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LED lighting cable calcs

Baldyhugh
Hi all


when calculating cable size for an LED lighting circuit is there a multiplication factor to be applied like the 1.8 for the old fluorescent lighting? I’m aware of the high inrush currents of led fittings but what I understand of this it is of such short duration that it only affects the curve rating of the mcb selection. I have a couple of 150 disco Lowbays to fit in a warehouse and would appreciate some advice.


many thanks
  • 0
    There is no simple figure. - the old 1.8 was based on the ratio of tube voltage (about 100) to supply voltage (about 240) and some assumptions about how much current flows through the series choke while  the tube is shorted out by the starter to get the heaters warmed up and the mercury from drops into vapour.

    LEDs just do not work that way, and the only sure fire way to find out is to check with the makers of the fittings. Do you have a part no, or link to maker's data ?


    Once up and running the current should be very close to the theoretical  2/3  of an amp for 150watts, as proper fittings over 25watts should  have proper power factor correction built in if they are allowed to be sold in the EU.

    Inrush may be very high (tens of amps), but it will be all over in less than one half cycle, so a C-type MCB will be more than adequate, and cable can be thin... If you had a lot of them, then I'd also say keep an eye on the earth leakage, and if need be  break out the lighting wiring onto their own RCBO, for a couple, less of an issue.


    Edit An example driver from  far eastern maker  Meanwell     common inside fittings from companies without an electronics reputation, says 75A inrush for a 150W driver.

    A few ohms in series would reduce that a lot, but add a few watts to the standing losses so they tend not to do that.


    You may need a 10A C type if both come on at once. The exact shape of the under hang of the timing curve for times shorter than 100ms is not tested or guaranteed by MCB makers but is typically a bit like this example


  • 0
    Thanks mike


    so as regards the current calculations I basically use the ‘running’ current rather than the ‘starting’ current if you can call it that?
  • 0
    yes. Note I have edited the first post while you replied. Sorry.
  • 0
    It is unlikely that these LEDs will have a high inrush current, and as they are only 150W it won't be much anyway. Fit a 6A type B and see what happens. Electronic PF correction limits the input current anyway, its not like charging a big capacitor via a bridge rectifier, although at such low power even that would not be a large current for long.  Design ELFI for a type C just in case.
  • Former Community Member
    0 Former Community Member
    Hi Hugh, on a recent job I had 11 of 100W led floods and 12 of 50W led floods, out of interest I temporarily put them all together on one switch and they started up fine on a B6 MCB. They are now on multiple circuits/switches.

    Hope this helps! :)
  • 0
    Hi everyone


    Just the info I was looking for. Very helpful as always, much appreciated 


    Peter
  • 0

    Looking with interest at the differing views of inrush current of LED drivers.


    I personally think it depends on whether the manufacturer has addressed the issue. Someone mentioned a particular make of driver having a high inrush current - well, some of their switched mode power supplies (SMPSUs) have the same problem.


    But it's not the same with other manufacturers.


    I'm sure that davezawadi will confirm that inrush current is something the driver manufacturer could have largely addressed for a few pence with a varistor inrush-current limiter, or either zero-voltage or zero-current switching for startup (dependent on the swtiching transistor arrangement in a switched-mode converter).


    If a varistor is to be used, the truth of that is a little more complicated from a driver manufacturer's perspective, because the load attached to the driver or SMPSU needs to be known, so that you can calculate the required power dissipation and resistance range of the varistor.


    Some of these drivers are probably sold as a universal component, perhaps with either full specifications, if not instructions of how to integrate them into a final product or system (and I would guess, from previous experience, the manufacturer stated previously in this thread would be in the bracket of providing full specs as their products have a very wide range of uses). If an LED luminaire manufacturer just packages the universal product without considerations such as inrush, and omits inrush limiting up-front, then you might have problems.


    Similarly, if you purchase your own drivers separately to luminaires, it's something to check for, and address by adding inrush current limiting if you need to.


     


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