Why low voltage is not low risk
You cannot get a meaningful shock off a 12V battery through dry skin. That is true, it is why people are relaxed around it, and it is the reason 12V systems burn houses down more often than mains ones do.
The hazard is not the voltage. It is the current. Mains wiring is protected by an entire industry of standards, and the supply behind it is fused at the street. A 12V battery bank is fused by whatever the person who built it decided to fit, and behind that fuse sits a device capable of delivering thousands of amps without blinking.
Three things cause almost every 12V fire:
- A fuse sized for the appliance instead of the wire.
- A fuse that physically cannot interrupt the fault current available.
- A connection that was never quite right, heating a little more each year.
None of these announce themselves. All three are avoidable in an afternoon.
The fuse protects the wire
The rule for sizing: 125% of continuous load, rounded up to the next standard fuse size, and never above the wire's rating.
A 10 A continuous load wants 12.5 A, so a 15 A fuse, on wire rated for at least 15 A, which is 14 AWG or 1.5 mm2.
Sizing the wire
Use stranded cable, not solid. Solid core belongs in fixed building wiring. Anything that will be flexed, vibrated or worked on wants fine strands, which also crimp far better.
Use proper cable. Automotive or marine grade is designed for exactly this duty and is tinned against corrosion in the marine version. Mains flex and speaker wire are not substitutes.
Voltage drop, the thing the ampacity table does not tell you
At 12V, the ampacity table is often not the binding constraint. The voltage drop is.
Target 3% or less. At 12V, 3% is 0.36 V. That is a tiny budget, and long runs eat it quickly.
What this means in practice:
- Doubling the run length doubles the drop.
- A cable that is perfectly safe at two metres can be useless at twelve, not because it overheats but because too little arrives.
- The symptom is not a fire. It is dim lights, a pump that will not start, a controller that never quite reaches its charge voltage, and a battery that mysteriously never fills.
A battery that never reaches full charge is often a cable problem, not a battery problem. The controller is producing 14.4 V at its terminals and the battery is seeing 13.9 V, so it stops short and you blame the battery.
When in doubt, go one size heavier. Cable is cheap compared with the system it feeds and vastly cheaper than diagnosing this twice.
Interrupt rating, and why a cheap fuse can fail to blow
This is the failure mode almost nobody knows about, and it matters specifically because of lithium.
Every fuse has an interrupt rating, sometimes written AIC, which is the largest fault current it can actually break without failing violently. Exceed it and the fuse element vaporises but the current keeps flowing as an arc across the gap, so the circuit is never actually broken.
Lithium changes the arithmetic. LiFePO4 has very low internal resistance, so a single 100 Ah cell can deliver 5,000 to 10,000 A into a dead short. Several in parallel can exceed 20,000 A. Lead acid banks simply cannot do this, which is why the problem is newer than the wiring practice most people inherited.
The practical rule: for the main fuse directly on a lithium battery bank, fit a Class T. For a modest single-battery 12V system an MRBF at the terminal is adequate and is far better than an ANL. Blade fuses belong on branch circuits, never as the battery main.
What happens when you get this wrong is worth stating plainly, because "the fuse blew" is not the outcome. The fuse element vaporises, an arc sustains across the gap, and the arc continues until either the cable burns through or the battery is exhausted. It is a sustained high energy fire at the fuse, which is by definition right next to the battery.
Where every fuse goes
- Main fuse on the battery positive, as close to the terminal as physically possible. Ideally within 150 mm. This is the most important fuse in the system and the one most often missing entirely. Its job is to protect the whole length of the main cable, so any unprotected length before it is a length that can short and burn.
- Panel to controller.
- Controller to battery.
- Every branch circuit at the distribution block, sized for that branch's wire.
- Inverter feed, sized for the inverter's actual current, which at 12V is large. A 1,000 W inverter draws over 80 A at full load before efficiency losses.
A note on the negative side: in a simple negative-earth system you fuse the positive only. Fusing both sides can leave a circuit live through the negative when the positive fuse blows, which is a genuine hazard when someone assumes a dead circuit.
Making a connection that lasts
A poor connection is a resistance. A resistance is a heater. This is the slow version of the failure, and it is the one that turns up years after installation.
Crimping, done properly:
- Use a ratcheting crimp tool matched to the terminal type. The cheap pliers-style crimpers sold with terminal assortments do not produce a gas-tight joint, and gas-tight is the entire point: a correct crimp cold-welds the strands to the terminal so that no air reaches the metal to corrode it.
- Strip to the right length. Strands should fill the barrel with no copper showing beyond it and no insulation inside it.
- Do not nick the strands when stripping. Every nicked strand is capacity you no longer have.
- Pull test every crimp. A good one will not come off. Test it now rather than discovering it in two years.
- Heat shrink over the joint, adhesive-lined for anything that might get damp.
Solder alone is not the answer anywhere that vibrates. Solder wicks up the strands and creates a hard section, and the cable then fatigues and snaps exactly where the solder stops. Crimp for mechanical joints. Solder is for circuit boards.
For heavy cable, hydraulic lug crimpers can be hired or borrowed, and a properly crimped lug on 25 mm2 cable is not something to improvise.
Terminal torque matters. Loose is a resistance; overtightened shears the stud or crushes the terminal. Follow the battery manufacturer's figure where one is given.
Switches, isolators and the DC arc problem
Do not fit an AC-rated switch to a DC circuit.
Alternating current passes through zero a hundred times a second, and an arc drawn across opening contacts self-extinguishes at each of those crossings. Direct current never crosses zero. An arc struck across DC contacts keeps burning, and it will erode the contacts and can sustain until something melts.
- Every switch, isolator, breaker and relay must carry a DC rating at your system voltage. The DC rating is always lower than the AC rating on the same device, which is exactly why the substitution looks reasonable and is not.
- Fit a main battery isolator you can reach easily, and make sure everyone in the household knows where it is and which way is off. In a fire or a fault, one obvious switch that kills everything is worth a great deal.
- Label it. In the dark, under stress, with the wrong person operating it.
Routing and mechanical protection
- Anything passing through a hole in metal or wood gets a grommet. Vibration plus a sharp edge is a short circuit waiting for its moment, and it will happen in the one place you cannot see.
- Support cable every 300 mm or so so its weight is not carried by the terminals.
- Keep it away from heat, exhausts, flues and stove pipes.
- Never run DC power cable in the same bundle as anything you care about, and remember the bundling derate above.
- Leave a service loop at each end. Cable pulled tight is cable that will eventually pull a terminal apart.
- Label both ends of every cable. You will not remember, and neither will the person who has to work on it when you are not there.
The annual check
Ten minutes, once a year, and it catches nearly everything that develops slowly.
- Feel every connection after a period of heavy load. Warm is a warning. Hot is a fault. This is the single most informative check you can do and it needs no tools.
- Retighten terminals, which loosen with thermal cycling.
- Look for green or white powder, which is corrosion and means the joint is no longer gas-tight.
- Check insulation where cable passes through anything or rests on anything.
- Confirm the main fuse is still there and is still the right size. People borrow fuses.
- Test the isolator actually isolates.