Blade Fuse Calculation for 12v Auxiliary System

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Blade Fuse Calculation for 12v Auxiliary System

Post by NoPressure » 10 May 2019 12:42

This is a repost from another thread, as I didn't really get an answer.

I do think this is something rather important to get right. Hopefully it's helpful to all those who like to DIY.

Camping with friends the other weekend, they suddenly had their vehicle pouring smoke out as we were relaxing under the shade. It made me re-assess how my 12v setup is fused. (I had done it many years ago - which might be a problem as I know very little about the finer points of 12v systems).

My question is as follows- how do I go about calculating the right amp fuse to use? 

If I recall correctly you want the normal constant draw to be ~80% of the fuse rating, but this should obviously be less than the amp rating of the electrical wire? Is this seen from the gauge code on the wire?

Does this then for example rate to a 100amp fuse for your alternator cable to the aux battery?

What about the trickle charger - it fluctuates in voltage delivery - or do you just use an 20% greater fuse than the max amp output?

Thanks in advance to the Sparkies

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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by Peter Connan » 10 May 2019 14:27

Not a sparky, but I suspect you need to determine the power usage of whatever loads you have, pick a correct fuse for that load (maybe 50% greater than startup load) and then ensure that the wire (and other components) are better than that.

But that means you have to have a pretty good idea what each wire/outlet is meant to do...

I guess the most important bit of kit we all have is a fridge. They practically all draw around 5A at start-up. So I use a 10A fuse will do for that circuit (I don't think one gets a 7.5A?).
Most of the more complicated equipment (such as inverters and chargers) should have a sticker on the bottom detailing what power they consume, thus it shouldn't be too difficult to figure out what fuses to put on dedicated lines.

An auxhiliary point used for multiple purposes is a bit more tricky. But i think most of them are used for smallish loads (GPS's, phone chargers, camera battery chargers), so a 5A fuse should do?

As for the charging system, I have been using 50A fuses, but bolt-down ones, not blade ones. I know you get up to 50A blade fuses, but I don't think the connections have enough surface area to carry that much load. My reason is that my compressor had such a fuse, and the holder melted before the fuse blew. So I have put a bolt-down fuse in my compressor's lead as well.

The exception to the above is if you have one of those dual battery systems which allow you to "bridge" the system so as to start with a flat main battery, or for winching. Then I don't think you can have a fuse in that line. Which is why I don't like such systems...

Hope that helps a little?
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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by NoPressure » 11 May 2019 08:16

Thanks Peter, helpful as always. This is more-or-less what I had done. Although my concern is that our friends had their trickle charger directly connected to the aux battery - it is rated at giving max 4.9amps (it's from a brand I don't know). He had an 10Amp blade fuse in-line. The wires melted and almost set the car alight (imagine it was night and we were all asleep).

So if I take it that the fuse shouldn't get a constant draw that is higher than 80% of it's fuse rating then he theoretically needed an 6.5amp fuse, but you don't get that, so he got an 10amp fuse. This however as seen was greater than what the unit and wires could take. If he however had put in an 5amp fuse, that would of protected both, but you run the risk of it constantly blowing (leaving your battery uncharged).

I obviously advised him to get a better known brand charger...but still, it was an eye opener. I have never really looked at a 12v product and on purchase considered how accurate you would be able to fuse it. I usually just get the best/most/biggest/reliable for the price.

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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by Peter Connan » 11 May 2019 12:08

I guess fuses can't solve every problem. Sounds like in this case either the charger's wiring was not up to the job or perhaps had been damaged.

I may once have nearly had my car burn out as well, when a wire for an LED strip-light got pinched in such a way that it created a short. It was laid under a carpet, and burnt a black line in the carpet before the 5A fuse blew...
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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by mvcoller » 11 May 2019 22:24

I follow the lead of the suppliers. National Luna are experts in the field of dual battery charging systems and they make specialised fuse holders for 50 and 100 amp fuses. They also supply these two aperage fuses.

