What amp rating do I need?

[jtbsc]

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

 

[kchriste]

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.

 

[audioguru]

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

 

[RadioRon]

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?

 

[Hero999]

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.