The Offshore Voyaging Reference Site

7 Checks To Stop Our DC Electrical System From Burning Our Boat

Not Fused

Before we dig into how to upgrade our boat’s DC electrical system, we need to check for potential boat-burners.

Two thoughts on that:

  1. I’m pretty sure that fires started by 12- and 24-volt battery-supplied systems are a common cause of boat losses.
  2. I would also bet that more fires are started by 12- and 24-volt systems than by shore power 120- or 240-volt systems.

No, I don’t have accurate statistics on this, and I’m guessing that, as is typical around recreational boating, no one does due to poor or non-existent reporting requirements. But using poor reporting as an excuse for inaction would be stupid…err…unwise. (If you know of useful stats on this, please leave a comment.)

What do I base this on?

The current (amps) in even a single small 12-volt lead-acid battery can turn any conductor with low resistance (think a length of wire) red hot, thereby starting a fire.

And the amount of dangerous amps lurking in the huge battery banks we see on modern cruising boats is positively mind blowing.

Wait, it gets worse. On many boats, even ones built comparatively recently, like our new-to-us J/109 (2004), the only over-current (short-circuit) protection is on a breaker/fuse panel, leaving the high-current conductors connecting batteries and alternators completely unprotected.

Contrast that to the shore power system that typically carries ten to twenty times less current (amps) and is pretty much always properly protected against a short-circuit-started fire by fuses and/or breakers.

(Shore power systems kill people, but that’s another article.)

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More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—Part 1, BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Part 2, Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Part 3, Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Part 4, Fusing
  34. 11 Steps To Better Lead Acid Battery Life
  35. How Hard Can We Charge Our Lead-Acid Batteries?
  36. How Lead Acid Batteries Get Wrecked and What To Do About It
  37. Equalizing Batteries, The Reality
  38. Renewable Power
  39. Wind Generators
  40. Solar Power
  41. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  42. Battery Monitors, Part 1—Which Type Is Right For You?
  43. Battery Monitors, Part 2—Recommended Unit
  44. Battery Monitors, Part 3—Calibration and Use
  45. Battery Containment—Part 1
  46. Lithium Buyer’s Guide—Budget: High End System
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Petri Flander

Hi, here’s some stats that I had run into. Nigel Calder had these 2009-2013 BoatUS stats on his new website, that indicate ~55% of fires start from electrics, and roughly third from DC side.

Then, here’s more recent 2015-2019 statistics on BoatUS article:
Again, roughly third from DC.
Cheers, PF

Dick Stevenson

Hi John,
I came across this in my notes for some past writing, perhaps from the same source:
Boat Fires
1.    26% start off the boat
2.    20% due engine electric
3.    15% other DC elec
4.    12% AC elec
5.    9% eng
6.    8% batteries

Ernest E Vogelsinger

I want to add that it doesn’t suffice to add breakers, especially for switchable/resettable breakers. These should be from a reputable manufacturer, e.g. BlueSea Systems, and not some cheap breakers built in ~somewhere~ 😉
For my two house circuits I had purchased two cheap 50A resettable breakers. Then I made an error in the cabling, producing a direct short circuit, which I initially didn’t notice, however when flipping the battery main switch the aft cabin (where the breakers are located) quickly filled with smoke. The affected breaker did not open but instead started to boil, and more or less welded its internal contacts, making things worse…
See attached images

Ernest E Vogelsinger

Hi John,
directly behind the house battery positive terminal there is a 500A Mega fuse. A 70mm2 cable goes to a T-Bone and then to the 1-2-both master switch.
The circuit with the failed breaker originated at the master switch with 16mm2, approx. 15cm (~6″) to the failed breaker, then continuing with 15mm2 for cm (~7″) to a bus bar, from there 25mm2 approx.7m to the switch panel at the nav station.
The wiring fault occurred at the said bus bar where I connected a preexisting red-coded cable which I believed would power the front cabin, but actually was connected somewhere to neg.ground… I know, I should have used my multimeter before.
The breakers have no additional AIC or similar, only printed “50A” on them. As I do not want to advertise them here I’m sending you the product link by mail.
Since the failure I exchanged both of the cheap no-names with this model of BlueSea Systems, having an ACI of 5000A @12V:

Ernest E Vogelsinger

John, you’re absolutely right – I once (mis)calculated and never challenged – will change these fuses when I get back to the boat.
As to the BlueSea PDF, 2/0 inside the engine room would allow for 300A, which should be sufficient for even the starter circuit (28hp Hatz Diesel).
Thank you for the heads up!

