rezdiver wrote:all the switch should have to do is energize a low current coil which closes the main contacts in a relay which carry a high current. a switch should never have to carry full load current if wired properly. most switches should handle up to 10 amps
[... snip ...]
.
The point of a relay is to use a very low current circuit to switch a high current circuit. So, you're absolutely correct that there is not much problem with an off-the-shelf switch, within reason.
However you should be careful if you use a switch in an automotive application without a relay. " ... most switches should handle up to 10 amps ..."
It is not necessarily true that most switches can handle 10A of DC. Generally switches are rated for Alternating Current (like in your home) and might be used, for example, to operate a lamp in your living room or a blender in your kitchen.
The Direct Current in your vehicle is not AC, however. With AC, there is zero voltage across the switch 60 times per second (in North America, 120V 60 Hz AC) and less than full current most of the time as it swings from zero to full wave and back down to zero again. This serves to halt an arc across the switch terminals in 1/60th of a second or less, even if you happened to trip the switch when full power was across the terminals. Once the arc is halted, then the distance between the terminals only has to be sufficient to prevent an arc from jumping the switch terminals.
DC is "always on" ... there is never a time when the power is zero in the closed circuit. Some AC-rated switches cannot handle any DC loads without arcing (if the distance between terminals is not sufficient to prevent an arc from jumping the gap).
If you examine the switch body you will rarely find one that is DC rated at all. This is true even with switches found in automotive shops.
Others are built to handle both AC and DC. If that's the case, the switch will be expensive, and will have both ratings marked on the switch body. A switch that is rated for 10A DC would cost at least $20 and usually more. They are also physically large to allow for the gap needed to stop a DC arc.
As a rule of thumb, a switch that is rated for AC only can only be expected to switch 12VDC at 1/20th the AC amperage rating at 120V. So a switch rated 10A 120V AC is only safe for a DC circuit of 1/2 A (500 mA) at 12V DC. As a rule most DC rated switches are rated for 32V DC, but this doesn't mean the rating would be higher for 12V; the criteria for voltage and current limits are different.
With a relay, this won't be an issue ... generally a relay low-current side will have perhaps 100~200 mA across the switch terminals. Just avoid an AC 120V switch rated for about 6A or less. That indicates the switch doesn't have the distance across the switch terminals necessary for 200mA or so; these switches are generally small in size.
If for some reason you need a higher current DC switch, try to look for aircraft surplus switches, eg military surplus. The price for surplus goods would be reasonable enough and they will be rated for DC current (it's illegal not to use a DC rated switch on a DC circuit in aircraft). OEM automotive switches can also be expected to be DC rated, but it's common for OEM parts to not have the ratings marked, since there is no need when used for direct replacement. Therefore you might need to do some research to determine if the switch is adequate for your use.