Switch ratings; how to compare?

Probably around 10 amps.   The voltage rating is more for the insulation and how the contacts make and break, but lower voltages don't increase the switch rating a great deal.  You'll see some that have a somewhat higher rating for lower voltage, but many don't.

There will also be separate ratings for AC and DC, as DC is actually harder to switch, the current never goes away to break the arc as the switch opens.

I have also wondered about this.   I assumed that the capability of the switch would be related to the wattage (and therefore heat buildup) it could handle, so with a lower voltage, you could handle more amperage.  But there is more to it with arcing and insulation issues.  Thanks, John

Bob

Actually, the switch is going to see the same voltage drop and thus heating of the contacts, regardless of the voltage being handled.

Ohms law:

Watts = Amps x Volts

1200 = 10A X 120 V

1200W/20V = 60A

It's 10 amps x the voltage drop across the switch when it is closed that determines the watts being dissipated by the switch. 

I wouldn't think any switch would consume energy unless it is a lighted handle switch. Amperage ratings allow for the proper switch to be used in a circuit.  You wouldn't want to use a 5 amp rated switch in a lighting circuit that draws 16 amps.  Best to have all the components of a circuit appropriately rated, the weakest link always fails first. As John noted 10 amps is 10 amps either at  20 volts or 120 volt.

  Also note there are switches (toggle type wall switches) with higher voltage ratings for 277 volt circuits. I would think a 277 volt switch would allow for a slightly larger contact point gap. I've been wrong and often. Mike CT 

The voltage rating of a switch is the voltage the switch contacts

can handle without holding an arc as the contacts open.

The rating of a switch can also depend on the kind of load, resistive

or reactive ( inductive or capacitive ) being switched.

So the switch could take more if it wasn't switched under load ...?

Again, in general, there wont be a huge difference in current ratings at different voltages, especially switching AC voltages.

Our local ACE Hardware has a variety it turns out; thinking the 15amp SPST will work well for turning off yard tracks.

Ended up getting some Heavy-Duty 20amp rated toggles at ACE Hardware for under $5 each, in both DPST and SPST types.  These should hold the load.

If you switch with no load there is no current, so no issues.  But you do want it rated for the load it will carry.  So 15 or 20A is a good safety margin based on transformers sourcing 10A max.  G

Switching with no load is called switching dry.Reed relays are used

to carry a load but are switched with no load  because the arcing at

the contacts will weld the contacts closed. The load is added after

the reed relay is closed. Contacts are made with a tungsten coating

to protect the contacts from the heat from the arc at opening and

closing. A switch can carry much more current then it can switch.

Some motor starter contact points are replace-able.  At one time there was a fair amount of silver in these contacts.  Point arcing is a problem.  Motor circuit designs were at least 125% of full load rating on the motor, which was still way short of Locked Rotor listed on the motor. There are charts, usually on the inside cover of a motor starter that lists the correct over current protection (heaters) to be installed in the starter for a specific motor load.  Many of these assemblies are now adjustable.

Interesting note.  Kerrigan mentioned a Double pole single throw switch.  Caution.  A look-a-like switch is a common (4-way) switch, which is actually a Double Pole Double Throw Cross Over switch with the same (4 screw) configuration. A (4-way) switch has a different application.