Matt Marsh

I’ve seen that kind of breaker before. I’d bet a beer that you will find no CSA, UL, ETL, TUV, or other recognized certification marks on its case, and that if there is a brand name, it’ll be something that nobody’s ever heard of.

Steve D

The common abbreviation is AIC, ampere interrupt capacity. As an aside, class T fuses should be used with any primary battery bank supply wiring, they have an AIC of 20k amps, compared to ANL’s which are commonly and incorrectly used, and cheaper, which have an AIC of 6000 amps, which is easily exceeded by two 8D batteries’ fault current.

Steve D

I like the MRBFs in 12 volt applications, where they have the necessary AIC, they have made life much simpler in many installations. For 24 volts the AIC is lower. On that subject, the 24 volt discussion is an important one, to many builders, IMO, cling to 12 volt where 24 makes much more sense. Any vessel over about 35 feet benefits from a 24 volt system. When I built 36 foot lobster yachts, I calculated we saved about 1000 lbs. in wire weight by switching to 24 volt.

Arne Mogstad

Just a quick comment. When I used to work as an electrician, we had to do calculations to avoid this exact problem in case of a high load or a short far out on a wire, that they break fast enough for the cable to not exceed a specified temperature, to balance between the different fuses/breakers so that the correct fuse/breaker would trip and that the whole spectrum was covered, and that they have the breaking-power of the current that can be generated.

These are Norwegian requirements (which are somewhat special since we use high power heaters that can overload a wire without the current tripping the fuse/breaker), but I would assume there are similar stuff in English. Sadly I don’t have a link, as it’s all in Norwegian and in paper.

Steve D

Well-done and an important subject, especially the over-current protection aspect. Lack of OCP, or improper placement of OCP, is the number one electrical issue I identify in vessel inspections I conduct. I can find it on virtually every vessel, it’s the thing that keeps me up at night more than any other onboard short-coming (pun intended). I too worry about DC fires, and AC electrocution.

You are understating the severity of the amperage supplied by a short from batteries. The fault current supplied by a battery can be 4-5 times its CCA.

I may be mistaken, however, the small gauge red wire shown in the third photo looks like it is much longer than 7″ before it is fused. Also, if it’s smaller than AWG 16, it is in violation of ABYC Standards.

If ABYC compliance is the goal, alternators actually need OCP at their output, as well as at their connection to the batteries if…

ABYC E- #4. Overcurrent protection is not required in conductors from self-limiting alternators with integral regulators if the conductor is less than 40 in (102 cm), is connected to a source of power other than the battery, and is contained throughout its entire distance in a sheath or enclosure.
#5. Overcurrent protection is not required at an alternator if the ampacity of the conductor is equal to or greater than the rated output of the alternator.

Essentially, this means after-market alternators with external regulators whose output cable leaves the engine, require OCP at the alternator.

A common issue, and ABYC violation I encounter all too often, is starter positive cables making contact with the engine (if they are fused, as you recommend, it’s less of an issue but still a concern, I never rely 100% on a fuse). The positive starter cable should leave the starter and not make contact with any part of the engine, brackets, transmission or motor mounts.

Balmar does call for the voltage sense to be placed at the alternator output stud. This is safer, but offers less accuracy.

Re. #6, this issue can be avoided by placing the regulator near the batteries rather than near the alternator. Voltage drop from the regulator to the alternator is a non-issue as it is compensated for, while voltage drop from the regulator to batteries is an issue, for which there is no compensation, at least not automatically.

I deal with so many OCP violations, with boat owners and builders alike, I wrote this article on the subject, to try to educate them and save myself some typing time when writing reports.

Again, you’ve covered a critically important subject that will likely prevent fires.

Steve D

I have actually tested/measured this voltage drop, on a larger vessel it can be significant, which in this scenario can be as little as a tenth or two of a volt. If the sense wire is run directly to the battery, my preference the highest accuracy is achieved, and it avoids the voltage drop issue you describe, and locating the regulator near the battery makes that run short, more reliable and easy to protect.

Andrew Reddon

I am puzzled by the statement: “#5. Overcurrent protection is not required at an alternator if the ampacity of the conductor is equal to or greater than the rated output of the alternator.”. From what context is this taken? The “conductor” referred to cannot be the voltage sense wire, so that leaves the wire from alternator to battery/buss. I must be missing or misreading something.