I therefore use these (100 amps between the main battery and the solenoid and 50 amp at each of the auxiliary batteries) on the feed cable (25sq mm) between the main battery and the solenoid and then another fuse between the solenoid and auxiliary battery. I fit this fuse no more than 200m away from the battery.

I have two aux batteries in my Patrol, so the same applies for the second battery. These fuses are sufficient for the alternator to charge the batteries up to and even above 50 amps, yet is still able to pop in case of a dead short anywhere along the route of the thick supply cable.

The only of my 12v devices that is plugged directly into that high amperage supply line is the National Luna freezer. That is fused (20 amp iirc) on the electrical panel of the freezer, protecting it from a short in the supply line and protects the supply line in the event of a short in the freezer.

To run all other devices, I have a seperate supply box near the rear auxiliary battery. This box is not fused as a unit, but inside this box I have two cigarette lighter plugs, one Hella plug and two 35a Mini Harisson Plugs fitted. Each of these are fused individually, based on what I am likely to plug into the specific plug. I have therefore a 10 amp fuse for each of the cigarette lighter plugs, a 15 amp fuse on the Hella plug and a 20 amp on each of the Harisson plugs.

The male plugs are fitted to my devices according to the amps they need to operate and according to where I plan for it to be plugged into.

Most chargers ( phones and cameras) and my in-tent fan, I run off the cigarette lighter plus. My 300w 220v inverter which I use for charging my laptop and my hand held Bosvark two way radios (up to 4 at a time) is also done from these cigarette lighter plugs.

Currently I have no use for the Hella plug.

The only units that I plug into the Harisson plugs are the solar panel and the 10a Battery Charger to top up my auxiliary batteries at campsites where AC power is available.

What must not be forgotten, is that each device that is connected and is drawing current should also be appropriatly (correct amperage) fused individually, along its own supply cord if possible or feasible.

I managed to get a Bush Power battery charger that will work with an AC supply as low as 90v AC. I have in the past run into trouble where my older battery chargers would not work with the 180v that was available where we were camping in Zinbabwe and the 175v that was on offer on another occasion in Zambia.

This is quite important, as your Danfoss compressor run fridge will not run on AC lower than about 200v. The ability to charge your auxiliary batteries at times like this, ensures that you can run it off 12v from your batteries.

PS
I always have jumper leads in my vehicle (stored in the engine bay) which I can use to jump start from my front auxiliary battery.

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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by Tinus lotz » 12 May 2019 07:41

The thing to watch the most for on this is volt drop . On 12v dc we use even 6mn wire for a fridge that runs at 2.5 amps but we have 1% volt drop then . Rule of thumb is to make sure all source off ignitino is fused ....cable from alternator and second battery ...it can feed back from the back to the front when there is a short in the supply cables .
There is a lot of diff opinions out there but you cannot charge a battery properly with a solenoid system . You might get it to 70% but not easy .
The modern dc to dc system controol the charge voltage and thats what makes it work. Modern batteries needs 14.8 to charge and your alternator is lokking at your crank battery that is already full and then the regulator drops the voltage to 13.4 v or even lower. Anyway charge wires on these needs about 30a fuse becouse they work o 20a normally 6mm to help with volt drop . Ave 10a socket can have a 10a fuse otherwise they melt .
More important is to make sure all positive sources of energy has a fuse that allow the working amps plus 35% and protect sockets with its rated current .
Wire max amps stays the same for ac and dc the voltdrop just becomes to great on dc
1.5mm 17 a
2.5mm 27 a
4mm 37a
6mm 45a
10mm 57a
16mm 84a
25mm 101a
This is the current carry capacity of the wire without overheating to much . A person must take into consideration that a wire running under a carpet is going to overheat much easier then one open on roof rack .
Also tipe of insulation makes a big difference on rubbing holes in it and taking a beating . Silicone is great for automotive use but split tube makes most wire o k .

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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by NoPressure » 12 May 2019 08:16

Great thanks Tinus

Would you reccomend replacing all wires that are less than 6mm (6mm2 ?) off any device (chargers etc) with 6mm wires? I would rather that any device go kaput than the wires setting something alight.

