What is the limiting factor of a switch?

I know for wire, current dictates if the whole circuit will go up in smoke. However on switches I often see [250v, 10amp|125 volt, 20 amp|3/4 horsepower]. This seems to suggest switch ratings are based on power, not current.

I am interested to understand this situation because I am using 1.2 volt rechargeable batteries in parallel that will deliver ~40 amps. They will never be switched on for more than a minute at a time. Depending on how this works I will either have to buy expensive, large 12 volt 50 amp switches, or inexpensive, common 125 volt 6 amp (750 watts, compared to 1.2 volt * 40 amp = 48 watts).

Perhaps it is somewhere between these two choices. What do you think a reasonable switch for this load (1.2 volt, 40 amp) is?

Thanks for any help.

A 125volt 6amp switch will probably melt or weld closed with 40 amps. It's not the wattage that's the problem it's the current, also the type of load you'll be switching matters because if it's switching an inductive load, especially at 40 amps you're liable to get the contacts welded closed from contact bounce. You also have to make sure that the listed current is the switching current and not for disconnect usage. Disconnect usage means the switch can handle the current but isn't meant to switch on or off during full current. The horse power rating is base on the motor being an inductor, and when it's switched off it can generate it's own voltage which can produce arcing. The switch in your case is only rated to switch motor loads up to 3/4 horse power. Switching 40 amps isn't a trivial thing, either make your own contacts or buy a rated switch.

The load would be an electromagnet winding (so really just a few feet of wire shorting out the batteries). There is a very large price jump from 20 amps -> 40 amps, and there is very little selection at that high current range. The switch only has to hold out some test runs and a competition. I am not looking for a permanent solution.

Would 20 amp/24 volt switch be able to survive?

Get a Ford starter solenoid,
http://www.ytmag.com/store/parts/pics/wm_D2AF11450AA.jpg
the kind that bolts onto the engine compartment wall.

You'd need a separate 12 v battery to power it. The rules might allow it.

Trisorion said:

Would 20 amp/24 volt switch be able to survive?

Click to expand...

bump......

Seems to me a starter solenoid is a bit of an overkill. Why not use a typical 40 amp automotive relay. Smaller, (much) cheaper. I have found that most of these relays will trip at above 9 volts (connect 8 of your 1.2 batteries in series for the relay coil).

Welcome to the forum Fatfenders.

Switch rating

Hi trisorion,

the rating information on the switches and relays is a bit misleading. They are absolutely not rated for power.

A switch rated 125VAC/10A won't last long switching 250VAC/5A, which is exacly the same power. (At least not for the given switching cycles in the datasheet)

The contacts are much too close and will burn up very quickly. Also the springs inside a switch or relay are dimensioned (tension) for the power rating.

Miniature relays are often labelled 24VDC/2A. Using that kind of relay to just switch 10mA at 250V it will go up in flames before the first switching cycle.

So given information makes sense.

For high power applications 500A at 400V three phase switches are replaced at regular intervals because of the tiring of the springs. If they become too tired the switch closes slower and the contacts start burning up, and in many cases they are baked together.

Boncuk