How to calculate switch inrush current for florescent light circuit

[logikman]

We are trying to determine the current rating for a switch to operate a bank of flourescent lights.
Is there a simple formulae to calculate the operating and inrush current rating for the switch.

 

[DRWeig]

logikman,

It depends on the type of ballast and lamp you're using (and, of course, how many). If you're considering more power than what we'd be allowed to put on a 20-amp circuit, you might consider a pilot switch and lighting-rated contactor.

More info from you will yield a better answer...

What sort of ballasts? What sort of lamps? How many?

Old Dave

 

[logikman]

Hi Dave.

Thanks for your prompt response !!

As you probably have gathered I am not an electrician, but have been asked to design a door post (next to glazing)to accomodate a "slim" light switch. The normal Architrave switches (such as the MK Gridswitch) is just are too large and cumbersome. We have identifed some smaller rocker type switches and I was looking for a more general formula to determine the switch rating covering a typical office lighting circuit. Typically this would have 4 - 5 modular light fittings containing 4 x 36 watt lamps.

We are able to obtain 16 Amp rocker switches with an inrush current of 150A Inrush to IEC 65 but need to find out how many modular fittings it could safely operate.

The other option that we have is a Rocker switch rated at 20 (4) A 250 V~ with an inrush of 120A. With the lowwer inrush figure does this mean it would operate less lights ? or is the general current rating the more significant factor?

 

[BJC]

I think I just said something like this in another thread. If your trying to put a swtich in the post of a "store front" system good luck.
I recomment doing the actual switching in a lighting relay somewhere above the ceiling. The actual switch in the door frame then can switch at 24 volts DC.
I can't recall the name of the switch we used for such appllications but that's where you call the local lighting drummer and let them help you. With 24 ( or some work with 12 ) you can go down the door frame or whatever to a switch that looks more like a door bell switch than you regualr wall switch. It's best to get one from a lighting manufacture because then you can avoid hassels with electrical inspectors.

 

[DRWeig]

Logikman,

I agree with BJC on this'n. The switches you identified are probably both good for your load, but they really should have a specific "ballast" rating like lighting contactors and mainstream snap-type toggle switches.

Inrush to a ballast can be anywhere from 1.5X to 10X steady state, depending on type, age, etc... You'd need info from both the ballast manufacturer (for the specific ballast you want to use) and the switch manufacturer if it doesn't carry a true ballast rating.

Again, based on what you're telling us (4 or 5 luminaires, each probably about 1 amp maximum steady-state, your switches you found are probably good for the task. Using the low-voltage plus relay solution is a lot slicker though.

Let us know what you wind up trying!

Old Dave

 

[DRWeig]

Oh, and I almost forgot to answer one of your questions -- the steady-state (or full load) rating is more important for fire protection, while the inrush rating is more important for switch life (although the switch contacts can become a fire hazard as they die).

The inrush and arcing upon opening due to inductive load in the ballast will try to pit and weld the contacts. When properly designed and applied to a load within their rating, the arcing will actually help keep the contacts clean.

So both items are important. If a switch is rated for lighting loads (example: 20A ballast or 20A tungsten), you don't have to be concerned with calculating inrush. Just keep within the rating for steady-state.

Old Dave

 

[logikman]

thanks for all your responses.
I will try to fid out the "Ballast" rating of the lighting, but I am not too sure of the switch manfrs give this figure or not. It does seem odd to me that the lighting unit mnfrs and the available switches seem to use different rating methods. I guess frm the repsonse that there is no hard formula for this (like ohms law) to determine the figures when there is AC and a ballast involved (or is there ?)

regards

Logikman

 

[advidana]

You need to get a switch that has been tested and rated for use for light switches by UL because if you get a fire from the bad switch you will be held legally responible for all the damage. if the building burns down then the cost of this will be yours. I Do not want to rain on your proect but you should known this. good luck.