A C type 3A would make more sense. Same inrush as a B6, but lower circuit rating.

    Make it an RCBO, and no-one will care too much about the length.

    many hundreds of metres depending on the load distribution along the length.

    M.
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    As we are all using led lamps / fittings now it seems this should be a standard circuit.

    3A MCB with 1.0/1.0 cable



    Why? 1.00 sq.mm. T&E, method C, has a CCC of 16A.
  • 0
    Indeed, and 1.0 mm twin with earth is still well over 6 amps under all but extreme conditions of de-rating for grouping and thermal insulation.
  • 0
    geoffsd:
    As we are all using led lamps / fittings now it seems this should be a standard circuit.

    3A MCB with 1.0/1.0 cable



    Why? 1.00 sq.mm. T&E, method C, has a CCC of 16A.


    Why 1.0mm2?


    Because it is readily available and commonly found. It is probably the smallest practical size to be used on site that will not easily break or be damaged too much by being chucked in, errr, I mean installed. We could use 1 Amp bell wire or alarm cable, but it may be broken by other trades bashing it, tugging on it or walking over it in their hob nail boots. Also 1.0mm2 allows for future extension to circuits without too much trouble.


    But having said that, a 1.0mm2 copper conductor is easily broken if previously accidentally nicked or scored. Perhaps we should consider that.


    Z.


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    Yes, inrush currents may well be a problem.


    However, I think this is a vary valid question that requires a little more thought.


    The distance of 68 m is limited by on 3 % volt-drop for a current of 5 A, roughly evenly distributed along the circuit. So, initially, you might think we could get away with a longer distance if we assume say a 3 A load current - and with an RCD protecting the circuit where Regulation 411.3.4 applies, the  circuit length for a 3 % volt drop could increase to over 100 m even with a 6 A protective device.


    However, if a longer circuit is used, a designer would have to be careful that the impact of inrush currents does not cause a voltage drop which temporarily (for a couple of seconds) affects driver stability downstream of one or more drivers  being switched on ... and I've noticed occasionally this can be a problem with circuit lengths shorter than 68 m. There are ways round this, which could, perhaps very cheaply, be built into LED drivers themselves (e.g. MOV inrush current limiting and/or larger reservoir capacitor) - and there are probably drivers on the market that have this kind of approach built-in.


    So,  LED lighting may well be more energy efficient, but unless inrush currents are managed, it doesn't necessarily translate to smaller cross-sectional area of cables, or longer circuit lengths.
  • 0
    What advantage would there be of using a 3A MCB? I can see none at all, and just a pile of possible snags. Close fusing may be used in certain other situations, but why on a simple lighting circuit? Just who floated the idea, because it needs a proper explanation? Perhaps it's the latest EU idea to save energy, turn on 3 lights and the whole lot go out. Even less sensible than a 500W kettle! Whilst a 3A type C might work Mike, please explain the advantage and the price differential? But then they might get cheaper in the future, like AFDDs. Ah I know, a 3A AFDD/MCB/RCD combination will mean the AFDD doesn't have to work at all, pretty much as they don't if the arc is low current < 3A! Still in the regs though.
  • 0
    Zoomup:
    geoffsd:


    Why? 1.00 sq.mm. T&E, method C, has a CCC of 16A.


    Why 1.0mm2?





    Hi Zoom.


    You have moved the question mark and completely altered the meaning of what I wrote.

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