Earth wire rating from secondary fuse box

[fooster]

Having fitted a ripca 12 point fuse box as a secondary fuse box for my leisure battery and wired it up (Ti the leisure battery with 10mm 70amp cable) I am now need to wire the negative earth of the fuse box. I am a little stumped as to the size of wire I need for this. The stock westy leisure negative cable from the battery to the chassis is much thinner than the positive cable. Can I use a 30amp cable as my negative from the fuse box to the chassis, or does it have to be the same rating as the positive from the battery to the fuse box?

Slightly confused

[MidLifeCrisis]

The earth wire should be of a sufficient gauge to handle the current in the same way as the positive wires are.
So, if you have used a 70amp wire for the pos to the fuse box then I would expect a 70amp wire to be used for the ground.

However, will you be running all the grounds back to the fuse box or will you be grounding the items running off the leisure battery to the chassis. If you are grounding items to the chassis then I would question whether you need the negative at the fuse box?

(Note - you'll obviously still need to ground the lesiure battery to the chassis with the 70amp wire.)

[bigherb]

I'm confused as well, why do you need an earth to the fuse box?

[fooster]

The earth wire should be of a sufficient gauge to handle the current in the same way as the positive wires are.
So, if you have used a 70amp wire for the pos to the fuse box then I would expect a 70amp wire to be used for the ground.

However, will you be running all the grounds back to the fuse box or will you be grounding the items running off the leisure battery to the chassis. If you are grounding items to the chassis then I would question whether you need the negative at the fuse box?

(Note - you'll obviously still need to ground the lesiure battery to the chassis with the 70amp wire.)

[/quote]

That's the thing- I am looking to run the grounds back from assorted led light installations, USB sockets and the submersible pump- its not a whole heap of stuff really and not everything that us running off the fuse box will have grounds running back. It's more about convenience and accessibility really- I can run a 2 core 17amp wire out from the ground terminal to a central location which can act as a grounding 'hub' for the LEDs and USB sockets. That way I don't have to screw into the chassis in awkward places. Does it matter if I'm only grounding 15amps worth of stuff through the fuse ground terminal? If not then can I get away with a 37 amp wire for the purposes of grounding?

Maybe I should just go with a 70 amp... Just trying to avoid endlessly over speccing when it's not necessary but obv still be safe.

[fooster]

I'm confused as well, why do you need an earth to the fuse box?

I think its my rubbish wording- I meant ground- its a secondary Ripca 12 point fusebox- I was just trying ti avoid grounding everything to chassis and keep it tidy. My knowledge/understanding is somewhat basic as you've prob gathered

[bigherb]

If you want to run the earths back to the fusebox just connect them all to the body near the fuse box or if it is near the battery just run them to the battery negative terminal.

[Mocki]

It is much more efficient to use chassis ground, and better for reducing interference too, but which ever way you choose, make the grounding point big and clean to avoid problems in the long term .

[fooster]

Brill, thanks for all your replies much appreciated by a 12 v learner driver!

[California Dreamin]

As usual...I turn up and toss a granade in the mix.

I think we are all forgetting that this is the leisure circuit and looking at the starter battery and it's massive cables and making an assumption that the sizes of cables should be similar.
Doh!
Forgive me for pointing out the obvious but on the coldest of mornings the starter battery may have to supply up to 200 amps to the starter (Diesel engine).
The demants on the leisure battery are far less dramatic:
1amp to each USB connector
5 amps if ALL the interior camping lights are turned on.
A few amps to the CD radio.
A couple of amps to the blow heater.
3 - 4 amps to the cold water pump.
12 volt TV? 8 amps perhaps.
Turn everything on...and I mean everything...and you are probably talking less than 25 amps?

Thats why the earth on the leisure battery looks so thin in comparison to the starter battery.
Personally I would earth locally to the socket or consumer (remember that your dash is METAL so earthing is easy in this area)
And use a 35 - 40amp main feed to the fuse box.

In terms of leisure battery earth and feed wires...again based on common split charge factory setups and my own observations/tests on leisure battery current draw from flat (typical one leisure setup), peak is typically 30 - 35 amps but this quickly falls into the mid 20's. This would suggest 50 - 60 amp cable is plenty enough (mines got a 90ah alternator BTW).



Martin