Fuse didn't Blow! Nearly a Fire!!

[btrayfors]

Lots of emoting going on here!

1. Shunts for battery monitors can be/should be connected to a BUS on the negative side of the house battery. Ideally, the bus is the ONLY connection to the neg post on the battery.

2. Similarly, low-amperage devices -- with their own in-line fuse or breaker as needed -- can be connected to the positive bus which, itself, has an appropriate high AIC fuse connected between it and the battery positive post.

There is a debate about battery sensing wires, and other very low current devices. If the wires are small enough, they may themselves serve as "fusible links" :-)

The debate over wire size for solar panels misses the point: solar panels are charging sources for the batteries and it is desirable to provide a very low resistance path to the controller and the batteries. "Oversize" wire is one way to avoid voltage drop, as is the use of robust fuses with large surface areas (not glass or little spade types or similar). Remember that 'voltage drop' = resistance = energy loss through heat.

The fact that the OP is seeing a 1.5A gain after replacing his fuse indicates that there was indeed a lot of undesirable resistance in the circuit, which led to heating and melting the fuse.

Bill

[Garrettw]

Originally Posted by sabray

The US code your quoting is for higher voltage.
No way should you have 14 awg on a 20 amp 12 volt dc circuit.
Amps and Wire Gauge - 12V Circuit

Went back and re read and saw that this was for a solar panel installation. I had another thread open about a mystery 120V system and got mixed up. I agree with your assertion that one should factor in voltage drop for a low voltage installation.

[Maine Sail]

Originally Posted by sabray

The US code your quoting is for higher voltage.
No way should you have 14 awg on a 20 amp 12 volt dc circuit.
Amps and Wire Gauge - 12V Circuit

ABYC E-11 Max Ampacity Chart (Non-Bundled)

[sabray]

fair enough but you would be on the edge of acceptable wire size at 5 feet of run using 14 awg. assuming total run length of 10 feet. I would not waste my time using 14 awg for a 20 amp circuit. Even if you showed me it was okay it just seems not the best choice. So on my boat no way am I running 20 amps across 14 awg.

Originally Posted by Maine Sail

ABYC E-11 Max Ampacity Chart (Non-Bundled)

[noelex 77]

This type of fuse holder comes with a short length of wire. This is all that is shown in the photo.

There are two considerations for any wire.
Voltage drop
Maximium current carrying capacity.

Normally the first factor is what governs the size of wiring on boats.
However the fuse holders only come with a foot or so of wire so the current carring capacity is likely to be the only consideration over this short distance.

As well as the wire itself that comes with the fuse holder the rest of the construction needs to be adequate.

This type of fuse holder comes in two models with maximium ratings 10 or 20A.
Obviously for 14A the bigger one is needed. If its the common Chinese no brand model I would still have some concern using it for a 14A in a hot marine environment.

Here is one from my spare parts collection. Note the endless loop of electricity

[btrayfors]

Exactly.

And, for solar hookups in particular, ampacity tables such as those above only tell a partial tale and it is misleading.

While AWG14 wire has a "current carrying capability" of 20A to 45A depending on insulation and whether inside or outside engine spaces, it would be ridiculous to use this guage wire for a solar installation of any size.

Consider the following.

Solar panel output 15A @ 12VDC. Length of one-way run of wire: 20ft (not at all uncommon on a boat).

An AWG14 wire would have a 13% voltage drop, i.e., 1.56 volt drop, with only 10.4 volts at the load end.

An AWG12 wire would have an 8% drop, or almost one volt drop.

It's not until you get to AWG8 when the percentage drop nears 3%.

AWG6 would be better in a solar circuit (2% voltage drop), and AWG4 would be even better (1.27% voltage drop).

In these types of circuits, it's voltage drop that really matters, considering current load and total circuit length.

Ampacity? Not so much!

Bill

[MarkJ]

Originally Posted by btrayfors

An AWG14 wire would have a 13% voltage drop, i.e., 1.56 volt drop, with only 10.4 volts at the load end.

An AWG12 wire would have an 8% drop, or almost one volt drop.


Bill

I have about 20 feet of very heavy battery cable from my solar panels to the battery. At the battery there is 4 inches of AWG12 and the new 30 amp fuse. (No other fuses are available here so I have what I have and thats it till civilization).

So does the 4 inches of AWG give much of a drop?


