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Lets say you're building a circuit. How do you figure out the amp for the fuse or the breaker. I know it's about 10% more than the amp going through the circuit, but what's the actual formula to figure it out?
You need to know the rated amperage draw for start-up and working amperage of the device that you are powering.
From that, you can select the appropriate fuse that protects the circuit from an overload. You will need to use the correct size wire to deliver the power to the consumer.
You can use a "copper wire current capacity" chart to determine the minimum size of wire that will safely carry the amperage load for the distance from the power source to the power consumer. This is important to deliver the power to the device without an excessive voltage drop or heat damage to the wire. The charts are on-line or published in many reference handbooks.
If you are selecting a switch for this circuit, be sure to use an appropriately rated device. If there's a high amperage draw, consider using a relay to power up the device controlled by a low amperage draw switch circuit.
Ohm's law is applicable from an engineering design standpoint, but from a practical application standpoint to install an effective and safe electrical device, you don't need to be engineering the installation. Simply get the amperage draw info from the device maker at the rated system voltage (use 15 volts as a guideline voltage in a 12V battery car), and select the wiring, fuse, and switch in accordance with the numbers you obtain. Easy to do .... don't forget to use an appropriately rated fuse holder for your installation, too.
PS: an ohmmeter will not give you results that are meaningful in the automotive world unless you are measuring a static resistance load, such as an incandescent light bulb. Nor will it give you an accurate reading of higher current resistance loads on a circuit; ie, a voltage drop test across large cable or ground connections is a far better indication of a high resistant connection under load than a very small voltage load placed upon the connection by a VOM. Further, if you are looking at electronics, such as a computer or audio amp or radio or player ... you'll not see the resistance of the unit as these can vary for many reasons under actual load. Again, your best guide to the amperage draw of these devices is the manufacturer's rating.
I know how to figure out the Amps, Volts, and Ohms. that's not my question.
If I'm building a circuit from scratch, and I my battery is xVolts, I use xResistor so I would only use xAmps in the circuit...now I need a fuse to protect the circuit, obviously it can't be same Amps as the current in the circuit. So what's the formula to figure out the Amps for the FUSE.
You need to know the rated amperage draw for start-up and working amperage of the device that you are powering.
From that, you can select the appropriate fuse that protects the circuit from an overload. You will need to use the correct size wire to deliver the power to the consumer.
You can use a "copper wire current capacity" chart to determine the minimum size of wire that will safely carry the amperage load for the distance from the power source to the power consumer. This is important to deliver the power to the device without an excessive voltage drop or heat damage to the wire. The charts are on-line or published in many reference handbooks.
If you are selecting a switch for this circuit, be sure to use an appropriately rated device. If there's a high amperage draw, consider using a relay to power up the device controlled by a low amperage draw switch circuit.
Ohm's law is applicable from an engineering design standpoint, but from a practical application standpoint to install an effective and safe electrical device, you don't need to be engineering the installation. Simply get the amperage draw info from the device maker at the rated system voltage (use 15 volts as a guideline voltage in a 12V battery car), and select the wiring, fuse, and switch in accordance with the numbers you obtain. Easy to do .... don't forget to use an appropriately rated fuse holder for your installation, too.
Thank you, this is all very good explanation and good information, and I already know this. I know there is an actual formula to figure the fuse amp, which is what I'm looking for.
I know how to figure out the Amps, Volts, and Ohms. that's not my question.
If I'm building a circuit from scratch, and I my battery is xVolts, I use xResistor so I would only use xAmps in the circuit...now I need a fuse to protect the circuit, obviously it can't be same Amps as the current in the circuit. So what's the formula to figure out the Amps for the FUSE.
Your answer lies in the question: What safety margin do I need to have to protect the wiring circuit and the device that is being powered?
You need to consider the nature of the consumer and it's likely failure modes. A static load might require a standard fuse of nominal capacity over the amperage draw, while a more sophisticated electronics circuit might require a fast acting fuse to protect it against transients. Other electronics circuits may use yet an entirely different type of fuse for protection.
There is no single "formula" for selecting amperage of a fused circuit that will apply to all situations automotive.
There is no single "formula" for selecting amperage of a fused circuit that will apply to all situations automotive.
I'm sorry to disagree with you, but there is one, otherwise I wouldn't be asking. I have common sense and I can figure out what kind of fuse I'd need, that's not the problem. So hopefully someone here will be able to tell me
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