Which side of a fuse is Hot vs Cold?

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#1
I have gone up and down the internet trying to look for this but not getting a clear answer. When I type on how to find the difference of the 2 sides, all I get are explanations telling me what each function is like hot is loader and cold is receiver and not how to physically identify which one is the hot and which is the cold. I'm planning to get the Power Magic Pro but with no way to know which side is hot and cold so I make sure I put the fuse back correctly without blown the fuse or shorting the fuse box entirely.

If anyone can give me a simple way to test with a light meter or some identifying markers on a fuse, that would be extremely helpful. I want to avoid damaging my fuse box or a fuse but putting the fuse taps in the wrong way. And not the power magic pro installation, my car manual or the internet it feels can get me that answer. I was able to solve all my other questions I have so I have it down to know exactly what I need to do to install except for this last part. I know i need mini fuses, i know what to look for for ACC and BAT etc butt I just can't find which part is hot and which part is the bold and the light meter I read all it says is both are suppose to light up if its working, but nothing on how to identify which side is which. If anyone can gibe me an answer, that would be really helpful.
 

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#2
Fuses can be used either way around, there is no difference between the ends.

Except after it has blown, then you can put a multi-meter across the fuse and the end that is being supplied with power will be positive (hot) while the other end will be zero volts (cold). If you then turn it around the hot end will still be in the same place.
 

flip9

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#3
With a test light you need to pull out the fuse and poke one of the two legs till it lights up. The leg that lights up will be hot 12v, you might need to turn the key if its switched ACC/IGN.

There are two orientations/positions of using the fuse tap but either works for a low current draw of the dashcam.

Position A - the most common way used. It 'taps' before the factory fuse to create a new circuit. Its not recommended to put a high current load in this way as it will be possible exceed the factory wiring ampacity.

Position B - the new fuse runs inline after the factory fuse. The safe method as you will never exceed the factory wiring.

 
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#4
With a test light you need to pull out the fuse and poke one of the two legs till it lights up. The leg that lights up will be hot 12v, you might need to turn the key if its switched ACC/IGN.

There are two orientations/positions of using the fuse tap but either works for a low current draw of the dashcam.

Position A - the most common way used. It 'taps' before the factory fuse to create a new circuit. Its not recommended to put a high current load in this way as it will be possible exceed the factory wiring ampacity.

Position B - the new fuse runs inline after the factory fuse. The safe method as you will never exceed the factory wiring.

So the fuse has to still be in the box wit the fuse pulled out slightly but still hanging or you have to remove the fuse entirely from the fuse box when testing with light meter when you say pull out the fuse?

And thank you for the diagram, that does help. so is position A where both the fuse for dashcam and fuse for ACC/BAT both are running the hot side and Position B where the fuse for dash cam has the hot going opposite side, cold for fuse ACC/BAT?

Also, I know there is a lot of terminology im still learning but when this guy says "Loader" and "Receiver", he is saying hot and cold right and loader is hot, receiver is cold?
 

flip9

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#5
So the fuse has to still be in the box wit the fuse pulled out slightly but still hanging or you have to remove the fuse entirely from the fuse box when testing with light meter when you say pull out the fuse?
Pull the fuse out completely so all you see is the two legs of the fuse holder.

And thank you for the diagram, that does help. so is position A where both the fuse for dashcam and fuse for ACC/BAT both are running the hot side and Position B where the fuse for dash cam has the hot going opposite side, cold for fuse ACC/BAT?
Yes correct, but like i said either position will work for a low current draw like a dashcam. If your plugging in a parking mode battery pack like the iVolt (12.6A) or Cellink-B (7A) then Position B would be the safe way of doing it.

Also, I know there is a lot of terminology im still learning but when this guy says "Loader" and "Receiver", he is saying hot and cold right and loader is hot, receiver is cold?
The terminology he used on this clip is confusing and incorrect. He refers to the 'load' side as the 12v hot side and he calls the 'draw' side as the dashcam.

Its actually the opposite. You draw current from a battery to power the load.
 

SawMaster

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#6
@flip9- Well explained :D

Some 'caveats' about fuse taps-

Never tap into a safety-critical fuse. Some examples would be "Airbags" (sometimes called SRS), "ECM" or "PCM" (car engine computer), "wipers", "ABS", or any external lighting including brake lights. While a properly installed fuse tap should not be a problem with these, strange things can happen in life and a fuse tap is adding a possible point of failure. There are almost always safer fuses you can tap to get the needed power requirements.

