Max load when fuse tapping

[Tank Montana]

Hi folks,

So as the title suggests, I'm looking to understand what is the max load when tapping a fuse, either of the fuse slot itself, or the add-a-fuse.

To give context, my plan is to tap an existing fuse in my car which is 30 amps using an add-a-fuse, adding in a 3 amp fuse for my Thinkware F770, for the "ACC" line. My main question is if this too much draw for one port or is this okay? The fuse itself is for the following things, which makes me think it possibly is not suitable:

1. Fresh air blower control unit -J126-
2. Air conditioning system control unit -J301-
3. Fresh air blower isolation relay -J487-
4. Heater control unit -J65-

I've literally checked all other options in my fuse box and any other options are either always powered, obstructed by the frame/other fuses (when making sure hot side is on correct side) or are for important things. If this isn't doable I'll have to re-think the options.

Below shows the 30 amp fuse (red outline) I plan to tap, plus the 5 amp (green outline) I'm tapping for "battery" power, also with a 3 amp fuse.

If this process has taught me anything, it's that I'm terrible at electronics so any guidance is much appreciated.

Thanks,
TM

 

[SawMaster]

I thunk your plan will be OK. You're only drawing an extra coupe amps at the most which a 30A circuit fuse should have the capacity for.

Phil

 

[Module 79L]

One time I had to search for a power source controlled by the ignition to install an accessory in a '07 Ford Fiesta and I found out that some of those empty fuse holders had what I needed. I checked which of the terminals had 12V and soldered a wire with its own fuse on one side of a blown fuse, just to have something that fitted correctly in the holder to avoid bad connections.
I don't know what make your car is but it might be worth a shot.

 

[Ralph2]

You need to understand just.. how.. the add a fuse thing works. Installed properly.. the existing circuit is the same, as the same fuse goes into that slot that the add a fuse thing replaces. The additional fuse supplies power to the new circuit and is totally independent of the original. Providing.. the orientation of the add a fuse is right. the simple test is if it supplies power when.. the original fuse is NOT installed. This also proves the independence of the second circuit.

The limiting factor of the new circuit is the supplying buss which is part of the fuse box and you have no control over but for all practical purposes is not a problem. And a factor to consider is the size of the wire you will be running. Adding a 30 amp fuse with a wire the thickness of a hair will not give you the protection you are looking for.

 

[Tank Montana]

I thunk your plan will be OK. You're only drawing an extra coupe amps at the most which a 30A circuit fuse should have the capacity for.

Phil

Maybe I'm confused here but wouldn't the circuit be independent of the 30 amp fuse? From what I've read (and show in the image below), the original fuse is not really changed if you put it in the bottom slot and put the add-a-fuse the correct way around.

One time I had to search for a power source controlled by the ignition to install an accessory in a '07 Ford Fiesta and I found out that some of those empty fuse holders had what I needed. I checked which of the terminals had 12V and soldered a wire with it's own fuse on one side of a blown fuse, just to have something that fitted correctly in the holder to avoid bad connections.
I don't know what make your car is but it might be worth a shot.

I looked at the empty holders and they do seem half wired, where there is a "hot" side, but the other side (which would go to some device in the car) is not. This makes sense I guess because those get added as needed I assume, but is tapping the unused fuses safe without the full wiring?

You need to understand just.. how.. the add a fuse thing works. Installed properly.. the existing circuit is the same, as the same fuse goes into that slot that the add a fuse thing replaces. The additional fuse supplies power to the new circuit and is totally independent of the original. Providing.. the orientation of the add a fuse is right. the simple test is if it supplies power when.. the original fuse is NOT installed. This also proves the independence of the second circuit.

The limiting factor of the new circuit is the supplying buss which is part of the fuse box and you have no control over but for all practical purposes is not a problem. And a factor to consider is the size of the wire you will be running. Adding a 30 amp fuse with a wire the thickness of a hair will not give you the protection you are looking for.

I have tested the fuse slots with a multimeter to determine which side is the "hot" side (bottom in my case), which I am then aligning so it plugs into the left pin of the add-a-fuse, like below:

Not too sure what wire you mean though? I am using the Thinkware hardwire kit and an add-a-fuse adapter I got off Amazon, so these would be the only wires I'm adding.

Just so I'm getting the right idea, where the add-a-fuse is rated 20 amp, this then means for the new fuse, or a total of 20 amps shared between the two fuses?

 

[Ralph2]

I could not find quickly that image.. but it is a good one to show the path of electricity. When the prong (shown at the left with 12 v arrow) is connected to the live side of the buss.. ALL the power will go through that leg and feed both fuses. whatever size they are. I am not familiar with "rating" the actual add-a-fuse thing but would suspect it would be 20 amps for each. One could easily add-a-fuse thing to an existing 30 amp circuit and thus it would be overloaded before you even added anything to the second side.
All this is academic.. with the amount of power that your dash cam is going to draw..

