100a is a BIG load/ short. As said, hard for normal 16-14ga branch circuits/ wiring to pull 100a w/o melting in half... and that would happen only once.
I would attack two ways:
Using Ohm Meter
- Disconnect batteries
- Turn off all battery switches & panel
- Connect ohm meter across the
now pigtailed + & - battery wires
- Should read infinity (no meter
- Starting with each battery SW if
more than one... Slowly cycle
through each position while
watching for short (full meter
- If all OK, leave on and move on to
panel & flip breakers one by one
watching Ohm meter for short
If a short still exists this should reveal which circuit has issue
If you don't have Ohm meter or if this hadn't revealed any issues, connect the negative battery lead to the negative battery terminal on only one battery. And temporary wire a 12v filament type light bulb between the position battery lead and the positive battery post. The higher wattage the 12v bulb, the better... like a spare 55w 12v spreader light. If you trigger a short/ high load the bulb (now in series will glow dim to bright depending on load/ full short circuit. This will save blowing a costly $17 fuse as you progress through finding
the short with the boat's 12v system softly energized.
- With the 12v system now being activated, but with limited current
availability because of the series 12v bulb, repeat the above sequence. I wouldn't expect any difference, but if you have to do some/ all this working alone... it's a good technique to be at two places at the same time because you can position the bulb to be seen as you go to other location on the boat.
Assuming you haven't found the short yet, now go back to your most recent projects (statistically new problems are usually triggered by new work... directly/ or indirectly by misplaced pliers left behind fallen and shorting something unrelated to the project
at hand) and shake, rattle, roll the wiring/ new device you just installed. It won't be the first time a new light fixture came with shorted bulb base, or a wire got pinched, hidden wire got drilled through,... doing an otherwise simple project
. Stop, look, listen, smell around each job site for any clues. While it's more likely that your project somehow triggered the current
issue, it's possible that long term chaffing of wiring just picked this time/ or so slight nudged and failed. So grab and tug on any/ all wiring harnesses/ bundles whether you were around them or not.
As part of this process, pay particular attention to/ trace (every inch) of each of your high current battery cables to their destinations.
While further down the list of probable cause, if your batteries are wired in parallel or the battery SW was left in the Both position and one battery developed an end of life shorted cell it could possibly trigger a catastrophic event leading to a battery-to-battery high amp short through your fuses, which is one reason why they are there! So while the batteries are separated check their standing voltages (no charger/ loads) to see that both are in the range of 12-13v'ish.
Many boats, despite ABYC guidelines run their bilge
pump wiring directly to a battery to ensure it never accidentally gets flipped off with the rest of the breakers on the panel. Of course, even wired this way there is usually a 'twist & lock' glass cartridge fuse next to the ubiquitous ''Auto-Off-Run Bilge SW to protect the boat from direct shorts that might cause a fire/ dead batteries. But it is possible that the bilge pump wire from the battery somehow got pinched/ shorted before it got to that protective fuse. Again, I doubt that the typical bilge wiring gauge could cause a 100a fuse to blow... but if you haven't found the issue by now, it's worth tracing that bilge wire from the battery to its destination
. We should all do these checks once in a while anyway, including the shake, rattle, roll of wiring harnesses. Better to find issues at the dock
than coming into a marina chased by a storm and the lights all go out.
Circling back... I'd really especially check all the big cables from the battery... and really check the ones to to the starter and the electrical
panel... because generally they are the only big red wires that end up going to places where there is a clues by negative terminal/ metal that could cause such a significant 100a short if it got loose/ came off its stud.
Most boats now used switching technology based chargers... lighter because no big heavy iron transformer. But w/I that isolation transformer, when switching power supplies fail it can result in the 120v AC it's connected to at least temporarily connected to your battery. Again, I sorta doubt that this could fry a 100a fuse, but if you have exhausted other options, it's worth disconnecting the charger wires and checking it's output voltage.
I saved this last one for last because it's very unlikely, and I hope it isn't the cause, and because if it caused a 100a fuse to blow, there could be a serious safety
issue. Our boat are all usually 'grounded' to the salt water
via shaft/ prop mechanically connected to the engine which in turn almost always has a big negative battery cable bolted to it to complete the starter circuit. Also many boats have their safety
green ground wire from their shore power cord eventually also finds itself connected to the shop's 'ground.' If your dock
ground (green) or neutral (white) wiring has lost
its continuity your boat may have temporary taken the dock's return path to ground when you re-plugged in your boat. Again, this is highly unlikely, as by now many boats would be suffering some all kinds of low voltage, hum in electronic audio circuits, erratic operation of most electronics equipment
. But if this has occurred, you should disconnect your shore cord until the issue is resolved by a professional.
Give us a status update/ what you found, or if you need more ideas.
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