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?
 

CodaMan

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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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TJim

TJim

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If you used smaller and the device reached it's maximum current draw you would blow the fuse and burn up the relay.
So I must use a 45 A fuse and 45A Relay?
Exactly the maximum amperage draw of each accessory?
 

CodaMan

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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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CodaMan

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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.
 

pagrey

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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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TrueTexas

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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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TJim

TJim

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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?
 

Apparition

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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.