Thanks


Mark

[model 10]

I have a large common ground running to the arch (lights, solor wind etc,) at the back of the boat and individual + leads running back forward. How would you determine the proper size cable for the +'s from these charts?

[Maine Sail]

Originally Posted by sabray

fair enough but you would be on the edge of acceptable wire size at 5 feet of run using 14 awg. assuming total run length of 10 feet. I would not waste my time using 14 awg for a 20 amp circuit. Even if you showed me it was okay it just seems not the best choice. So on my boat no way am I running 20 amps across 14 awg.

I was not meaning to suggest anyone push 20A through 14GA wire, especially in a charging circuit where minimal voltage drop is critical. I only show that for the OCP.

Fuses add voltage drop. People often use the smallest fuse to do the job. In reality a larger fuse, but not above the wires safe max ampacity, will result in the least voltage drop across the fuse.

Here is some data I collected while doing some fuse testing. These are Cooper Bussman ATC fuses in an in-line fuse holder:

* 5A Fuse / 5A Load = .11 volt drop
* 10A Fuse / 5A Load = .051 volt drop
* 30A Fuse / 5A Load = .019 volt drop

[noelex 77]

Originally Posted by MarkJ

So does the 4 inches of AWG give much of a drop?

No virtually nothing about 0.02v. I think some posters don't understand the photo only shows the short length of wire that is integral to the fuse holder.

However I do have some reservations about using this type of fuse holder for 14A especially if it is a no name Chinese version.

[MarkJ]

Originally Posted by noelex 77

No virtually nothing about 0.02v. I think some posters don't understand the photo only shows the short length of wire that is integral to the fuse holder.

Thanks mate

Originally Posted by noelex 77

However I do have some reservations about using this type of fuse holder for 14A especially if it is a no name Chinese version.

You are right. The old one looks very cheap.

The new one is a marine one with a waterproof cover. Looks more robust.




Thanks for your help


Mark

[sailorboy1]

Originally Posted by MarkJ

I have about 20 feet of very heavy battery cable from my solar panels to the battery. At the battery there is 4 inches of AWG12 and the new 30 amp fuse. (No other fuses are available here so I have what I have and thats it till civilization).

So does the 4 inches of AWG give much of a drop?


Thanks


Mark

Did you put it in a cold box from the frig to keep it cool and connect a sensor to an automatic fire extinguisher for when it catches fire next.

It not people here are not going to be satisfied.

[muskoka]

We used a big Blue Sea fuse holder (the 30A) and just run the big cable from the panels all the way through. It holds the large format blade fuses. The twin 2A fuses are for the battery monitor.

Cheers

[MarkJ]

Originally Posted by muskoka

Blue Sea fuse holder (the 30A) and just run the big cable from the panels all the way through.
Cheers

good one. Thanks

[svlamorocha]

Originally Posted by btrayfors

If I understand you correctly, Mark, that fuse was connected directly to the house batteries.

That's a no-no. Only three types of fuses have high enough AIC ratings to connect directly to the house bank: ANLs, MRBFs, and Class-T. For this application, I'd use either an ANL or a MRBF type.

Your post is an excellent illustration of why the ABYC has specified Circuit Protection Devices (fuses or breakers) with at least a 5,000 amp interrupt capacity for connecting directly to a big battery bank. Devices with less than this rating can possibly fail in the closed (shorted) position which can easily cause a fire.

In your specific case, and assuming no huge transient currents (like lightning or power surges), my bet would be surface corrosion had built up on the blades of the fuse, causing significant resistance and heating.

And, yes, that can cause heat, melting, and a fire with far less than 20A current.

Bill

I agree that the the absence of a CPD with large interrupt capacity (thousands of amps) close to the battery is a no-no.. That said, IMHO we should clarify that in this case (where the photo suggests that the current was at most not much higher than the rating of the blade fuse, at most dozens of amps) that absence does not seem to have been a factor .

+1 on the warning about bad connections that can have enough resistance to create excessive heat under normal currents. Here the heat seems to have come not from the thin part of the fuse (which is the one that fused at X times the rating) but from the interface between fuse blades and the female fuse socket as if caused by rust, loose connection, etc... IMHO the action item for all of us is to check that under design loads nothing in the electrical system gets hot (say hotter than 10 degrees C over ambient temprature or 60 degress C regardless of ambient temperature, I am sure we can refine these numbers but you get the flavor). I bet this check would have picked up the issue a while ago. There you have a good use for infrarred thermometers/cameras!


C