 

[tabetha]

You can indeed "populate" the empty FUSE bays/slots by simply inserting from behind a correct fuse box terminal, or use the add a fuse taps you have, the cam F770 when I measured it drew 356ma, 1/3 of an amp roughly so adding to a 30 amp circuit will cause no issues at all the serial bus that supplies the power "behind the scenes" won't even notice.

 

[Module 79L]

I looked at the empty holders and they do seem half wired, where there is a "hot" side, but the other side (which would go to some device in the car) is not. This makes sense I guess because those get added as needed I assume, but is tapping the unused fuses safe without the full wiring?

First let me explain that ALL fuse holders obviously only have one "hot" side when the fuse is not there. This is basic electricity. Second, it is safe. You would be using it only as as power source, so all you need is that "hot" terminal, regardless if there's wiring or not connected to the other fuse holder's terminal.

I only suggested this because one of the unused fuse holders could have switched power, meaning you would avoid the "hassle" of using an existing circuit.

 

[Module 79L]

Just so I'm getting the right idea, where the add-a-fuse is rated 20 amp, this then means for the new fuse, or a total of 20 amps shared between the two fuses?

Add-a-fuses don't have any rating, it's the fuses you put there that give the protection rating to the respective circuit. The original fuse receives the 12V from the "hot" end of the add-a-fuse and lets it through to the other end, whereas the extra fuse receives the 12V from the same point and lets them run to the wire that's coming out of the add-a-fuse. You put the original 30A fuse in the lower holder and the new fuse (1A is more than enough, I don't think those types of fuses have ratings of less than that) goes into the top holder. The original circuit will be protected by the original fuse and the hardwire kit will be protected by the new fuse. Simple as this.

 

[SawMaster]

Maybe I'm confused here but wouldn't the circuit be independent of the 30 amp fuse? From what I've read (and show in the image below), the original fuse is not really changed if you put it in the bottom slot and put the add-a-fuse the correct way around.

Yes and no.
Yes, the added circuit when installed correctly is not drawing through the existing fuse. But no, there is still a connection here.

The existing fuses are powered by a 'bus' or wire(s) which are rated to carry the designed load of the fuse rating. In other words, a wire feeding a 30A fuse will sustain at least a 30A load- therefore all is safe when 31A comes along as the fuse will blow before the wire feeding it dies. And standard engineering practice on fused circuits is to allow at least 10% more fuse rating than the maximum expected load. Often 20%+ more will be allowed. This is done so the fuse doesn't blow under adverse operating conditions, but only during a serious overload or short.

Therefore in a worst-case scenario, a 'bus' or wire feeding a 30A fused circuit will be capable of handling at least 3A more draw. And most of the time this would be 6A more. So you can safely tap something like a dashcam in there without any worries at all

Phil

 

[Module 79L]

Therefore in a worst-case scenario, a 'bus' or wire feeding a 30A fused circuit will be capable of handling at least 3A more draw. And most of the time this would be 6A more. So you can safely tap something like a dashcam in there without any worries at all

Phil

That's why I suggested using one of the unused fuse holders, as long as the car as a switched one. At least there won't be any more things drawing power from it except for the dashcam.

 

[SawMaster]

If you draw power from an unused fuse slot for your dashcam you OK since so little power is needed. Just be careful that you don't exceed that slot's fuse rating if you're adding larger items The owner's manual should show those if it's not marked at the fuse panel

Phil

 

[Tank Montana]

Firstly, thanks to all of you for replying, really is much appreciated and has educated me a lot in this matter, as well as making me a lot more confident in what I was doing was actually mostly correct.

Secondly, I spoke to the supplier of the add-a-fuse taps and he told me that the ratings in their description are for the additional fuse, meaning the new fuse can be a maximum of 20 amps. He also mentioned that this rating related to the wire that the new fuse feeds your device through, based on the gauge the manufacturer has used. To add, he said that it can actually take "much more" than the stated 20 amp, but for safety they state that as the figure.

As many of you have stated about the empty fuse slot option and that it would be a better route to go down, I will check this weekend if any of those are fit for purpose. I am assuming that I can still use the add-a-fuse and just use only the top slot with the 3 amp fuse, leaving the bottom (original) fuse slot empty?

 

[Ralph2]

Using an empty slot with an add-a-fuse would work.. providing that underlying buss provides the power you are looking for.. as in ACC or always on. Using an existing named fuse location sometimes avoids that guessing.
The trouble with the add-a-fuse thing is that the fuse box cover will not likely still fit.