Fuse on house battery bank?

[S/V Alchemy]

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.

You may find this of interest. The size of the fuse is related to the size of the wires carrying the load, not just the cold cranking ampacity of the battery. Also, I follow MaineSail's lead (pun intended) on these matters.

[S/V Alchemy]

Originally Posted by a64pilot

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

Yes, basically. Read the Maine Sail link I posted: all is revealed! By the way, I compared his advice to that in some books I've read (Charlie Wing and Nigel Calder) and it checks out.

[Cadence]

Originally Posted by 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.

I would believe it is more likely a breaker would only be good for one interrupt and then would have crispy contacts as a result of arcing upon opening. Although I guess welding would be possible if a better gap is maintained than I can. I don't recall seeing DC relays weld but burnt contact are a norm..

[Wotname]

Originally Posted by Cadence

I would believe it is more likely a breaker would only be good for one interrupt and then would have crispy contacts as a result of arcing upon opening. Although I guess welding would be possible if a better gap is maintained than I can. I don't recall seeing DC relays weld but burnt contact are a norm..

I concur burnt contacts are the norm with DC relays although I have had a couple of welded relay contacts over the years .

For the OP.
The problem with the circuit breaker is that if the I.C. is exceeded, it is possible for the contacts to weld before the breaker manages to trip. Result is the breaker "trips" but to no effect as the contacts are already welded. All happens in milliseconds (or less) and then the next weakest link in the circuit fails .

As many have stated, fuses are way less prone to such issues.

[BozSail]

So, what size fuse should I use? I want to be able to switch to both the start and house banks if I need an extra oomph to start the motor, and if I use too low amperage on the house back fuse it will just blow when I hit the starter.

Sent from my XT1080 using Cruisers Sailing Forum mobile app

[Wotname]

Originally Posted by BozSail

So, what size fuse should I use? I want to be able to switch to both the start and house banks if I need an extra oomph to start the motor, and if I use too low amperage on the house back fuse it will just blow when I hit the starter.

Sent from my XT1080 using Cruisers Sailing Forum mobile app

OK, the short answer is read the links posted by others in this thread as the answers are in there .

The longer answer is "what size cable are you using to both your main house DC bus and to your starter motor? "
The answer to this question will allow us to give you an informed response.

While not entirely necessarily, it could be helpful to know what engine or starter motor you have.

[Stu Jackson]

Originally Posted by BozSail

So, what size fuse should I use? I want to be able to switch to both the start and house banks if I need an extra oomph to start the motor, and if I use too low amperage on the house back fuse it will just blow when I hit the starter.

Switching to "both" to get extra oomph means something is wrong. You should never combine banks. EITHER one bank or the other should be able to start your engine.

And the wiring from BOTH banks (one at a time) must be adequate to do so. If the wiring is adequate, then logically the fuses would be the same size.

And you size these fuses based on the wire size.

[mitiempo]

If you have separate switches for both house and start bank you have a dedicated start bank, which ABYC doesn't require to be fused. On the other hand if you have a 1/2/both switch either bank can be used for loads other than starting so both banks should be fused.

As posted wire size determines fuse size. I wire battery banks with 1/0 as a minimum, more often 2/0. I usually try to stay within the wire's ampacity but ABYC does allow a fuse up to 150%.

2/0 is rated at 281 amps in engine spaces so a fuse of 250 or 275 amps works and there should not be nuisance blows starting a diesel under about 75 hp.

[Cadence]

To the OP. Much has been said about determining the correct cable size and thus fuse size for your needs. Someone previously mentioned securing a couple of spare fuses right near the one in use. Don't let that get lost in the mix of the discussion. In a seaway trying to find a spare fuse isn't what you want.

[BozSail]

I know that a fuse size is based on wire size, for protection of the wire. I also know that I should have a fuse on the house bank. What I want is to be able to put my battery switch on "both" in the event I should need to have extra battery power to start the engine (just in case I need the extra juice - not expected under normal circumstances, but available if needed). And, I know the
current generated by the starter is quite high. So I want to fuse the house bank with a fuse that will not blow if the engine starter current runs through it.

Sent from my SM-T530NU using Cruisers Sailing Forum mobile app

[skipgundlach]

FWIW, in our setup, we normally run both (rather than engine alone) to start.

That kicks in the much higher short-time output of the start battery, and allows both to recharge from the alternator.

And, while I believe our switch would not break-before-make, if I never switch when it's running, I don't have to worry about it

[skipgundlach]

FWIW, in our setup, we normally run both (rather than engine alone) to start.

That kicks in the much higher short-time output of the start battery, and allows both to recharge from the alternator.

And, while I believe our switch would not break-before-make, if I never switch when it's running, I don't have to worry about it

[skipmac]

Originally Posted by BozSail

So, what size fuse should I use? I want to be able to switch to both the start and house banks if I need an extra oomph to start the motor, and if I use too low amperage on the house back fuse it will just blow when I hit the starter.

Sent from my XT1080 using Cruisers Sailing Forum mobile app

Use a dual terminal fuse block on the battery. One terminal use a smaller fuse and smaller wire, I use 50 amps, for feeding the DC panel. On the other terminal use a larger fuse and cable, 200-300 amps wired to a 1-2-Both switch that also connects to the starting battery and output to the starter.

This allows you to protect both systems correctly. The part to do this.

https://www.bluesea.com/products/215...k_-_30_to_300A

[mitiempo]

Originally Posted by skipmac

Use a dual terminal fuse block on the battery. One terminal use a smaller fuse and smaller wire, I use 50 amps, for feeding the DC panel. On the other terminal use a larger fuse and cable, 200-300 amps wired to a 1-2-Both switch that also connects to the starting battery and output to the starter.

This allows you to protect both systems correctly. The part to do this.

https://www.bluesea.com/products/215...k_-_30_to_300A

The only problem with this solution is the wire to the panel is unswitched.

A better solution is a single fuse on the battery post protecting the heavy (2/0?) wire leading to the switch. From the switch a heavy wire to the starter and a fused smaller wire (4 gauge?) to the panel.

[skipmac]

Originally Posted by mitiempo

The only problem with this solution is the wire to the panel is unswitched.

A better solution is a single fuse on the battery post protecting the heavy (2/0?) wire leading to the switch. From the switch a heavy wire to the starter and a fused smaller wire (4 gauge?) to the panel.

Sorry, for brevity I didn't post the entire wiring setup. Of course the wire to my panel is switched.

In your solution, a single very large 200 amps or so, must be used on the battery to feed the switch since it must carry starter current. Then you run a small cable to the panel from the switch but that cable is only protected by the large fuse on the battery so depending on the size of that cable, might not be protected in a short circuit situation.

Also, how would you set up that one switch? If an On/Off switch then the panel and starter would always be activated together. I can't think of a simple way a 1-2-Both switch would work in this configuration.