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Old 12-11-2017, 12:41   #1
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Inverter 12V Fuse Question

I have a Freedom 25 Heart charger / inverter that was in place when I got my boat. I am rewiring the DC electrical system and need advice about fusing.

The installation manual for the charger calls out the following:
"Battery Cable Fusing
A fuse is required by the NEC to protect the battery cables. The fuse must be installed in the positive battery cable, within 18 inches of the battery. Recommended Fuse: Class T JLLN 300A."


The current installation has a Class T fuse located on the positive side of the inverter, not near the battery as recommended.

I have replaced the house bank and incorporated a blue sea battery terminal fuse (250A). Is this fuse sufficient for protection? I can't see why but is an additional fuse on the positive side of the charger lead needed?
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Old 12-11-2017, 13:47   #2
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Re: Inverter 12V Fuse Question

You need a fuse at the positive battery terminal. Those blue sea fuse blocks make it all easy to do. Only one extra connection. You do not need the fuse at the inverter/charger. Will 250 amps be enough? I would use the recommended 300 amps. Blue Sea Systems 5190 Battery Terminal Fuse 300 Amp Genuinedealz.com
You also have to consider the size, awg, of the cables between battery and inverter/charger. Bigger is better.
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Old 12-11-2017, 14:00   #3
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Re: Inverter 12V Fuse Question

By putting the fuse on the terminal, you have satisfied the requirement of the fuse being withing 18" of the batteries.

There is the possibility that the 250A fuse may blow with a large inverter draw, so I would keep a 300A replacement on hand.

The only wire connected to this terminal fuse should be your inverter. Any other loads should have their own primary circuit breakers or fuses within 18" of the battery.
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Old 12-11-2017, 14:49   #4
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Re: Inverter 12V Fuse Question

If Freedom recommends a 300A fuse, the odds are that with the inverter pulling full power to run a maximum load, it might need 300A, and would blow a 250. But. You'd have to run your own numbers on that. If you never load the inverter up to "full load", and you're pulling 3/4 of the max rating from it, then a 250A fuse should certainly do the job. And arguably better, since it would blow slightly sooner in a real fault.

Either way, you might want to make sure you have at least 3 spare fuses on board. I find that if a fuse blows, sometimes the real problem isn't found or fixed, so the only spare gets consumed in short order. Or, the spare gets blown (requiring a second spare) because something wasn't quite fixed right. Which makes three spares the minimum number to carry, since you can't just pick these up at the handy 7-11 store.

Of course if you have three spares, you'll probably never need any of them at all.
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Old 12-11-2017, 21:25   #5
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Re: Inverter 12V Fuse Question

Is this a 250a fuse for the whole bank? So now you have a 300a circuit and all your house loads off a 250a fuse?.... so when it blows you lose everything? you should seperate them. And have 2 fuses at the battery. Each with own cable leaving battery. Or it needs to be ~400a with correct cable. The inverter also needs its own switch.


You still need the class t. The mrbf will protect the wire. The clsss t will protect the inverter. All inverters request class t fuses.
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Old 13-11-2017, 05:20   #6
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Re: Inverter 12V Fuse Question

You only need the fuse at the battery terminal. That is what the instructions from your inverter manufacturer has told you. The terminal fuse linked is as good if not better than any class-T.
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Old 13-11-2017, 08:16   #7
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Re: Inverter 12V Fuse Question

Quote:
Originally Posted by smac999 View Post
Is this a 250a fuse for the whole bank? So now you have a 300a circuit and all your house loads off a 250a fuse?.... so when it blows you lose everything? you should seperate them. And have 2 fuses at the battery. Each with own cable leaving battery. Or it needs to be ~400a with correct cable. The inverter also needs its own switch.


You still need the class t. The mrbf will protect the wire. The clsss t will protect the inverter. All inverters request class t fuses.

To clarify the 250A fuse is for the entire bus. I do not regularly use the inverter and I don't have any requirements for anything near the max output capabilities. What is the purpose of a separate switch and fuse to the inverter? The inverter states that it has internal overload protection.
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Old 13-11-2017, 08:21   #8
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Re: Inverter 12V Fuse Question

Quote:
Originally Posted by AHoy View Post
I have a Freedom 25 Heart charger / inverter that was in place when I got my boat. I am rewiring the DC electrical system and need advice about fusing.

The installation manual for the charger calls out the following:
"Battery Cable Fusing
A fuse is required by the NEC to protect the battery cables. The fuse must be installed in the positive battery cable, within 18 inches of the battery. Recommended Fuse: Class T JLLN 300A."


The current installation has a Class T fuse located on the positive side of the inverter, not near the battery as recommended.

