Fuse on house battery bank?

[BozSail]

From all I've read, I should have a fuse on the house battery bank and it should ideally be placed within 7 inches from the positive battery terminal. Okay, so I'm wondering, why can't I install a properly sized circuit breaker instead of a fuse? So that if I trip the breaker, I can quickly reset it instead of the hassle of changing out the fuse (and having to carry spare fuses)?

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[Wotname]

Normally because it is difficult to source a suitably large circuit breaker that is affordable.

[transmitterdan]

Circuit breakers when used for DC applications have much less current interrupting capability. When you look at the specifications for fuses and circuit breakers there is the rated opening current such as 100A or 200A. That's what most of us look at. But there is another important rating known as short circuit interrupting rating. When a real short circuit happens the current can be very high especially in battery banks. If the current is higher than the interrupting rating then the device will not open and a fire could result. Generally circuit breakers have about 1/4 to 1/2 the interrupting rating of a quality fuse. For 12V applications the interrupting rating may have to be 10,000A depending on the size of the battery bank.

Read this and always check the short circuit interrupting rating of fuses and circuit breakers.

Circuit Protection

[a64pilot]

Question though, how big should the fuse be? If you fuse the engine starting circuit, that's gonna one big big fuse? I ask as I have 105 Amp 28V aircraft CB's, ought to be double that amperage at half the volts of course, is that big enough, too big?
I think though that a large fuse with a couple of spares mounted to a unused fuse block right beside the primary might be the ticket, that how we do our aircraft anyway.

[fallingeggs]

Out of interest, I have been looking at Blue Sea's website.

1) They have a 300 amp class C three pole breaker that runs $110. Has an interruption capacity of 5,000 amps at 65 volts (the only rating listing).

2) The fuse often used from Blue Sea is the terminal fuse. A 300 amp one runs $20-ish and the block is $30-ish. Has a interruption capacity of 5,000 amps at 32 volts and 10,000 at 12 volts.

I can't vouch for the needed interruption capacity, but the fuses are quite a bit higher in this regard. Probably almost twice as high.


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[transmitterdan]

ABYC does not require fuses on the starter circuit but it's a good idea. The size of the fuse is mostly based on the size of the wire after the fuse and not the current rating of the starter. So a rule of thumb is to find what is known as the "fusing current" of the wire. Then pick a fuse that opens at 10% of that value. Here is a fusing current chart:

Fusing Currents versus American Wire Gauge for copper, aluminum, iron, and tin (Cu, Al, Fe, Sn), wire melting currents

You can probably safely use a fuse at 15-20% of fusing current for starter circuits because starter service is very low duty cycle.

These are AWG sizes. The SAE wire sizes are smaller by about 15% but most boats don't have SAE sized wire.

[Wotname]

Adding to what Txdan first posted

It is possible for a circuit breaker to weld it's contact together and thus not interrupt the current if subjected to too large a current. This can't happen with a fuse.

[skipmac]

Originally Posted by a64pilot

Question though, how big should the fuse be? If you fuse the engine starting circuit, that's gonna one big big fuse? I ask as I have 105 Amp 28V aircraft CB's, ought to be double that amperage at half the volts of course, is that big enough, too big?
I think though that a large fuse with a couple of spares mounted to a unused fuse block right beside the primary might be the ticket, that how we do our aircraft anyway.

Blue Seas makes a terminal fuse with two outputs. Attaches directly to the battery terminal. I use one of these on the house battery with a smaller fuse feeding the DC breaker panel and a large one (200 amps or so, forget exactly) to the starter as a backup to the main starting battery.

[a64pilot]

Originally Posted by transmitterdan

ABYC does not require fuses on the starter circuit but it's a good idea. The size of the fuse is mostly based on the size of the wire after the fuse and not the current rating of the starter. So a rule of thumb is to find what is known as the "fusing current" of the wire. Then pick a fuse that opens at 10% of that value. Here is a fusing current chart:

Fusing Currents versus American Wire Gauge for copper, aluminum, iron, and tin (Cu, Al, Fe, Sn), wire melting currents

You can probably safely use a fuse at 15-20% of fusing current for starter circuits because starter service is very low duty cycle.

These are AWG sizes. The SAE wire sizes are smaller by about 15% but most boats don't have SAE sized wire.

Got it, your protecting the wire of course, not the applience

[noelex 77]

If the fuse ever blows you will likely have a major short and it is going to take some time to diagnose the problem. The extra time to replace a fuse is not likely to a major concern.

You also have to watch the "interrupt capacity" with a CB. At very high currents the CB (or even fuse) can fail to break the circuit. I think the ABYC require an Interrupt Capacity of at least 6,000 A. There are some CB that will meet this requirement, but many that will not. A large battery bank with a short in large cables can (briefly) generate very high currents and this is exactly the situation when you want the current reliably interrupted.

Edit: I see others have already mentioned the IC

[GordMay]

... I think the ABYC require an Interrupt Capacity of at least 6,000 A...

ABYC's I. C. requirements for DC systems up to 24 volts are as follows:
Battery Banks:
Up to 650 CCA - 1500 Amps
650 – 1100 CCA - 3000 Amps
Over 1100 CCA - 5000 Amps

[noelex 77]

Thanks Gord.
Our house batteries don't usually have CCA listed, but it looks like an IC of 5,000 A should be OK (until you install lithium )

[a64pilot]

Logically battery bank capacity should determine wire size, and wire size determine fuse size?

[Saltyhog]

Originally Posted by a64pilot

Logically battery bank capacity should determine wire size, and wire size determine fuse size?

The load on the battery bank should determine cable size. A windless may be supplied by a single 100AH battery, but require very large cable to supply the high current demands. A house bank may be comprised of many batteries to build a large AH capacity bank, but if the house loads are small, the cable may also be accordingly smaller. Cable size does determine the fuse rating. You're protecting the cable. But you also must select a fuse with a max current interrupt rating exceeding the short circuit capacity of the bank. This will be bigger for AGM's than for flooded bateries (and even bigger for Lithium).

[skipgundlach]

I see that this thread has migrated from house to start batteries.

Bringing it back, I have had experience with on-the-battery fuses. One (I have two x 2x6V) looked fine - but tested open.

Determined to make it simpler, I removed the fuse and bought a nice housing and fuse block. I'd also picked up a spare fuse at a SSCA Gam, years ago, and knew where it was, but I wanted to go up a bit, to 500A for the whole bank, so ordered the block/housing/500A package.

So, then I went looking for how I was going to integrate this into my positive terminal block, protected under what used to be a telco nid cover. Imagine my sheepish grin when I saw that there was already such a (not covered) terminal block, complete with 500A fuse. Imagine my embarrassment when I discovered yet ANOTHER spare fuse in a baggie, hanging from the 4" screw that held the cover on.

Basically, I want a load which is a great deal bigger than I'm likely to ever directly engage, but one in which a dead short will immediately blow. I've had the pleasure (long story about a third or lower tier charter) of doing some reconstruction after a battery fire (and battery explosion) caused by a dead short; I figured 500A would easily do the job.

Our battery bank is 740AH...

L8R

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