12-14 volt electrical system

In a 12 volt system the alternator is 13.8 or 14 volts right? When buying a switch to close a circuit (12 volt pump), do I buy a 12 or 14 volt switch?

Also the circuit breaker, should it be the exact amperage of the pump (i.e., 4 amps) or more?

Hi Abraham,

Buy a switch rated for DC current at an amperage above what the load will be drawing - usually by a good margin - you're probably going to end up with a switch rated at least 5, 10 or even 15A or more.
Also make sure the circuit is protected with an appropriately sized fuse or circuit breaker for both the load and wire gauge. There are lots of good articles, postings and publications on this topic. Suggest reading until you are confident you've got a good understanding and/or have a local EAAer (tech counceler SP?) check your plans and work. Regarding breaker sizing - you will likely size the breaker with some margin above the rated current draw of the load, allowing for variation due to temperature, 'inrush' (startup current) etc.

Chris

Voltages interchangeable

For the most part, 12V & 14V are interchangeable numbers. The components are rated as 12V, the 14V number is generally a charging voltage (13.8V) most often. I wouldn't get too wound up on the voltage numbers. As the previous poster noted, current ratings on wires, switches, and fuses/breakers are much more important. The rule of thumb I use for fuse/breaker ratings is 150% of the device the fuse/breaker protects. Also, remember - fuses/breakers really only protect the wire, not the component!

Quote from Bob Nuckolls' ARTICLE

As a general rule of thumb, I've told builders that the 115 VAC rating is directly translatable to 14 VDC applications.

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Even though a switch manufacturer might not publish the DC rating of a switch, every switch has a DC rating. Of course it is going to be a fraction of the AC rating if the voltages are equal. But 14 volts is only 6 percent of 230 volts. So switches installed in our airplanes are automatically derated by being operated at reduced voltage. Look at the switches that B&C sells for aircraft use. They publish the AC rating, but not the DC rating. My rule of thumb is that any snap action switch can handle the same current at 12 volts DC as it can at 115 volts AC. This rule does not apply to switches that open or close slowly such as sliding switches.
For 12 volt systems, not to worry about the voltage rating. The current rating is the one to be concerned about.
One might think that gold plated switch contacts are better than silver plated contacts. But look at the current rating. Gold contacts conduct less current than silver contacts. Gold contacts are good for small electrical signals because gold does not corrode. Silver contacts are better for larger currents. Switches need to be exercised to wear away corrosion. An unused switch can fail due to corrosion causing high resistance.

Thank you everyone for the explination.

Here is another question that I have:

In this diagram from B&C, look at label number 6 it lists the part as "S700-1-3", why would a fuel pump have an (ON-ON) type of switch?

The switch can be wired for on-on, but in the case of the illustration, it is wired to be on-off.

Notice the third terminal is unused.

Mike S said:

The switch can be wired for on-on, but in the case of the illustration, it is wired to be on-off.

Notice the third terminal is unused.

Click to expand...

Got it! So it is basically a single-pole single-throw..