It will probably also be prudent to put a fuse in under my solar panels on the roof of the vehicle while I'm busy.

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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by Anthony Forgey » 12 May 2019 10:19

load current X 1.35 then choose a fuse which has the closest higher rating

eg. if the fridge draws 6 amps....
6 X 1.35 = 8.1
You don't get an 8.1 amp fuse so you go to the closest higher fuse rating which would be 10 amps.

caveat: your wire size must have a higher current rating than the fuse (if that wasn't obvious :lol: ) use Tinus' chart as a guide
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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by Tinus lotz » 12 May 2019 11:42

NoPressure wrote:
12 May 2019 08:16
Great thanks Tinus

Would you reccomend replacing all wires that are less than 6mm (6mm2 ?) off any device (chargers etc) with 6mm wires? I would rather that any device go kaput than the wires setting something alight.

It will probably also be prudent to put a fuse in under my solar panels on the roof of the vehicle while I'm busy.
No not at all a usb plug pulls like less than 1a on 12v so 1mm or so is more than good for it . I think if its a 3m long wire and its on a 10a socket try stick to at least 1.5mm wire if it goes past 2meters always go 1 or two sizes up . If you have 3 plugs at the back put a 6mm with 35 a to a point and then 10a in 1.5mm to each point fused separately .
:imsorry: Biggest problems that i have seen is high resistance faults .
Like led strips going up in flames but doesnt blow a 15a fuse ect ....try buy good quality stuff and if it pulls 1 amp and is permanent installed like led strips put a local 2amp inline by the strip

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Re: Blade Fuse Calculation for 12v Auxiliary System

Post by mvcoller » 12 May 2019 12:16

I agree 100% with Tinus "try buy good quality stuff and if it pulls 1 amp and is permanent installed like led strips put a local 2 amp inline by the strip" I like to put those glass tube inline fuses (I like the 1, 2 and 5 amp fuses) on just about every device I plug into my battery system. Very few items need more than a 10 amp fuse. Wire size is also very important, Voltage drop on a DC system is a real issue, only overcome by thicker gauge wire, as Tinus also pointed out.

I however do not agree with his statement on solenoid charging. Yes, it is true on some older vehicles (maybe 2000 and older??), but not on the more modern vehicles. For instance, both my Patrol and my Pathfinder (and all modern vehicles) the alternators/voltage regulators of today run at 14.4 volts due to all the electric and electronic devices that they have to run. I have physically tested my Patrol, and the output at the Cole Heresee solenoid bounces between 14.3 and 14.4 volts. That is good enough to charge any battery 100%, and fairly fast too....

I had fitted a 40 amp DC to DC charger to the Patrol and I was convinced it was not performing as well as the previous Solenoid. in fact, on my Terrano this DC to DC unit sucked my main battery dry and I only realised this when the radio stopped playing and the headlamps resembled two sleepy glow worms (night trip). After disconnecting the DC to DC unit, all returned to normal!!

On the 4x4 Community forum I met a chap by the name of Dave Rickson who is a 12v electrics and electronics expert, and he came over to my place and measured my charging with his fancy gauges and measured my charging rate is initially at 30amps to the Aux batteries and later tapered off to first 20a, later to 10a and then to just 2 or 3 amps. He suggested I do not put the DC to DC charger back.

I questioned Peter Slement, Owner and Founder of National Luna about this (they manufacture both systems) and he told me that on modern vehicles, during the first 4 to 5 hours of a run, the solenoid outdoes the DC to DC charger in performance and only after about 8 hours of travelling does the DC to DC overtake the performance of the solenoid charging system. They have proven it many times in their test environment and on Peter's own vehicle when he goes travelling in Africa.

In an article in I think SA 4x4 Magazine, about a year ago, he actually recommended their Solenoid charging system over their DC to DC charging solution in most cases

Also see the post below regarding the advice given by the National Luna experts to members of SA 4x4 Community Forum

https://www.4x4community.co.za/forum/sh ... ost4142833

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