battery cable size guage?

[stevelec]

hi guys , just helping my family convert a campervan and im strugling to get my head around what size battery cable size to use for batterys and to connecting equipment .

12volt side is all run in 1.5mm
240v is all in 2.5mm

there will be 3x 130AH AGM batterys connected in parallel (what size wire?)
this will then connect to a 40amp solar mppt and also to a dc dc charger and 20amp shore charger

estimated usage to about max 90amp a day but i may be way off. 12v fridge/roof fan/extractor/phonecharger/6 led lights/water pump

any ideas or point me in the correct direction of how to work this out

thanks

steve

 

[stevelec]

just made a quick mock up without the dc2dc charger and the 20amp 12volt charger

 

[The Wino]

I'm no electrician but I think in general you need much thicker wires at 12 v than 240v for a certain load.

 

[stevelec]

I'm no electrician but I think in general you need much thicker wires at 12 v than 240v for a certain load.

1.5mm should suffice for the lights/fridge/12volt sockets/fan/waterpump/diesel heater. cable instalation method would be rated to 16A. max fuse for those circuits will be 10A.

 

[Tombola]

1.5mm should suffice for the lights/fridge/12volt sockets/fan/waterpump/diesel heater. cable instalation method would be rated to 16A. max fuse for those circuits will be 10A.

in theory yes, but take a butchers at this may help more if extending
.

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[stevelec]

in theory yes, but take a butchers at this may help more if extending
.

Thanks, ive already seen that video. it is a great video. but i noticed on the diagram he has 3 12 volt panels in series giving 36v connected to a 12volt mppt controller and battery in 12v parralell . And secondly the use of a 6mm cable for a fridge? presume he must have buried surrounded that cable in insulation for that rating. in regards to my original question the video does explain well why he has chosen that cable (inverter amps) but i wonder about backfeeding/heat and the different combinations of cable used connected to the battery .

 

[autorouter]

There's a cable size calculator on the 12VoltPlanet Knowledge Centre, (scroll down to it) and some good advice there if you have the time to read it.

Cable Size Calculator - Cable sizing and selection | 12 Volt Planet

Cable Size Calculator - Read our guide to selecting cable of the correct type and size for your automotive, marine or leisure wiring application. Auto and Marine Electrical Components from 12 Volt Planet

www.12voltplanet.co.uk

There are two things that affect the cable thickness - current flow (amps) and voltage drop. Too many amps and the wire heats up. Too much voltage drop and the appliance doesn't work properly. In 12V wiring, the main problem is voltage drop. If your wire is OK for voltage drop, it will be more than OK for amps rating.

An average MH has a main fuse of about 50A on the leisure battery, and some even have 30A. Your system is bigger than average (40A of solar, 390Ah of battery) so could have a bit larger fuse. The recommended maximum amps into/out of a battery is about a fifth of its Ah capacity, so that's about 80A. So a main fuse of 100A would be OK for your 3 batteries. The battery wiring will need to take 100A without heating up or too much voltage drop.

For a 1 metre return path, the minimum size would be 10mm2, but the next size up, 16mm2, is about the minimum most people would use. If it was my MH, I'd be using 35mm2 for the wiring near the battery.

You'll be connecting multiple chargers and loads to the battery, so it would be a good idea to fit a positive terminal that has fuses built in, like this.
https://www.12voltplanet.co.uk/battery-distribution-fuse-box.html
If you don't like that idea, maybe a main fuse box like this.

Mega & Midi Fuse Power Distribution Box

Compact power distribution & fuse box that distributes power theough a single mega link fuse into 4 midi or strip link fuses.

www.12voltplanet.co.uk

Fit a 100A Megafuse on the main feed to the batteries, and some smaller Midi fuses to the various chargers. After these smaller fuses you can use thinner wire, maybe 10mm2 to the chargers.

Also fit a standard blade fuse box for your load circuits. Something like this, either 6-way or 12-way.

Standard Blade Fuse Box With Positive & Negative Busbars - 6 Way

6 way standard blade fuse box with 2 positive busbars & 1 negative busbar. The -ve busbar is useful to common cables back to the battery -ve terminal.

www.12voltplanet.co.uk

This type has convenient negative connections too, which is useful in a motorhome where the usual negative return through the bodywork and chassis is problematic because everything is wood, plastic or fibreglass.

 

[Revolvor]

When I started adding all the "Goodies" to my van, I wish that I had planned ahead better, and fitted Positive and Negative Busbars to accomodate all the extras I might possibly need. As a result it's become a little bit messy, although I have just connected all Negative cables to a common busbar, so I can then connect it to the leisure batteries via a Victron SmartShunt.

