Fuse Rating Selection

Marty K

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Hi, all.

I am no electrician by any means, but know the basics of hard-wiring various accessories in a vehicle. Anyway, one topic comes to mind that I have seen different opinions for, but does not seem to provide a definite one-for-all accepted answer - what is the appropriate fuse rating (amps) to be used when hard-wire LJs?

For an example, I have TMG duals system installed. The CPU is rated to have 75W max output. If I hard-wire it, what fuse amperage I should go with? I understand that in normal operation, the current drawn will be very low, but what happens when there is that short time when the system is under full or close to full load?

I am asking, because I am a little confused by the fuse ratings supplied with the system. The 12V DC power adapter has a 1A fuse in it, just as my first hard-wire kit (the in-line fuse) when I purchased my system. I recently got a replacement CPU, and the hardwire kit that came with it has 2A in-line fuse.

What are your thoughts on this?

Thank you!
 

Godowsky17

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ALP is designed to have a maximum draw of 1.5A of current. I believe TMG's specification is similar. So any fuse over 2A should work well, I decided to put in a 5A fuse so I have some buffer.

A 2A should work fine as long as the TMG is the only accessory running off that fuse.
 

gwing

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OP you should be looking for the power input requirements, not the output. Input will be stated in volts and amps or volts and watts. It is from this information that the fuse size is determined.
 

Marty K

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ALP is designed to have a maximum draw of 1.5A of current. I believe TMG's specification is similar. So any fuse over 2A should work well, I decided to put in a 5A fuse so I have some buffer.

A 2A should work fine as long as the TMG is the only accessory running off that fuse.
Thank you for the input!
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OP you should be looking for the power input requirements, not the output. Input will be stated in volts and amps or volts and watts. It is from this information that the fuse size is determined.
Didn't think of that. Good point!
 

Deacon

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As a reminder, fuses are not really there to protect equipment (or whatever the “load” is on the circuit). They’re there to protect the wires themselves. When too much current passes through wires that aren’t big enough to handle it, they get hot, and they melt the protective sheathing, and they can start a fire. Usually this is from a direct short of some sort, but it can happen due to wires getting pinched between a seam, rubbing on something over time due to vibrations from a typical automobile environment, or both. For most of what we do, 5A is probably mostly low enough to prevent disaster. But if something comes with a 2A fuse, which is plenty sufficient for normal operation, why replace it with a fuse that doesn’t go until it hits more than double that?
 

Transporter

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As a reminder, fuses are not really there to protect equipment (or whatever the “load” is on the circuit). They’re there to protect the wires themselves. When too much current passes through wires that aren’t big enough to handle it, they get hot, and they melt the protective sheathing, and they can start a fire. Usually this is from a direct short of some sort, but it can happen due to wires getting pinched between a seam, rubbing on something over time due to vibrations from a typical automobile environment, or both. For most of what we do, 5A is probably mostly low enough to prevent disaster. But if something comes with a 2A fuse, which is plenty sufficient for normal operation, why replace it with a fuse that doesn’t go until it hits more than double that?

WHAT??

Fuses on power cables are there to protect the equipment from internal shorts and excessive damage so if 1) it was a simple overload the fuse can be replaced and one is back using their equipment or 2) the damage is kept to a minimum so the unit can hopefully be repaired!

In main wiring, a circuit breaker at the point of higher power protects the power line/power harness and an additional fuse either in the devices power cable or point of power cable connection on the equipment protects the equipment with one last feature to the whole system, the determined wire size is the final fuse should everything that could go wrong goes wrong and so much current is drawn that the wire acts as a fuse.

In a vehicle with multiple power distribution points (fuse box/fuse panel), one can pull power by adding a Add-a-Circuit/Add-a-Fuse to create a new protected circuit. Since a vehicle has an easy to get to direct DC power source, the battery, a fused or circuit breaker main new drop could also be added. If a fused/CB new line is used, still use the provided fused equipment power line just as you would with an Add-a-Circuit. On a new main line the Fuse/CB should be as close to the positive battery terminal as possible as the fuse/CB is there to protect the battery and the new line from excessive current draw (yes protecting the wires and battery, but remember this is not the fuse for the device).

