DC-DC Fuse help

Submitted: Sunday, Jan 15, 2017 at 09:40
ThreadID: 134105 Views:10524 Replies:9 FollowUps:6
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Hi had a fail in the wiring from my main battery to my dc-dc charger.The fuse cooked it self as i down graded the cabling around it from 6mm to 3mm .What i would like to do is place a breaker in its place what size should i use knowing the charger is 25amp.
cheers Warren
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Reply By: Erad - Sunday, Jan 15, 2017 at 09:54

Sunday, Jan 15, 2017 at 09:54
The fuse probably blew because your 3mm wire had become so hot that the insulation melted and the wire then shorted out, thereby blowing the fuse. You need a cable and fuse sized to suit the load being applied to the charger. Determine what the maximum load is, and get a fuse and wiring sized to suit.
AnswerID: 607571

Reply By: gbc - Sunday, Jan 15, 2017 at 09:56

Sunday, Jan 15, 2017 at 09:56
My 25a dc charger was blowing the 30a breaker that the trailer builder put in there. I read the instructions and upgraded to the recommended 50a and all is well. In those instructions is also cable section size charts for the length of cable run. Interesting reading.
AnswerID: 607572

Follow Up By: warren h1 - Sunday, Jan 15, 2017 at 10:14

Sunday, Jan 15, 2017 at 10:14
Thanks Gbc
DH me on the 3mm cable will keep 6mm away to breaker
FollowupID: 877271

Reply By: Member - abqaiq - Sunday, Jan 15, 2017 at 10:51

Sunday, Jan 15, 2017 at 10:51
Refer to a DC wire size table and remember the length to look at is the total (out and back) length, NOT one way. If possible do not use the vehicle body as the ground for charging loads. Run double wires (out and back, two core, etc). Also remember to fuse + side at both the source battery and the second battery. You have major power sources at both end of the circuit. Look for some wiring articles on the "Frig and Solar" website.
AnswerID: 607573

Follow Up By: Member - Dave63 - Monday, Jan 16, 2017 at 21:53

Monday, Jan 16, 2017 at 21:53
Don't skimp on your wire size for the charging circuits, best to buy 8 or 6 B&S depending on the length of the run. I can send you the formula to measure the voltage drop across any wire if you like but anything more that .2 of a volt is too much. As suggested by others spend the money on the wire on the charge side as it is well worth it.

Depending on your vehicle type, some newer vehicles with smart alternators require you to use the vehicle body for earth return and not direct wired (running earth from each accessory back direct to the battery). The reason for this is that the smart alternator will only produce what it needs, this saves the alternator producing power that is not needed, hence saves fuel. (How much is debatable or even worth while) How the alternator finds out what it needs to produce to replace what is used, is by measuring the current in the earth strap from the body to the earth terminal of the battery (actually at the battery).

Not all vehicles use this method but if it is, you must ground each accessory to the vehicles body for this to occur, because if you run a direct earth cable it by passes the earth return current detection. Therefore doesn't produce what you are actually using which can flatten your main battery. This could be from radios, driving lights etc, not just BCDC charges.

Older vehicles, I have always run a return earth direct to the battery as it much more reliable circuit to guarantee a good circuit but some newer cars are just getting to clever (problematic I would say). You just need to be careful which way you wire by the type of alternator you have.

Hope this helps and doesn't confuse the issue.

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Follow Up By: Allan B (Member, SunCoast) - Tuesday, Jan 17, 2017 at 13:53

Tuesday, Jan 17, 2017 at 13:53

It is still good practice to use a negative return wire, especially for longer runs (back of vehicle) or higher currents.
If such return wires are connected to the common earth stud, where the battery negative strap connects to the chassis, then the 'smart alternator' system is not compromised as the current returns to the battery via the sensing earth strap. What is important is to not return directly to the battery negative post.

Retuning current via the chassis or body introduces a higher resistance and less reliability into load circuits. Alright perhaps for small loads such as tail-lamps etc.

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Reply By: swampy - Sunday, Jan 15, 2017 at 11:41

Sunday, Jan 15, 2017 at 11:41
dc to dc 25amp at least 8mmsq cable
AnswerID: 607574

Reply By: Dean K3 - Sunday, Jan 15, 2017 at 12:00

Sunday, Jan 15, 2017 at 12:00
Need to upgrade wiring, as idea with my smart solenoid system (which by today's standards is obsolete) we used 16mm2 cable

I am yet to upgrade to a BCDC charger unit -some will say this is size is overkill but larger the core mm2 less resistance less voltage drop less heat produced (in theory)

Without knowing what brand of bcdc charger you have hard to give a exact answer but its installation booklet/technical section should have info you need

you can get manual rest and automatic reset circuit breakers with various ratings

try this narva link (hope it works ok) that will tell story in pictures than my rather vague rambling


will be other brands around think i got mine from altronics as they are a supplier to company i was working for. realistically they probably all made in same factory and then have brand/logo put on the delivery carton
AnswerID: 607575

Reply By: The Bantam - Sunday, Jan 15, 2017 at 13:10

Sunday, Jan 15, 2017 at 13:10
Firstly it must be understood that you can not afford to undersize cable in 12 volt systems. ... for a number of reasons.

