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Light globes

  1. Apr 24, 2005 #1

    This is a very simple question and it should only take one sentence to answer. It's not worthy of these forums, lol, but despite my efforts I cannot find an answer through google.

    If a light fitting says '40 watt max' and I stick a 60 watt bulb in it, will that have any adverse effects? I assume what will happen is that only 40 watts of power will be fed into the bulb thus not fulfilling the bulbs potential brightness, but I'm unsure and thought I better ask before I do it.

    Thanks in advance.
  2. jcsd
  3. Apr 24, 2005 #2
    No, it's a heat consideration. A 60-watt bulb generates more heat than a 40-watt bulb, and if you use it you'll be putting more heat into the globe than it is designed to handle. You'll still get the same light output from the 60-watt bulb that it would give in any other circumstance.

  4. Apr 24, 2005 #3
    If you want to use a larger wattage bulb, leave the globe off. Otherwise you might have a fire problem.
  5. Apr 24, 2005 #4
    Thanks for your response. I'll simply go and buy some 40 Watt bulbs, lol.
  6. Jun 5, 2005 #5
    Assume you have a 120volt supply. The bulb resistance will cause a particular current to flow ([tex]I=V/R[/tex]) and this will result in a certain amount of power being dissipated ([tex]P=VI=V^2/R[/tex]). This means that a 40watt bulb has a specific resistance resistance ([tex]R_{40W}=V^2/P=120^2/40=360\Omega[/tex]) and that the resistance of the 60W bulb is different ([tex]R_{60W}=V^2/P=120^2/60=240\Omega[/tex]). The 60W bulb will cause ([tex]I_{60W}=V/240=0.5 Amps[/tex]) to flow, compared with ([tex]I_{40W}=V/360=0.33 Amps[/tex]).

    The fitting doesn't know the difference, but it is possible that the difference in current could damage light wiring or that the extra heat could damage plastic fittings. A more significant effect depends on the cold resistance of the bulbs (before they are turned on) because this determines the initial current. The higher wattage bulb has lower hot resistance and so probably has lower cold resistance. The higher starting current may cause you to blow more bulbs.

    It probably won't hurt (I have just replaced a 25W bulb with a 60W bulb because that was all I could get) but if you are the careful sort then follow the manufacturer's directions.
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