Switch rating - how to read the printed stuff - Page 1
Forum section: electronics / beginners

author: bitman
author_rank: Unknown
timestamp: May 26, 2018, 06:08:34 pm
content: Hello,I have a handful of these switcheshttps://photos.app.goo.gl/HjhdVYrgLoEsdMX03- a simple rocker that I want to use for a few 120V lights I have on my work-desk. The rating says 2/250~ which I interpret to mean 2amps 250volts max. First of, is that the right reading?Second, if that's true it looks a bit "flimsy" inside to support that load - but I guess 500 Watt isn't high enough to be of concern? My lights are definitely not 500W even though I'm on 120V. So my reading is that I'm fine, but I wonder if this is just Chinese made up stuff on the back?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 26, 2018, 07:07:46 pm
content: The switch does seem to be rated for 2A~ and these indeed often look (and are) flimsy.Main concern is probably not the switch itself, but the housing.How good is the stress relief for the cable?How easily does the housing fall apart?

author: dmills
author_rank: Super Contributor
timestamp: May 26, 2018, 09:08:07 pm
content: @120V, ~2A is only 240W, not 500W!That said, if 240W will do for your use case, and if your wire works with that style of rather pony strain relief, then the swich will probably work fine.Regards, Dan.

author: bitman
author_rank: Unknown
timestamp: May 26, 2018, 09:18:19 pm
content: Quote from: dmills on May 26, 2018, 09:08:07 pmAs I wrote and as the picture says, the print says 2/250v - not sure where you get the 120v from?

author: glarsson
author_rank: Frequent Contributor
timestamp: May 26, 2018, 09:55:44 pm
content: Quote from: bitman on May 26, 2018, 09:18:19 pmProbably from you. You wrote "I'm on 120V".The switch does not care about the W of the load. It only cares about the voltage it can safely break and the current it can safely conduct. The power of the load can be 24W at 12V, 240W at 120V or 500W at 250V. If you gave 120V then the max is 240W.

author: Zero999
author_rank: Super Contributor
timestamp: May 26, 2018, 10:34:10 pm
content: Quote from: glarsson on May 26, 2018, 09:55:44 pmActually switches are often rated in terms of power, because the switch has to dissipate some power, when it's breaking or making the connection. Theoretically a switch controlling a resistive load will dissipate half of the load's power, for a short period of time during turn on/off. In this case, no switching power rating has been given, only 2A at 250V, which would be 500VA, although on 110V it would still be rated to 2A, so the maximum power would be 240VA.

author: Ian.M
author_rank: Super Contributor
timestamp: May 26, 2018, 11:34:50 pm
content: However if you are using it with incandescent or CFL lamps, you need to allow for inrush current so u sing it for more than 50VA of lighting at 240V or 25VA at 120V would really be pushing your luck.Tungsten filament lamps tend to draw a surge of up to 10x their nominal current at initial switch-on, and CFL lamps have to charge the reservoir capacitor for the DC bus of their inverter, so if they don't have soft-start power factor correction, they can also draw a high startup surge current. Unfortunately the surge current occurs when the switch's contacts are bouncing on closure, so if you load it with 500VA of 240V lighting or 250VA of 120V lighting, both with a surge of up to 20A, the odds are high that it will soon fail welded closed.