Steve D

It does refer to the main output wire. Indeed, if only one fuse is used, at the battery end, the short circuit remains, pumping current into the block for instance, without overheating the wire, the alternator simply sees it as a load. If you are OK with that, I’m not, then only one fuse is required. Interestingly, Caterpillar uses a CB on the output of most of their alternators even though they are internally regulated and the output goes from the alternator to the starter, a short, harnessed run.

Matt Boney

I’d like to add a comment on the importance of checking any existing cables already fitted. A friend upgraded his shore power charger from 30 amps to 120 amps and asked the yard to fit it whilst he was away. Before he returned after the winter break he asked them to turn it on – and his boat caught fire. They hadn’t upgraded the cable to carry the extra current.

Steve D

Possibly avoided by prefacing any (electrical in this case but applicable to many areas) work requests with, “And all work must comply with ABYC Standards and manufacturer installation instructions” and conforming the yard is a member and preferably has certified staff on hand. That is a rookie error of epic proportions. Was the boat a total loss?

Seth Winnick

So here’s another question or two on #5. This is timely for me since I am upgrading my battery side DC wiring after burning out a 120 amp alternator. The Amptech alternator is on a Westerbeke 35B engine with a serpentine belt and is regulated with a Balmar ARS-5. I believe the cause of the alternator failure was either too much heat (I have now installed a heat sensor on the rebuilt unit,) or a bad connection on the alternator field terminal.

In the upgraded wiring, the current path from the alternator hits the first fuse at the always-on positive bus with the next fuse on the house battery bank positive terminal. The Balmar ARS series has a single power input/sensing voltage wire. If I connect that wire on the alternator side of the positive bus fuse, then I am missing the voltage drop between the positive bus and the battery bank. (Do I also risk losing power to the regulator if the alternator fuse blows?) If I connect the power/sense wire to the battery bank fuse, then I could have the aforementioned separation problem. Seems a conundrum.

Seth Winnick
S/V Nomad
(Sabre 38 Mk I)

Iain Dell

This post offers to serve as a lesson to other readers who may be as electrically numpty as me.

In contrast to the many who know what they’re talking about here, I’m an ‘electrical dyslexic’ – but I’ll bet I’m far from alone in the cruising community. My eyes tended to glaze over in any discussions about power onboard; there was always a marine electrician who’d handle that black art for me.

Until one day I smelled burning at sea.

I simply could not imagine the hazards around even professionally-installed low-voltage gear such as my AIS. With the instruments on but under sail I was astounded to see acrid smoke and molten plastic dripping from the back of the AIS straight on top of a folded genoa beneath the unit. I was single-handed well offshore at the time and had I been elsewhere but in the cockpit and able to react quickly the results might have been much worse than an early trip to the sailmaker. The picture doesn’t really reflect the volume of smoke or smell but it illustrates the point.

Despite now knowing my Ohm’s Law and able to infer the likely consequences, this has not cured my electrical dyslexia. However, I am much, much more aware and have regular and thorough electrical checks by someone I trust, with me following assiduously in the hope of something actually sinking in. I still can’t pretend to know what I’m talking about, but at least I’m very aware of a shortfall in my knowledge and I mitigate accordingly.

Terence Thatcher

Well, you have certainly worried me. But that may be because I have a hard time understanding this all. Not your fault. But let me try: My batteries are all fused at the big positive wires. But the Balmar regulator (I know, I should get a Wakespeed, but other projects have intervened) has both a sense wire and a temperature wire attached directly to the battery accepting the charge. (All batteries are combined when charging.) I can’t remember if Balmar told me to fuse the small sense and temp wires. If so, I am ok there. If not, I understand you to say these wires need to be fused on their way back to the regulator. Similarly, any smaller wire attached to the batteries (or positive bussbar) and going anywhere should be fused or have a breaker sufficiently sized to carry the normal load, but small enough to blow if the small wire might get dislodged or damaged. Am I correct on that? And does that fuse/breaker need to be right next to the battery? Or can it be a couple of feet away? And, you have convinced me, I will join ABYC as a “boat owner.” I have an ABYC electrician coming down soon to install the big shore power ELCI. Hope he knows the rules. Scary to learn that those we rely on may often be ignorant. As you say, self-reliance may be advisable . . . until it isn’t.