Fuse sizing, the fuse/CB on the power feed line should be larger than the fuse for the device(s) it is feeding power to it, as should the wire gauge be larger than the wire gauge on the devices power harness. Example: I find a 10 Amp fuse slot in my passenger foot well power distribution panel, I pull the 10 amp fuse and place it in the Add-a-Circuit main slot and put a 5 amp fuse in the new circuit slot to wire that I will Posi-Lock the CPU wiring harness to for power, than place the Add-a-Circuit where I pulled the 10 Amp fuse. So I have a 10 Amp circuit with a 5 Amp add-on that is powering a 2 amp CPU IE all fuses are descending in rated value from the main power supply because even the power distribution panel is on a 100 amp circuit breaker in the main power box after the battery. So always go lower as you get to your devices rated draw and always go as low in rating as you can for the fuse the devices power harness is connected to as in using a 5 Amp circuit to provide power to a 2 Amp draw CPU.

Of course in direct power wiring there are some exceptions but only as long as the devices power harness is properly fused (but I still believe one should go to the additional work of still adding a fused power point). This would be wiring the fuse CPU cable directly to the battery IE it would be a fuse power line with direct source power. Another example is using a cig lighter plug to power a CPU, the socket is a 15 Amp power source, one would properly solder the positive and negative to the proper terminals of the cig lighter plug (using the fused CPU power cable), than put a 3 to 5 Amp fuse in the cig lighter plug so it cascades from 15 Amp source, to 5 amp cig plug, to 2 amp CPU power cable.

Fuses protect Equipment, main Circuit Breakers/Fuses nearest the main power source protect both equipment but mainly the wiring with the caveat that there could be much damage to the equipment if it is not properly protected by a fuse because the main line will draw much more damaging current before a dead short cause it to trip or blow doing its final job of protecting the wiring.

.
 

Deacon

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always go as low in rating as you can for the fuse the devices power harness is connected to as in using a 5 Amp circuit to provide power to a 2 Amp draw CPU.
That’s not as low as you can go. When the original device comes with a 2A fuse (much less 1A), there’s no reason to increase it to a 5A fuse to “have some buffer.” Hell, a 2.5A fuse would be an unnecessary increase.
 

Marty K

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@Deacon and @Transporter, thank you very much for your inputs here! Much appreciated.

Like I said earlier, there is always more than one opinion or "best" approach in this matter. I am still confused of what the right fuse for my case would be. So here is the situation as simple as I can state it:
The TMG CPU states the max power output is 75W. I do understand that this is a peak output, not the normal (most of the time) power draw. I get that. But what about that extreme moment, when it will draw the 75W? If I understand this correctly, the peak current will be about 5.43A (for 13.8V, not corrected) or 6.79A (corrected, by 1.25 coefficient). So how does that work? If the CPU needs this much output, it must draw it, as it is not a power generator on its own. How brief that max spike really is? How does that affect the in-line fuse of 1A or 2A? They look like a fast-blow type.

Just to make it clear - I am not here to argue with anyone, just trying to make sense of how those systems work. I like to know the inner works of the electronics I use, that's all it is.

Thank you again!
 

Deacon

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The TMG CPU states the max power output is 75W. I do understand that this is a peak output, not the normal (most of the time) power draw. I get that. But what about that extreme moment, when it will draw the 75W?
It does not state that. It says “peak optical power” not electric input. Ignore the 75W marketing thing. It’s meaningless. It’s like how they measure the brightness of headlight bulbs or flashlights or whatever, with numbers all over the place. The only thing that matters is input current draw.

If it comes with a 2A fuse, then that means 2A is plenty of headroom.
 

Marty K

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It does not state that. It says “peak optical power” not electric input. Ignore the 75W marketing thing. It’s meaningless. It’s like how they measure the brightness of headlight bulbs or flashlights or whatever, with numbers all over the place. The only thing that matters is input current draw.

If it comes with a 2A fuse, then that means 2A is plenty of headroom.
OK, thank you for the clarification. Enough said.
 
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Godowsky17

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I think we're basically all in agreement that using the 2A fuse is the simplest and best solution given your current situation and the parts you have available.
 

GernBlanston

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Fuses on power cables are there to protect the equipment from internal shorts and excessive damage ...
Nope. Fuses are there to protect the vehicle from the users, not the users from the vehicle.
 

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