What fuses and relays should I use?

TJim

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Hey Guys!

I want to learn how to calculate which fuses and relays to use.

For example, my T-MAX Air Compressor mentions: Maximum Amperage Draw: 45A

Does that mean that I should use a 45A fuse and a 45A Relay? Or slightly smaller?
 
If you used smaller and the device reached it's maximum current draw you would blow the fuse and burn up the relay. so no, not smaller.

Edit: So no, not smaller. See below.
 
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The general rule is for a fuse to do its job correctly and protect the wires (purpose of a fuse), it should be rated at about 1.1 to 1.5 times the rated current value for the wires.
 
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and a relay is not a protection device, so it needs to be able to handle the 45 amps easily.

Edit: and that is across the contacts not the coil.
 
Personally I'd use a marine circuit breaker rated right about 45A. This way if you make a mistake it just pops and you press a button and move on. It sounds like you're the type that is going to make a few mistakes, buy a breaker and not a fuse. For the relay you can go right at the peak current or slightly under. T-max is giving you an absolute worst case and you don't need to worry too much about that. Personally I'd trust a 40A but that's a safety issue you should decide for yourself.

Edit: I use these Del City breakers.

breaker.jpg
 
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I am guessing that 45 amps Max refers to inrush current which is the initial spike in current when a circuit is turned on. Your fuse will need to be slightly higher than this. If lower, it may blow when turned on. @pagrey suggestion of a breaker is a good option as well.

For the relay, the fuse should be on the supply wire as close to the power source ( battery) as possible.

The coil wire on the relay should be fused as well but this should be small - around 3 to 5 amps should be sufficient.
 
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I am guessing that 45 amps Max refers to inrush current which is the initial spike in current when a circuit is turned on. Your fuse will need to be slightly higher than this. If lower, it may blow when turned on. @pagrey suggestion of a breaker is a good option as well.

For the relay, the fuse should be on the supply wire as close to the power source ( battery) as possible.

The coil wire on the relay should be fused as well but this should be small - around 3 to 5 amps should be sufficient.

about the relay, I should use >45A relay? Is there a problem if I use for example a 60A Relay? or Should I use a 45 A relay?
 
Personally I'd use a marine circuit breaker rated right about 45A. This way if you make a mistake it just pops and you press a button and move on. It sounds like you're the type that is going to make a few mistakes, buy a breaker and not a fuse. For the relay you can go right at the peak current or slightly under. T-max is giving you an absolute worst case and you don't need to worry too much about that. Personally I'd trust a 40A but that's a safety issue you should decide for yourself.

Edit: I use these Del City breakers.

View attachment 148193
Breakers also make troubleshooting much easier since they have a visual indicator.