Tony's had exactly one wiring fire in his life, decades ago in a boat that isn't around anymore, and it taught him more about fuse sizing than any manual ever did. Nobody was hurt, the damage was contained to a bit of melted trim and a very bad smell, but it's the reason he's fanatical about getting wire gauge and fuse ratings right on every 12V job that comes through the shed since, whether it's a dual battery setup in a ute, a caravan solar system, or just running a fridge off an auxiliary battery for a weekend camp.
The mistake people make most often is thinking any wire that physically fits the terminal will do the job, or that a fuse just needs to be big enough that it doesn't blow the first time you turn something on. Both of those get it backwards. Wire needs to be sized for the current it'll actually carry over the actual distance of the run, and the fuse needs to be sized to protect the wire, not to keep the accessory happy. If the fuse rating is higher than what the wire can safely carry, the fuse isn't protecting anything, it'll let the wire overheat and potentially start a fire before it ever blows, which is exactly backwards from what a fuse is meant to do.
Wire sizing comes down to three things: the current the circuit will draw, the length of the cable run there and back, and how much voltage drop you're willing to accept over that distance. This is where a lot of home installs go wrong, particularly in caravans and camper setups where the battery might be metres away from the fridge or the water pump. Thin wire over a long run causes voltage drop that starves the appliance of the voltage it needs to run properly, a fridge compressor struggling on low voltage draws more current trying to compensate, runs hotter, and wears out faster, all because the wire was undersized for the run length even though it looked fine for the appliance's rated draw on paper.
We use the wire size calculator at https://freeonlinetools.au/tools/wire-size-calculator.php for exactly this. Put in the current draw, the length of the cable run, and an acceptable voltage drop percentage, we usually aim for under three percent on 12V circuits, tighter for anything sensitive like electronics: and it gives you the minimum wire gauge needed. It's saved us from under-sizing more than a few runs where the straight-line distance to the battery looked short but the actual cable path, going around the chassis and up through a wall cavity, was two or three times longer than it first looked.
Once the wire's sized, the fuse comes next, and the rule Tony drums into anyone in the workshop is that the fuse protects the wire, always, not the appliance. You size the fuse based on the current-carrying capacity of the wire you've actually run, choosing a fuse rating at or below what that wire can safely handle, then check that rating is still comfortably above the normal operating current of whatever's connected so it doesn't nuisance-trip every time the fridge compressor kicks in. The fuse size calculator at https://freeonlinetools.au/tools/fuse-size-calculator.php walks through exactly that logic, wire gauge in, safe current capacity out, appropriate fuse rating suggested: rather than just picking whatever fuse happens to be in the glovebox.
Voltage drop deserves its own mention because it's the part people skip most, mostly because a circuit will often still technically work with too much of it, lights just look dimmer, a pump runs a bit slower: so it's easy to assume everything's fine. The voltage drop calculator at https://freeonlinetools.au/tools/voltage-drop-calculator.php lets you check an existing run rather than just size a new one, which is handy for troubleshooting when something's underperforming and you suspect the wire rather than the appliance. Feed in wire gauge, length, and current, and it tells you the actual voltage drop you're dealing with, which has diagnosed more than one "faulty" water pump in our experience that turned out to be a perfectly fine pump starved by an undersized cable run from years earlier.
The last piece, and the one people forget until the battery's flat after a weekend and they don't know why, is charging. If you're running a dual battery setup charged off the vehicle's alternator, the alternator charging calculator at https://freeonlinetools.au/tools/alternator-charging-calculator.php helps work out whether your alternator and wiring can actually keep the second battery topped up given your typical drive times and the battery's capacity, rather than assuming any alternator will do it because it's "just charging a battery." Long fridge runs on short drives between camps is the classic scenario where people find out the hard way that their charging setup was never actually keeping up.
None of this is difficult once you understand the order things go in, current draw, then wire size for the run, then fuse rating to protect that wire, then check the whole thing against what the charging system can actually deliver. But it's also not optional if you want a 12V system that's still working properly, and still safe, twelve months from now rather than something that seemed fine until it wasn't. If you're not confident interpreting the results or the job involves anything beyond a simple low-current accessory, it's worth getting a qualified auto electrician to check the install: these calculators are for planning and understanding the job properly, not a substitute for someone qualified signing off on anything that matters for safety.
