Interesting thread, what size cable does the jayco come "standard with". I have a 7pin flat on the tug as was told to get the 12 pin to run the 12v for the fridge. Can you option a bigger cable? say 8 B S which is i think 10mm2 through an anderson plug or is that an at home job to do. I am currently designing the dual battery system for my ute which will be in the tray with a CTEK dc to dc, so the run to the anderson plug on the towbar from the battery will only be 1m at most then whatever it is to the fridge. Rather get it right the first time than chase around an under performing fridge (150l) Also does this 12v cable just run straight to the fridge? or does it charge the in house battery as well.
cheers
matt
This is taken from an article by Collyn Rivers from the Campertrailers.org web site,
Auto Cable – a Trap!
Unfortunately there’s a huge trap. Automobile cable is commonly sold using a rating system that appears to correspond (but doesn’t) with appliance manufacturers’ cabling recommendations. Nor does it, nor indeed can it, correspond with other cable size standards.
Auto cable is that cable invariably sold by auto parts stores, auto-electricians and many hardware stores. This cable is rated in terms of
its outside diameter – insulation and all. In other words all that measurement tells you is the size hole it can be passed through!
But most appliance manufacturers and virtually all electrical trades specify cable either directly in terms of the cross-sectional area (in sq.mm) of its copper conductor - or by a numbering system (such as AWG) that relates to conductor area.
A typically recommended cable size is 2.5 mm.sq. Generally, the appliance maker will spell this out in full (eg. 2.5 sq.mm) – but not invariably. You can however be certain that if the appliance maker talks about 2.5 mm cable they refer to 2.5 sq mm cross-sectional area.
But when a buyer fronts up to the parts store and asks for the specified 2.50 mm cable what he will almost certainly be sold is not 2.50
mm.sq. cable, but 3.00 mm
diameter auto cable (the closest made to 2.5 sq mm). A 3.0 mm diameter auto cable’s conductor area however is typically a tad over 1.0 sq mm – about one third of the required size
If such cable is used for example for fridge wiring, the voltage drop across it will be close to three times that specified as the maximum by the appliance maker!
Unless that cable is exceptionally short, that fridge, which may be a top brand in perfect working order, will never work satisfactorily. This is the main reason why so many 12-volt fridges disappoint, and why trailer batteries are often grossly undercharged. Yet countless (in my experience,
most) trailers and motorhomes are, at least in part, wired this way! And that includes professional installations.
A Better Way
As mentioned previously is also very easy to work out the voltage drop for any given length of cable, current flow, and cable size.
I always use this rather than published tables (including my own) because once remembered it’s so easy to do – and it gives the right answer every time. The formula is simply:
Voltage drop equals (cable length (in metres) X current (in amps) X 0.017) divided by cable cross-section in mm.sq.
For example: 10 metres X 5 amps X 0.017 = 0.85. Divided by (say) 2.5 (sq mm), the voltage drop is 0.34 volt. This is just acceptable – but (here) 4.0 sq mm would be even better. That results in a drop of 0.212 volts.
Cable size tables and the above formula is for the voltage drop across a
single conductor path. Where, as is common with trailers, there’s twin conductors (one positive, one negative) the
total conductor length must be taken into account. In other words, if there’s a five metre run using twin cable, that’s 10 metres to be taken into account.
Hope this helps
Dave
