+ Reply to Thread
Results 1 to 5 of 5

Thread: What amp rating do I need?

  1. #1
    jtbsc Good jtbsc Good
    Join Date
    Sep 2007
    Posts
    1

    Default What amp rating do I need?

    Newbie here, hoping you can answer my question.

    I've got a small fluorescent light box that currently does not have an on/off switch. I'd like to add one. I've had to replace the power cord so the wires are fairly accessible.

    I know I need a 125V DPST switch, or I think I can use a DPDT if I have to, but what amp rating do I need?

    The lightbox has a sticker that reads "For line volt amps, multiply total lamp wattage by 2.5." There are two 9W bulbs in the lamp, so that makes the line volt amps 45.

    But if I do W=VxA, then I get less than one.

    As you can tell, my knowledge of electronics is enough to get me in trouble. Do I need a 45 Amp switch, and if so, am I ever going to find one? Or do I need a low-amperage switch that's likely available at Lowes or RadioShack?


    Thanks in advance,

    John


  2. #2
    kchriste Excellent kchriste Excellent kchriste Excellent kchriste Excellent kchriste Excellent kchriste Excellent kchriste Excellent kchriste Excellent kchriste Excellent
    Join Date
    Jul 2006
    Location
    Victoria BC, Canada
    Posts
    3,681

    Default

    It would only draw 0.410 Amp if it is 45 watts ( 9W x 2 x 2.5) and running on 110Vac. (45W/110V) A 1Amp 125V switch should be fine.
    Last edited by kchriste; 7th September 2007 at 03:37 AM.
    Inside every little problem, is a big problem trying to get out.

  3. #3
    audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent
    Join Date
    Mar 2004
    Location
    Canada, of course!
    Posts
    19,871

    Default

    You could use a 1A, a 2A, a 5A or a 10A switch.
    Uncle $crooge

  4. #4
    RadioRon Excellent RadioRon Excellent RadioRon Excellent RadioRon Excellent RadioRon Excellent RadioRon Excellent RadioRon Excellent
    Join Date
    Oct 2005
    Location
    BC, Canada
    Posts
    1,448
    Blog Entries
    1

    Default

    I notice that it is typical for compact flourescent bulbs to include both Watts and mA ratings on their labels. When you multiply the rated current by the rated voltage you always get a higher value of VA than the Watts rating. I contacted a major manufacture about this and received an answer from their engineering department saying that the typical compact flourescent has a power factor that is other than 1.0. The Watt rating is the true power consumption of the bulb, but the true current consumption is in fact as shown on the bulb, which is higher than (watt rating/line voltage).

    I gather from their label that the scaling factor of 2.5 accounts for this power factor issue as well as inefficiencies in the lamp ballast and so on. The only remaining concern is whether there is a significantly higher inrush current in such flourescents that might prompt me to go with a higher rating than 1A for best reliability of the switch. Does anyone have knowledge of inrush current in flourescents?
    RadioRon

  5. #5
    Hero999 Excellent Hero999 Excellent Hero999 Excellent Hero999 Excellent Hero999 Excellent Hero999 Excellent Hero999 Excellent Hero999 Excellent Hero999 Excellent Hero999 Excellent
    Join Date
    Apr 2006
    Location
    England
    Posts
    13,160

    Default

    It depends on what kind of ballast it uses and whether it's power factor corrected.

    Uncorrected solid state ballasts draw power in harmonic rich high current pulses and inductive ballasts cause the current to lag behind the voltage.

    Your switch should be adequately rated to switch the high current pulses or not arc over when interrupting an inductive load, depending on you ballast type.

    Now, as you're only talking about 45VA I wouldn't worry about this too much, a 1A switch should be fine.

    I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

    Please ask on the open forum if you have a question and I'll be happy to help,
    if I know the answer.

+ Reply to Thread

Similar Threads

  1. Capacitor voltage rating
    By skyymanz in forum General Electronics Chat
    Replies: 7
    Latest: 8th March 2009, 10:36 AM
  2. 1000V power supply with low amp rating...
    By Souper man in forum General Electronics Chat
    Replies: 17
    Latest: 16th April 2007, 07:12 PM
  3. Capacitor rating
    By Electronics4you in forum General Electronics Chat
    Replies: 1
    Latest: 9th April 2007, 12:59 PM
  4. Current rating of power supply
    By shiv in forum General Electronics Chat
    Replies: 3
    Latest: 28th May 2003, 05:03 PM
  5. Thermistor max power rating
    By nicksmale in forum General Electronics Chat
    Replies: 5
    Latest: 4th April 2003, 05:28 AM

Tags for this Thread