Fuses for 10 mm wiring?

[dave friday]

Morning all,I have a 30 amp on the leisure battery,what size fuse (30?) should I use for the feed to the split charge relay? And what size fuse for the fridge wiring (30?)
I did have a 2 volt drop on the fridge wiring !
Ta.

[itchyfeet]

yes 30 A fuse max for a 30A relay
wire size needs to be big enough for 30A too but 10mm2 is well over that, usually wire is increased over minimum because you want to reduce voltdrop in a system thats on only 12V, if it were mains you would not care of you lost a few volts, but with 12V every little bit helps.

[dave friday]

Thanks Itchy,I was thinking of measuring the voltage drop across the fuse and then make a decision wether to put another 30 amp fuse in parallel ( making 60 amps )
Kr.

[itchyfeet]

no one fuse per circuit, never parallel fuses.

volt drop is seen over the wire because it has a resitance and at 30A it is significant.

Volt drop is current in amps X resistance of cable in ohms,
10mm2 will be fine but if you used 4mm2 and had a 5 meters of cable you could loose 1V ( assuming return is through body and low resistance)

[dave friday]

Why not parallel fuses(the electricity board do,although they take the 240 from two different points).
But I do see that paralleling fuses could cause problems and be dangerous!
Thanks for your help.

[itchyfeet]

Why not parallel fuses(the electricity board do,although they take the 240 from two different points).

dunno what you mean?

a fuse should blow when current exceeds the rating, have two fuses in parallel and they must each be half the value i.e. for a 30 A circuit 2x 15A, one may age and blow first what the point, no benefit , you then have a 15 A circuit left.

[tobydog]

How much current does the fridge draw? Or put another way how many watts is it rated at?

Fuses ain't parralled

[dave friday]

A house I worked in had problems with the mains voltage dropping to 190volts,the bloke from the electricity board replaced a fuse ( else ware ) that cured the voltage drop,so almost two fuses in parallel !
I know a bit of a red herring!!
The fridge draws 100watts ( 8.5 amps ish )
Anyway I'll probably one feed to the fridge and one to the leisure battery.

[itchyfeet]

A house I worked in had problems with the mains voltage dropping to 190volts,the bloke from the electricity board replaced a fuse ( else ware ) that cured the voltage drop,so almost two fuses in parallel !
I know a bit of a red herring!!.

yes red herring.

[California Dreamin]

Going back to the original question, quote: Fuses for 10mm wiring and your first statement: I have 30 amp on the leisure battery ...I see nothing indicating the load capability of your split charge relay?

30 amp fuses tend to be commonly used in split charge systems but that means everything else: relays,connectors and wiring, must be rated substantially higher (45 amps or higher) so the fuse does its job and blows before damage occurs elsewhere.

As someone else pointed out: Absorption fridges in camper vans typically draw around 7 - 9 amps so fusing at 10 amps would be too close, a 15 amp fuse for the fridge would be better.

The 10 mm2 wire is typically rated at 70 amps so that's good, however standard 6.3mm male/female connectors and standard fuse holders are rated around 30 amps. Your split charge 'setup' is only as good as its weakest point and with that in mind I would advise you avoid using these standard sized fuse holders and connections for the main wiring (battery to battery) .
You should be using something like this:

http://www.autoelectricsupplies.co.uk/p ... ategory/32" onclick="window.open(this.href);return false;

http://www.autoelectricsupplies.co.uk/p ... ategory/62" onclick="window.open(this.href);return false;

http://www.autoelectricsupplies.co.uk/product/291" onclick="window.open(this.href);return false;

http://www.autoelectricsupplies.co.uk/p ... ategory/36" onclick="window.open(this.href);return false;

Or dedicated relay with isolated fridge feed like this:

http://www.autoelectricsupplies.co.uk/p ... tegory/149" onclick="window.open(this.href);return false;

Martin

[dave friday]

Hi Martin,and thanks that's just the info and links I need.
I've ordered two 100 amp relays and a screw type fuse box.
The existing fridge and leisure battery wiring looks like speaker wire ( installed by " leisuredrive")

[irishkeet]

great advice Martin, will revisit this when Im at the interior stage

[California Dreamin]

Just avoid getting too 'carried away', not that there is any harm over rating circuits, just bare in mind the highest current flow on the leisure side is likely going to be less than a quarter of that seen on a starter circuit.
I think the issue here is an assumption to treat them both the same (big thick earth & live feeds similar to the starter side) and whilst minimizing voltage drop is essential to ensure the leisure battery reaches full charge, you need to remember that during the final 30% or so (70 - 100%) charge, the split charge circuit will be carrying only 10 - 15 amps if that, voltage drop over a 1 - 2 metre 70 amp cable at that rate is negligible.
Most OEM split charge circuits I've seen have typically used 35 amp wire (thereabouts) on the main battery to battery connections and whilst I accept that is a little weak, it gives you an idea of what they thought was necessary.
In reality, I believe with around 80 - 120 amps of leisure batteries, the 70 amp cable you are using is about right.


Martin

[dave friday]

Yes your right about getting carried away,I want to get the maximum voltage to the fridge ( element cold resistance of 1.7 ohms ).
I don't like the push on crimp tags and couldn't find 10 mm of good quality here in Spain.
The leisure battery is less of a problem,a 20 watt solar panel tops that up.
The Suns just come out so I'm off outside!

[irishkeet]

is there a special tool requires for these bigger terminals?
http://www.autoelectricsupplies.co.uk/p ... ategory/32