Dick Stevenson

Hi all,
In a recent issue of Professional Boatbuilder, Jonathan Klopman (Marine Surveyor), comments on the state of lithium batteries on boats. His particular expertise is accident reconstruction and failure analysis.
Of particular interest in the article was his description of the suggestions generated by ABYC’s technical information report which (as I understand it) is a precursor report prior to ABYC developing and publishing their standards for a Lithium install.
I am loath to c&p copywrite material but “Loose Cannon”, with permission of the author, has re-printed the article and it can be found at
The article also has some compiled fire statistics.
My best, Dick Stevenson, s/v Alchemy

Terence Thatcher

Don’t know if this in on-topic, but here goes: A few years ago O bought a Sterling Power alternator open circuit protector from Rod Collins at Compass Marine. I never installed it, however. I was about to wire it in, but thought I should ask you, John, before I do. Yes or no? Thanks.

Terence Thatcher

what happened to the 2 minute edit feature?

Stephen Hershman

I’m down a rabbit hole on circuit breakers and trying to understand the difference between UL489 and UL1077. The way I’m reading it, all of the Blue Sea electrical panels are built incorrectly. They only have UL1077 devices for both the main and branch breakers. However, UL1077 is not rated for standalone circuit protection, it’s only meant for protection downstream of a UL489 branch circuit protection device. It’s also only meant to protect a single device, not for example a series of outlets. Am I reading this right, or does ABYC provide different guidance? Maybe it’s a topic you’d like to tackle?

Andrew Reddon

Re: which end to fuse? As the article indicates, one scenario for shorting the alternator wire is vibration causing the wire to fall off the alternator terminal. Since the shorted battery current is more than the alternator’s output, a fuse at the main positive buss (which I have never had before) makes the most sense. I am implementing the improved fusing that seems so obviously necessary now that I have read this article (thank you, John), but I am thinking about the chafe/shorting that could theoretically happen anywhere along the wire, while the alternator is putting out current. The compact terminal fuses designed for battery posts are very convenient to install and very affordable. It seems tempting to install one on the alternator stud in addition to fusing at the buss. This adds two bolted connections at the alternator instead of just one, so doubles the vibration risk, I guess. It will also more than doubles the resistance present. I am not sure that those are compelling reasons not to add the fuse, but I feel there must be a reason not to, or it would have been alluded to somewhere. I am wondering iof the answer might be that the alternator will fail in the case of a mid-run short just as safely as it would fail in the case of opening a fuse on its output while running. I am not sure about that. Any comments would be appreciated.

Terence Thatcher

Two late questions. My Balmar regulator has a sense wire connected directly to the battery that first receives the charge from the alternator. (Combiners connect the other batteries at 12.75 volts or thereabouts.) Balmar includes a 1 amp fuse in the sense wire. Do I understand you to say that if that fuse blows for some reason, all hell will break loose? Second question: The positive post of all three of my battery banks are protected by MRBF fuses. I take it that is satisfactory as long as the fuses are sufficient to protect the wires in case of a dead short. The few smaller wires connected to the batteries are all separately fused next to the batteries. (Yes, I know, I intended to get a Wakespeed, but I ran out of time and money this year.) Thanks, as always.

Craig Buchner

Re:#5 Alternator sense wire. Thank you for raising this important issue. Attaching the wire on the alternator ( one of several suggestions in the Balmar installation manuals (mc-614 and earlier ?) , which would ensure this issue cant happen,but sacrificing accuracy. So if one has the ability to switch alternator output to a different bank , perhaps in an emergency, what suggestions do you have to ensure the sense wire “follows the alternator output”

Eric Klem

Hi John,

Craig’s comment got me reading your point 5 again and I found it more than a little alarming that you saw 22V with a disconnected sense wire. Do you mind sharing what alternator regulator that happened with? If I have the same one, I may need to think harder about how to protect against that failure modality.

The reason that I ask is that it seems like really poor risk management was done on the design of that unit. If doing something like design failure modes and effects analysis, some of the first lines I would expect to see would be what happens if each I/O becomes disconnected or shorted to something. You would then find that a disconnected sense wire would result in dangerously high voltages that could not only harm electronics but cause some real exciting stuff to happen in the battery compartment. But this also shouldn’t be that hard to mitigate against in design. The whole point of a sense wire is to deal with voltage drop so you could limit the regulator to a maximum mismatch of something like 1.5V or just set the max alternator voltage to 1.5V above commanded and that should take into account even poor wiring practice. I think that all the regulators I have used have always had a direct connection to the alternator positive terminal for powering the regulator so you are already electrically connected to do this and it becomes a firmware implementation only in all likelihood.