Among other fuses, don't tap low-rated fuses (say 5A or less) as doing so can possibly overload the car wiring.

Keep your wiring tidy and secured where no possible movement of people in the car or movement of any car part (like doors) might cause problems such as pulled or pinched wires. This includes not running wires across sharp edges.

If you make a wire-to-wire connection, either solder it or use a crimp-type connector, then use electrical tape or heat-shrink tubing to insulate the connection. Also, if you make this kind of connection on a multi-wire cable, stagger the connections so that if the entire cable pulls loose or the insulation fails it will still be impossible for those wires to short against each other.

Lastly, if you are not certain that you can do the job correctly, then enlist some help from someone who can or let a pro do it for you. Car audio shops are good with this kind of thing though any mechanic should be able to handle it too.

Phil
 

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#8
Yeap spot on Phil.

I was gonna write up a comprehensive guide about wiring and fusing that included all this but i just haven't had the time.
That would be a good thing indeed and feel free to quote any of my ramblings here without the need to credit me if you want :) I also have some good stuff bookmarked on this subject; PM me and I'll send you those.

Given the high number of requests for wiring assistance it would be nice to have both a main-page-linked general reference guide as well as car-maker specific forum or subforums or threads where folks can share specifics on each car too. That might be a lot of work for @DashCamMan though, so I'm just tossing the idea out there ;)

Phil
 
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#9
So what would be a good BAT fuse to tap into? I know for ACC that the radio would be a good choice. My car uses most 5A, very few 10A. I was thinking of used the E-light for the BAT but don't know what else I could try. I would have to take another look to see.


Also flip9. So you are saying the cold side is where the dash cam 'load' power? sorry o got confused on that response. because you said hot is load but then you said he had it backwards in the video so is hot the receive? Sorry just got confused there
 

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#10
Use what is available other than those I mentioned if possible. Sometimes you have little choice to get the characteristics you desire and have to tap them anyway. Done correctly that should still be safe. Those restrictions I mention are simply exercising best practices. You're probably OK tapping into a 5A fuse if that's all you've got to work with.

One problem which is becoming larger is that more and more new cars are using computer-controlled power schemes which do not make it easy to determine which circuit is switched and which is not :( That is why I suggested DCT might want a specific forum or series of threads on the topic of obtaining power for a cam and/or powerbank system ;)

Look at flip9's diagrams again. Trace the path from the "hot leg" or car battery supply to the cam. "A" goes only through the added fuse while "B" goes through the original fuse first then through the added fuse. See his reasons for choosing one over the other after you understand that. Either way should work though :)

One last thing- your cam fuse should always have a smaller rating than the fuse being tapped into.

Phil
 
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#11
Use what is available other than those I mentioned if possible. Sometimes you have little choice to get the characteristics you desire and have to tap them anyway. Done correctly that should still be safe. Those restrictions I mention are simply exercising best practices. You're probably OK tapping into a 5A fuse if that's all you've got to work with.

One problem which is becoming larger is that more and more new cars are using computer-controlled power schemes which do not make it easy to determine which circuit is switched and which is not :( That is why I suggested DCT might want a specific forum or series of threads on the topic of obtaining power for a cam and/or powerbank system ;)

Look at flip9's diagrams again. Trace the path from the "hot leg" or car battery supply to the cam. "A" goes only through the added fuse while "B" goes through the original fuse first then through the added fuse. See his reasons for choosing one over the other after you understand that. Either way should work though :)

One last thing- your cam fuse should always have a smaller rating than the fuse being tapped into.

Phil
Definitely understand the low rating. by the look of the videos where they put the dashcam for BlackVue, they have 3A. But always wondered what if they are equal amps?

so make sure I got this right, I just want to be careful I don't blow my fuses lol The hot leg is the loader that is the one a light meter will light up on and the cold is the draw that does not light up on a light meter? Position B does not have the hot leg of Dashcam fuse going parallel with the fuse to the default ACC/BAT fuse? And that the hot leg has to be the one furthest from the piggy back cables wire on the wire tap?

I'll also probably post a picture on here when I get it and see if, with the picture, you can tell me if it is correct or not for the 2 fuses so I make sure I put it back correctly.
 
