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When my towbar was wired in, the guy wired the voltage-sensing relay for battery charging/fridge into the boot by taking a feed off the wiring for the rest of the trailer electrics. This was a bad idea because when the engine starts the relay comes on, and the fridge draws enough current so that the voltage drop across the wiring TO the relay is enough to make it click off again, so it cycles continuously. The internet seems to agree that this can happen if you don't wire the relay close enough to the car supply i.e. at the fuse box.
So I need some more thick ish cable to handle enough current for the caravan fridge and the battery charging circuit, so I can wire the relay into the fuse box area with an appropriate fuse.
a) What size/spec cable for 12VDC and I guess 20A?
b) What name/terms should I then be searching for to buy online?
Get some amplifier cable/fuse form Halfords?
You need to know the length of run and what voltage drop you will be happy with to do the calculation for sizing. That or just buy the biggest you can justify. I'd be surprised if the fridge plus leisure battery charging is as low as 20A. You could look at finding a way of holding the relay open a little longer. Maybe an RC network
Cable is usually rated by it's cross sectional area in millimetres squared, but most suppliers will also list it by it's approximate current carrying capacity. Vehicle Wiring Products are a very helpful supplier:
http://www.vehicle-wiring-products.eu/section.php/198/1/single-core-pvc-thin-wall-cable
The actual current capacity depends on a lot of variables, the temperature, length, proximity to other cables, etc. But the easy way to deal with it is simply to get a cable much bigger than you actually need.
As nickjb says, are you sure all you need is 20A?
leisure batteries + fridge in a camper guidelines state should be at 110A / 16mm^2 to take account that your leisure battery may be flat and the alternator can put out 90-100 Amps.
from what i see folk dont do this but stick in a fuse as close to the source as possible that will burn out if the alternator tries to send max current to a flat leisure battery.
Equally for a vehcile application you need *flex* not house hold cable which goes brittle due to repeated flexing from vibration.
As nickjb says, are you sure all you need is 20A?
Well. It's not exactly clear.
The fridge alone is about 13A, so that's not a big deal. But the battery charging, I'm not actually sure. On the old 'van a 20A fuse blew immediately when I replaced it. So maybe that was the problem. I'd assumed that the caravan had a battery charger, that would feed the leisure battery with a sensible amount of current rather than simply connecting it straight to the car alternator.
Unfortunately, for this van I don't have the handbook so I've got no wiring diagrams.
"I’d assumed that the caravan had a battery charger, that would feed the leisure battery with a sensible amount of current rather than simply connecting it straight to the car alternator."
They do not .
the charger in the circuit inside the caravan unless you have something incredibly modern is 240-12v step down
The newer stuff has to use B2B chargers like a sargent or a ring or it upsets the canbus on modern cars.
So why on earth was there a 20A fuse in the battery charging circuit on my old van? That was a 1994 van.
because if there wasnt then it may have tried to draw 100amps and fried the cable - see my first post on the matter , the fuse wires are often rated so that anything over normal draw ie a flat battery blows the fuse rather than fires too much current down the wire , over heats and burns down the van.
It will be uncommon for anyone to have a 100amp cable from their battery to their caravan in a normal set up - can you even pass that through a 7 pin plug? , its likely the wire is more or less set up just for 20amp hence the 20amp fuse.
because if there wasnt then it may have tried to draw 100amps and fried the cable
I understand that, but it's pretty useless wiring up a battery charging circuit to blow fuses when the battery voltage is a bit too low. Surely there should be a better method of controlling the current?
there is but it costs more money - well there are a few ways but as i said the stick a fuse in is the cheapest way to achieve it safely especially in 1994. B2B is only really becoming common place now because Canbus systems drove it.
you could use a sargents B2B charger a Ring RCDS10 B2B charger
you could rewire the whole car to have an appropriate cable front to back (how would a van manufacturer be able to mitigate that you had done this when you buy it second hand and fry your cars wiring)
hence the 20 amp fuse . TBH a bit too low usually correlates with completely banjoed will never live again in a useful capacity battery where the car battery and the van battery then try to equalise instantly.....
Canbus? My car has canbus..? Or are you talking about something specific?
something specific - specifically canbus driven smart alternators that really dont like split charge systems.,
Ok, ta.
Since you are on here, Trailrat:
I bought some 30A cable. Am I right in thinking I need a single run from the fusebox up front (via the voltage sensing relay) down to the socket at the back - and then I can wire the earth to the nearest earth point at the back?