These are my Before and After Battery Schematics:

 

[cmcardle75]

1.5mm might be OK for occasional consumers such as water pumps and low power devices like lighting. However, for a compressor fridge (and possibly diesel heater), I'd go higher. The lower voltage drop will increase energy efficiency on what will probably be your biggest consumers of energy.

 

[stevelec]

thanks for the posts guys , some good information .

yeah looks like 35mm2 is going to be connected for battery parallel connections.

fridge cable is in situ unfortunately but cable is good for 16 amps and vault drop over the length i have worked out to be 0.173 volts so should be okay

did my calcs for diesel heater and ive worked it out for a 5kw but i think they ordered a 3kw. cable is only 1.5m run and max draw on startup should be around 9 amps settling down to about an amp. however cable isnt in for this yet so will prob feed this in spare 2.5mm anyway.

the solar cables provided from controller to battery will be 10mm and fused with the 40 amp recommended fuse .

12 volt blade fuse and busbar from battery will be fed in 16mm and fused at 50amps at present (leaving room for future expansion)

ac>dc charger will be used when on shore and has the fuse in the plug (allthough a 20amp charger i feel this will really need to be 40amp to charge adequalty and have advised them to swap it )

i like the idea of the mega fusebox - but im unsure on any future additions so will probably go with a 35mm cable into 100amp fuse and then into a standard 4/5/6 way positive busbar and hook all connections to that at present.....i think lol. and then add the same without fuse for the negative.

i need to ask them what they are doing with shunt/b2b charger yet

for earthing there will be one earth connection from battery negative busbar to chassis in 10mm and the same on the 240v 1 10mm to chassis no other cross bonding.

appreciate your input guys , will mock up a new diagram. but if you think ive made an error with anything please let me know.

cheers

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[autorouter]

Another point, can't remember if this was mentioned in the video but the 240V wiring needs to be in stranded flexible wire, not solid core as used for house installations. Solid core is liable to long-term failure due to breakage in a vibration environment like a moving vehicle. And use 2-pole MCBs not single-pole as in UK house wiring. Or RCBOs.

 

[mitzimad]

the fridge is a 10 amp load this is what dometic say in their instal manual
12 Volts DC Supplies The connection is made to the terminal block marked “12 volts DC” located at the bottom left corner on the back of the refrigerator. To avoid a voltage drop, the cross sectional area of the connecting wires between battery and refrigerator must be at least 6 mm2 . To ensure safe operation, the positive lead must be fitted with a fuse rated at 20 amps. Correct polarity must be observed when connecting to the 12 V DC supply. Note! Do not use the body or chassis of the vehicle as a substitute for either of the two conductors. Electrical leads must be routed and secured so that they cannot come into contact with hot or sharp parts of the refrigerator. Do not connect any other electrical equipment or lighting to the refrigerator circuit. The connections must be clean, tight and free from corrosion. If not, a resulting voltage drop will cause a decreased cooling capacity
thats for a three way fridge

https://www.dometic.com/assets/49/30/installation%20and%20operating%20manual_64930.pdf

table for wire size for a compressor in the link above but even 1m from the battery requires a min of 2 mm

 

[Langton]

Hi guys very useful information, one question that is bugging me is why do you see camper builders in the US use a lot bigger gauge wire in their systems than we do in the UK. The only difference I could imagine is when they use shore power. Thanks

 

[cmcardle75]

Hi guys very useful information, one question that is bugging me is why do you see camper builders in the US use a lot bigger gauge wire in their systems than we do in the UK. The only difference I could imagine is when they use shore power. Thanks

Mains cable actually needs to be much thinner, as it is more efficient to transfer power at higher voltage. However, the 12V systems depend on the same laws of physics both sides of the pond.

As with all engineering, you need to see the entire problem to determine the most efficient gauge of cable required for an application. Having too thin cables can lead to resistive power losses, or even overheating. Too large can be expensive, heavy and difficult to install. There is generally a Goldilocks zone. But you need to do calculations to ensure the correct design.

 

[Lenny HB]

Hi guys very useful information, one question that is bugging me is why do you see camper builders in the US use a lot bigger gauge wire in their systems than we do in the UK. The only difference I could imagine is when they use shore power. Thanks

Because they do it properly also RV's are much larger so much longer cable runs. When working out cable size you have to take into to account the length of the cable run and aim for a volt drop of under 3%.

Cabling in most Brit built vans is way under size if you look a German vans you will see they use much heavier cables.