I have replaced the house bank and incorporated a blue sea battery terminal fuse (250A). Is this fuse sufficient for protection? I can't see why but is an additional fuse on the positive side of the charger lead needed?
Code and good wiring practice requires overcurrent protection as close as possible to the source of power. So, connecting a bilge pump, for example, the fuse would be close to the battery.

The cables from a battery charger to the battery are unique in that they are actually hot at both ends. If the fuse at the charger blows, the cable is still hot because it's connected to the battery. If the fuse at the battery blows, the cable is still hot because it's connected to the charger.

So - overcurrent protection is required at the charger (often internal) and at the battery. The fuses that connect directly to the battery terminals are ideal for this, they eliminate any unprotected cable.
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Old 13-11-2017, 08:25   #9
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Re: Inverter 12V Fuse Question

Fuses protect the wire behind. If you do not have a fuse at the battery and the insulation gets damaged somewhere a shortcut can burn your cable and vessel. So ALWAYS place a fuse in the positive wire at the SOURCE.

If you have an plain Inverter, a single fuse at the battery is sufficient.
If you have an Inverter / Charger you would have 2 fuses (one in or near the charger and one near the battery plus pole.)

The dimension of the fuse must mach the wire diameter. The thicker the wire the higher the Amps he can carry.

The internal fuses / safety circuits of the inverter are not sufficient to protect the wire.
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Old 13-11-2017, 12:09   #10
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Re: Inverter 12V Fuse Question

I don't think an extra fuse is needed at the charger. I can't find that in any installation instruction. Example; My ProNauticP has internal, easy to replace fuses. Fuses are needed at any battery positive terminal.
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Old 13-11-2017, 12:14   #11
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Re: Inverter 12V Fuse Question

fuses are not needed at charger end. alternator end etc. they are all self limited.
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Old 13-11-2017, 12:20   #12
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Re: Inverter 12V Fuse Question

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Originally Posted by AHoy View Post
To clarify the 250A fuse is for the entire bus. I do not regularly use the inverter and I don't have any requirements for anything near the max output capabilities. What is the purpose of a separate switch and fuse to the inverter? The inverter states that it has internal overload protection.
everything should be fused seperate for redundancy. or a main should be able to handle all normal loads.

if you "acidently" run too much off the inverter. and blow that 250a main battery fuse. you just lost your nav lights, vhf, etc. untill you figure out what happened and fix it. if you put 2 MRBF's on the battery. one for the house loads, and one for the inverter. you won't lose power to the whole boat if you blew the inverter one. plus you probably don't want a 250a fuse feeding your house bus as it's probably too big. depends if it can crank your engine or not. so each one is sized properly

also the switch is required as per ABYC. and is likly in the inverter manual as well. though that was is pretty old and rules change. like the chassis ground rules. which are likly different now that what your manual had. if defiantly can not be after the main house switch.
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Old 13-11-2017, 13:14   #13
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Re: Inverter 12V Fuse Question

I think there's something wrong with your picture, smac.

Suppose he replaces the 250A fuse with a 300A fuse. Ignore the wiring, pretend he got lucky and it doesn't need any changes.

THE 300A FUSE CAN STILL BE BLOWN WITH THE SAME RESULTS!

It doesn't really pay to argue whether the inverter alone can blow it, or whether a defect blows it, or whether anything is rated improperly. The fuse can STILL BE BLOWN.

And that's what happens, yes, when the primary battery fuse blows, you lose everything. Also known as "You protect everything."

If he doesn't have enough toys on board the boat to blow the 250A fuse, it certainly won't matter.

Now, on other hand, if everyone in his harem lights up a 1600 Watt hair drier at the same time, well hell, they just could blow any size fuse he can put in there. (Have you seen the size of that harem? And they ALL pack hair driers!)
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Old 13-11-2017, 15:13   #14
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Re: Inverter 12V Fuse Question

The Freedom 25 is an inverter charger combo. The manual (RTFM!) calls for a 300A fuse close to the battery.

This assumes he or someone else sized the wire properly.

One fuses for the load and one also fuses close to the battery for the wire size.

BlueSea website has tables for both amperage loads and wire size fusing.

:::::::::::::::
Blue Sea Tables https://www.bluesea.com/support/reference Scroll down to find the PDF download: Protect Your Boat with the Correct Size Wire & Fuse Chart. Handy to keep around.
:::::::::::::::::::::::

This is NOT that hard.
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Old 13-11-2017, 15:43   #15
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Re: Inverter 12V Fuse Question

I would fuse the battery with an MRBF (terminal fuse) - 300 amp - with a very short run to a heavy duty bus. From the bus I would run wires to the main switch, inverter, bilge pump(s), alternator, as well as any other charging sources. Wires should be sized for any maximum load they will see.

A switch is needed after the fuse to the inverter for service as the AC system will be energized without shore power!
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