Now when you are talking about 6mm and 3mm cable ....... is this autoelectrical sizes or real electrical sizes ( 3mm automotive is about 1.1mm2 (10 amp cable) 6mm automotive is about 4.5mm2 (good for about 40 amps in very short runs))...... even then if you are talking about propper 3mm2 cable you are pushing the friendship with a 25 amp circuit on a 12 volt system.

If we are talking 3mm automotive hell way too small ... if we are talking 6mm automotive ..... over the common distances in a car or 4wd I would not use it for more than a 15 amp circuit

My choice for this sort of work would be 6mm2 solar twin ... even then being careful of distance.

Now to fuses ... what sort of fuse and fuse holder where you using ...... if you are talking standard blade fuses .... the limit of the format is 30 amps, even then a lot of the fuse holders are not rated for 30 amps ...... so a 30 amp fuse may be fine, but if the holder is only rated for 20 amps ( typical of plastic fuse holders) .... there will likely be a problem.

Personally I won't use standard blade fuses over 20 amps, I will step up to a maxi-blade fuse, wafer fuse or a breaker.

to fuse selection ...... unless there is a specific requirement otherwise, we select fuses and breakers bassed on the cable size.

It is fairly safe to work on 10 amps per square millimeter as a current rating for cables in automotive applications ..... most car manufacturers push things way further than that.

So if you have a 6mm2 cable a 50 or 60 amp fuse or breaker will protect a 6mm2 cable quite adequately.

AnswerID: 607578

Follow Up By: warren h1 - Sunday, Jan 15, 2017 at 14:16

Sunday, Jan 15, 2017 at 14:16
Thanks Bantam
Look like i have 4.5mm2 before i ran off and rerun say 8mm2 the numbers are
Main Battery to dc dc 3m
Solar to dc 1.5m
Do you think the voltage loss would be say 20%
Protected by 50amp breakers and it is a projector dc-dc changer
FollowupID: 877276

Follow Up By: The Bantam - Sunday, Jan 15, 2017 at 15:34

Sunday, Jan 15, 2017 at 15:34
Without knowing the exact currents flowing I can not calculate voltage drop.

While the dc to dc charger might output 25 amps it may draw more ..... 6mm2 solar twin would probably be sufficient, but if you wanted to go heavier I would go 10mm2 solar twin.

At the moment solar twin seems to be the best bang for bucks copper for this purpose.

If you got 20% voltage drop you have a problem

FollowupID: 877280

Follow Up By: warren h1 - Sunday, Jan 15, 2017 at 15:45

Sunday, Jan 15, 2017 at 15:45
Sorry from dc charger to 2nd battery is only 300mm
FollowupID: 877281

Reply By: Frank P (NSW) - Sunday, Jan 15, 2017 at 15:19

Sunday, Jan 15, 2017 at 15:19

Just wondering why you downgraded cabling from 6mm to 3mm. Did you actually replace 6mm with 3mm, or have you added a 3mm circuit and are now having problems with that?


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AnswerID: 607585

Reply By: Batt's - Sunday, Jan 15, 2017 at 23:30

Sunday, Jan 15, 2017 at 23:30
Using a fuse would be a better option circuit breakers don't like heat under the bonnet especially on hot days as well. I tried one in my patrol it tripped out on a 38 deg day my neighbour had the same type it lasted 2 yrs in his BT50 then tripped out on a hot day recently running around town both vehicles are turbo diesel we both run fuses now.

Use the correct size wire and fuses the manufacturer recommends.
AnswerID: 607606

Reply By: Nomadic Navara - Monday, Jan 16, 2017 at 00:51

Monday, Jan 16, 2017 at 00:51
A 25 A DC-DC charger will draw around 29.5 A with an input of 13.5 V if you have heavy wire and your battery in down in a caravan. If you reduce the wire size to produce a voltage drop so as to only get 12 V into the charger you will need 33 A to drive it flat chat. Your thin wire will have an even greater voltage drop along it and you will feeding way less than 12 V into the charger, that will mean an even greater current is needed. Your charger input voltage could be 10 V (40 A needed) or even lower.

To get good results with a DC-DC charger you need cabling nearly as large as you do when attempting to charge your battery directly.
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AnswerID: 607607

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