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Requesting Help with Fusing my Car's ACC Socket Modification

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ArcWindsor

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Hello, its been a while since I was on this forum. Its good to be back.

I want to rewire one of the ACC sockets in my car (I have 2) to be constantly hot instead of only with ACC/ON like it comes stock. I accept the battery drain risk because I won't forget to unplug something.

Background: Below is the ACC socket circuit diagram for my car. My plan is simply to jump the relay. This way, the load is still drawn from the same source as it normally is- just no switching.

Notice underdash fuse#9 (15A) supplies the sockets. I plan to modify it like the below diagram. Only one socket will be rewired to be constant, and the other one will stay with ACC/ON. I would just be jumping the relay by cutting the wire and routing it directly to fuse#9 with an add-a-circuit.
**broken link removed**

My problem is the fusing. I understand that the add-a-circuit causes a single fuse slot to turn into two in parallel, and from what I understand, fuses in parallel actually increase the total amount of current possible without blowing it. So I'm worried about overdraw.

Without modification, if the sum of the two sockets' current exceeds 15A, then it'll blow. But if I use an add-a-circuit with two 15A fuses in parallel, then the sum of the two sockets would then have to exceed 30A for the fuse to blow. This sounds unsafe to me. If each socket rides at, let's say, 14.5A, then I'm going to have 29A running without either fuse blowing, right? I'm not an expert, so if anything is wrong I say, then please correct me.

Since each separate socket's line will be fused okay itself, I'm mostly worried about the line between the 15A and 40A fuses. Am I protected with two 15A fuses in the add-a-circuit? Or should I use two 7.5A fuses in the add-a-circuit? Using the 7.5A fuses will decrease the max capacity of the sockets, but who actually draws that much from the ACC socket with a phone, ipod, or aftermarket GPS anyway?

Thanks for your consideration.
 
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Fuses are sized based only on wire guage to limit the duration an over-current flows and to prevent a fire if a wire insulation rubs through to ground; they have NOTHING to do with protecting the LOAD!!!
 
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Thanks MikeMI for the prompt reply.

I understand, but that's what I'm worried about- overdrawing the wire between the 15A and 40A fuses. With the 15A add-a-circuit fuses in parallel, this line could get more current than its supposed to. I understand the 40A fuse is protecting that line from overdraw, but I just want to run this past you guys here.

Should I use smaller fuses in the add-a-circuit- or should I just use two 15A fuses in the add-a-circuit? Safety from fire/damages is my priority as of now.

Thanks again for the reply.

Also, fuses protect from a load overdrawing too (if its too big for the wire), right? So the normal current from a larger-than-normal load can cause fire/damages too, right? Correct me if I'm wrong.
 
What is the wire gauge?
 
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With 15A fuses, I would certainly use 16AWG or thicker. I'm basing this on a chart I seen, so if you think I should use thicker than 16, let me know. The stock wires will remain whatever they are since the 15A fuse was obviously selected based on that gauge by the manufacturer.

Edit: sorry, do you mean the gauge of the line between the fuses? I don't know that guage, but its definitely pretty low. Let's assume its just thick enough for a 40A fuse.
 
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Using typical automotive thermoplastic wire, I'd go with 16AWG, and a 15A fuse. Do not upsize the 40A fuse.
 
I don't see why you need to change the fuses at all. If you put the connection for the permanently live socket after the fuse, you can leave the fuses as they are. The only problem is that the an overload on one socket will stop both working, but you have that situation now.
 
1. You don't have any fuses in parallel in either schematic. Each end of the new fuse would have to be connected to an end of the old one.

2. If you make the modification to the block with one of those clip things, you COULD overload the tab. If there are 30 A fuses in the block, then you should not have an issue.
 
Originally, the combined maximum is 15A for both sockets. After modification, I will have two 15A fused circuits in parallel instead of one 15A fuse before both circuits, so its possible for me to draw 15A more from the source than I originally could without blowing either of the 15A fuses. This is what I'm worried about.

Originally, before modification, if I drew a combined total of just under 30A through fuse#9, then it would blow. But after modification, drawing just under 15A from each socket simultaneously (just under 30A combined total) would not blow either of the fuses. Am I right? This is what I'm worried about.

So if I put two 15A fuses in the green block on my schematic (in the add-a-circuit fuse-tapper), then is there any chance of overdrawing anything at all because of what I said above?
 
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There would be a chance of overloading the circuit with two fuses. What I don't understand is why you want to add an extra fuse.

My suggestion is attached.
 

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That's what I originally wanted to do, but that segment you connected to in the diagram is not accessible. Its part of the internal circuitry of the fusebox. The relay socket cavities and fuse slot are built into the fuse box. The only way to tap fuse#9 (before the relay) is with the add-a-circuit fuse-tapper.
 
Fuse #15, the 40 A fuse, is there to protect the cabling that feeds the two lower "other stuff" feeds. It should therefore be fine to take up to 40 A from that point.

Of course, lower value fuses won't hurt unless you are running something large.

The 10 or 15 A feed to accessory sockets is almost never run at full capacity.

What do you want to connect to the permanent power?
 
It would be for charging phones, ipods, Garmin GPS unit... nothing unusual, and it wouldn't be too often at all anyway. Just in case this is ever convenient one day, I'll be able to do it. I don't know for sure, but I think all this stuff draws tiny amounts less than 1A in order to charge.

Would any of the common peripheral devices, like what I named above, be a problem if I just went ahead and used a 7.5A fuse? I doubt they draw even close to that.
 
Most of those are USB chargable items. The currents re limited to 1 A max at 5 V or 5 Watts. Laptop power supplies could get to be say 120 W or 10 A. The Fuse tapper will probably be OK. 14 AWG for 15 Amps.

In theory, your A fuse could be at risk of blowing.

I've plugged in USB charger for days on an "Always ON" outlet. My car requires the ACC to be on all the time. Sometimes I don't like that method.

In my older vehicle, I ran an outlet to the trunk that was always on to charge my laptop that was located in the trunk.
 
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It would be for charging phones, ipods, Garmin GPS unit... nothing unusual, and it wouldn't be too often at all anyway. Just in case this is ever convenient one day, I'll be able to do it. I don't know for sure, but I think all this stuff draws tiny amounts less than 1A in order to charge.

Would any of the common peripheral devices, like what I named above, be a problem if I just went ahead and used a 7.5A fuse? I doubt they draw even close to that.

If you are only needing to take a low current, then having a 7.5 A fuse for each socket gives you a 15 A total as before, so that will be fine.
 
Hi ArcWindsor,

what you said the 15A fuses in parallel is not correct! because the fuse is connected to each load seperately! the 40A fuse in the underhood fuse box might be easier be burned if all loads' current are over 40A.

David
 
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