The mistake people make most often is thinking any wire that physically fits the terminal will do the job, or that a fuse just needs to be big enough that it doesn't blow the first time you turn something on. Both of those get it backwards. Wire needs to be sized for the current it'll actually carry over the actual distance of the run, and the fuse needs to be sized to protect the wire, not to keep the accessory happy. If the fuse rating is higher than what the wire can safely carry, the fuse isn't protecting anything, it'll let the wire overheat and potentially start a fire before it ever blows, which is exactly backwards from what a fuse is meant to do.
Wire sizing comes down to three things: the current the circuit will draw, the length of the cable run there and back, and how much voltage drop you're willing to accept over that distance. This is where a lot of home installs go wrong, particularly in caravans and camper setups where the battery might be metres away from the fridge or the water pump. Thin wire over a long run causes voltage drop that starves the appliance of the voltage it needs to run properly, a fridge compressor struggling on low voltage draws more current trying to compensate, runs hotter, and wears out faster, all because the wire was undersized for the run length even though it looked fine for the appliance's rated draw on paper.
We use the wire size calculator at https://freeonlinetools.au/tools/wire-size-calculator.php for exactly this. Put in the current draw, the length of the cable run, and an acceptable voltage drop percentage, we usually aim for under three percent on 12V circuits, tighter for anything sensitive like electronics: and it gives you the minimum wire gauge needed. It's saved us from under-sizing more than a few runs where the straight-line distance to the battery looked short but the actual cable path, going around the chassis and up through a wall cavity, was two or three times longer than it first looked.
Once the wire's sized, the fuse comes next, and the rule Tony drums into anyone in the workshop is that the fuse protects the wire, always, not the appliance. You size the fuse based on the current-carrying capacity of the wire you've actually run, choosing a fuse rating at or below what that wire can safely handle, then check that rating is still comfortably above the normal operating current of whatever's connected so it doesn't nuisance-trip every time the fridge compressor kicks in. The fuse size calculator at https://freeonlinetools.au/tools/fuse-size-calculator.php walks through exactly that logic, wire gauge in, safe current capacity out, appropriate fuse rating suggested: rather than just picking whatever fuse happens to be in the glovebox.
Voltage drop deserves its own mention because it's the part people skip most, mostly because a circuit will often still technically work with too much of it, lights just look dimmer, a pump runs a bit slower: so it's easy to assume everything's fine. The voltage drop calculator at https://freeonlinetools.au/tools/voltage-drop-calculator.php lets you check an existing run rather than just size a new one, which is handy for troubleshooting when something's underperforming and you suspect the wire rather than the appliance. Feed in wire gauge, length, and current, and it tells you the actual voltage drop you're dealing with, which has diagnosed more than one "faulty" water pump in our experience that turned out to be a perfectly fine pump starved by an undersized cable run from years earlier.
The last piece, and the one people forget until the battery's flat after a weekend and they don't know why, is charging. If you're running a dual battery setup charged off the vehicle's alternator, the alternator charging calculator at https://freeonlinetools.au/tools/alternator-charging-calculator.php helps work out whether your alternator and wiring can actually keep the second battery topped up given your typical drive times and the battery's capacity, rather than assuming any alternator will do it because it's "just charging a battery." Long fridge runs on short drives between camps is the classic scenario where people find out the hard way that their charging setup was never actually keeping up.
None of this is difficult once you understand the order things go in, current draw, then wire size for the run, then fuse rating to protect that wire, then check the whole thing against what the charging system can actually deliver. But it's also not optional if you want a 12V system that's still working properly, and still safe, twelve months from now rather than something that seemed fine until it wasn't. If you're not confident interpreting the results or the job involves anything beyond a simple low-current accessory, it's worth getting a qualified auto electrician to check the install: these calculators are for planning and understanding the job properly, not a substitute for someone qualified signing off on anything that matters for safety.
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