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SawMaster

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#12
Definitely understand the low rating. by the look of the videos where they put the dashcam for BlackVue, they have 3A. But always wondered what if they are equal amps?

so make sure I got this right, I just want to be careful I don't blow my fuses lol The hot leg is the loader that is the one a light meter will light up on and the cold is the draw that does not light up on a light meter?
Correct

Position B does not have the hot leg of Dashcam fuse going parallel with the fuse to the default ACC/BAT fuse? And that the hot leg has to be the one furthest from the piggy back cables wire on the wire tap?
Not sure how your fusetap is designed so I can't say how to turn it. There should be instructions included which explain that ;)

The "B" circuit is series wiring, not parallel, as the current flows through the existing fuse then to the dashcam fuse then to the cam in series. The advantage of this is that it is impossible to overload the wiring when done this way. The disadvantages are that you could possibly overload and blow the original fuse since it is now carrying more current than it is designed for, and that could adversely affect vehicle operation.

Since there is always extra capacity designed into the car wiring, and we are not drawing much current with cams and powerbanks, the "A" circuit is the one most often used and recommended. It is how I draw power from a fuse location. The advantage is that if the cam PS shorts, only it's fuse will blow leaving your car fully functional other than the cam. The disadvantage is that in theory you could overload the wiring with this configuration, but that never happens in practice with the small current requirements of a dashcam and/or powerbank and the over-rating of wiring as is done in all cars.

In reality either way works fine almost all of the time for dashcam purposes :cool:

Phil
 

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#13
Just an FYI, the picture of the fusetap in Position B in my diagram is not entirely accurate. Its just a horizontal flipped version of Position A.

In reality, you will actually see the back of the fusetap in this position. I only did this for better visual of the path the current takes.
 
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So when you say correct, you mean correct you can use equal amps on a piggy back or you mean correct . Or you mean which is hot and which is cold and that hot is the one furthest away from the piggy back cable? or are they both correct? Just want to make sure


And I'll try to make a visual here so I got this correct. Oh and i put the picture in accordance to both being position A if you were looking directly at it from top view

edit.png
 
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#15
The cam fuse must be smaller than the fuse being tapped into.

Phil
 
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So this is what I got from my manual and from the fuse box. Trying to see which 10A I can use for ACC and BAT 20170801_182936.jpg 20170803_113911.jpg 20170803_113344.jpg 20170803_113251.jpg
 

flip9

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#17
First you need to check which fuses are constant 12v and ACC switched.

With the car off, use the test light and poke the Radio fuse (20). No need to remove the fuse in this case.

If it lights up then this is where the BAT wire will go into as it is constant 12v. If it doesnt light up, flip the key to ACC and if it lights up this time then this will be where the ACC wire goes.

You will need to find a suitable fuse for which ever wire is left over. I would test the the unused fuse slots (5,9,11,13) if any power is going through them. These unused slots are usually for factory optional accessories.

Do you not have another fuse box on the driver side? I cannot seem to see the fuse for your cig lighter.
 
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#18
First you need to check which fuses are constant 12v and ACC switched.

With the car off, use the test light and poke the Radio fuse (20). No need to remove the fuse in this case.

If it lights up then this is where the BAT wire will go into as it is constant 12v. If it doesnt light up, flip the key to ACC and if it lights up this time then this will be where the ACC wire goes.

You will need to find a suitable fuse for which ever wire is left over. I would test the the unused fuse slots (5,9,11,13) if any power is going through them. These unused slots are usually for factory optional accessories.

Do you not have another fuse box on the driver side? I cannot seem to see the fuse for your cig lighter.
youre right, i don't see that either. ill have to check again
 
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#19
I believe the cigarette lighter is under the hood in the power disruption box that it says not to probe. ill have to make do with that i go.

So I want to try the 10A, for better safety as suggested. the 7 i have are 3, 8, 15, 17, 18, 20, 21

3, 8, 20 look okay to use but 15, 17, 18, and possible 21 look like not to mess with.

if i have to resort to 5A i think 7 or 14 will work, not sure what the cluster is tho for 16. If it comes down to that none of those work. could I just buy at 10A and use an empty slot? also how do i tell if the sloe is for the hot or cold side? so i test it the same way as with the legs of a fuse but for the fuse box where the fuse goes?
 
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Flip9, would it be easier if I just bough 10A and used the empty spaces? How would i know or make is so its constant or only when car is on?
 

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