quick help with fusing for dual battery

[shmick82 Junior Member Find all posts View Profile Close shmick82]

Hi, just a quick question, regarding wiring of dual battery and fuses, Im using 6B&S cable ultimately terminating at 50amp anderson which will charge a thumper battery to run a fridge.

I will use a 50A circuit breaker on the thumper (aux) side of the setup.

However, i have the projecta 150A isolator which came with a 150A fuse box which i will use between the cranking battery and the isolator. is there any point to use 150A worth of fuse as i think the cable with die before the fuse? the fuse box has a 100A fuse and 2x25A fuses. should i remove the 100A fuse, or maybe remove the 2 25A fuses?

Also, out of curiosity, wont the 50A anderson plug act as a fuse itself eliminating the need for a fuse or circuit breaker at the aux side of the setup?

(note: the isolator came with 4 gauge cable but i dont need it (hopefully) so thats why its 150A fused).

[Guest Close Guest]

The fuse is there to protect the cable and connections. If the maximum current the cable can handle over the run is 50A, then it would be wise to put a 50A fuse/circuit breaker in.

An Anderson plug can act as a fuse.... Once it melts and burns down your vehicle, the current should stop flowing by then

[AK7 Senior Member Find all posts View Profile Close AK7]

It sounds like you've got a Fused Battery Distribution Terminal (BT950-P1). It's not fusing 150A between the battery and the isolator. It's fusing 100A and giving you two fused distribution points (25A each) that you can tap into with other devices. Call the 25A distribution points "optional extras". If you look at the fuse box closely, you'll see that the fuses are linked via a bus bar at one end, but the other end of each fuse is free-standing.

When too much current goes through cables, connectors or devices, things burn and melt. Hence circuit breakers and fuses exist. Fuses should always be rated for a lower current than the cable and connectors are rated for - meaning that fuses will blow first, protecting the other components.


Why would you not need the supplied 4B&S cable?

[RanJ Avid PP Poster! Find all posts View Profile Close RanJ]

The fuse is there to protect the cable and connections. If the maximum current the cable can handle over the run is 50A, then it would be wise to put a 50A fuse/circuit breaker in.

An Anderson plug can act as a fuse.... Once it melts and burns down your vehicle, the current should stop flowing by then

Don't quite agree with the first point there.... although the second point is very true haha

That cable will hang on and melt it's insulation way last 150. Closer to 1.5KA!

The 150A fuse is ideal as it is there to protect against a short circuit. Your 50A circuit breaker is to protect everything from overload. The 150 needs to be fitted in the engine bay as close to the power source as possible.

If you put in a 50A fuse there and another further down (circuit breaker) Then you have two potential points of voltage drop. Since the fuse wire is much smaller than the thickness of the cable it can act as a small "choke" on the power. By putting in the whopping 150 fuse, the choke is far smaller, but should the short circuit take place, the 150 will blow in a fraction of a second and protect everything further down the line.

My rule of thumb has always been that the main protection fuse should be at least 150% of your maximum expected current load. This will help prevent voltage drop over the length of the cable.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

One should keep in mind overloading of the cabling. In the event of a short circuit
having a 150A fuse is ok as it will most likley blow, however if you overload the cable
to say 225A then the fuse is unlikely to rupture before everthing else goes up in smoke.

In the event of an overload condition 50A cable is going to be compromised if you have 100A
flowing through it for some time.

This is the reason it is a good idea to fuse the cable at its maxium rated current, this does not
mean a 50A fuse for 50A cable more likely a 30A would be more appropriate. Keep in mind fuses
are primitive devices, it takes around 150% - 200% times the fuses rated current flowing for 10
seconds or so to rupture the fuse, a 50A fuse could maintain 75A for around 100 seconds or so
depending on its design characteristcs. I have tested many fuses that have maintained 250% of
their rated current continuosly for 30 minutes and have not ruptured!

If I wanted to protect a circuit for a maximum of 1A for example I wouldn't use anything higher
then a 500ma high percision fuse and I would still have doubts as to wether it would blow at 1A
unless I had tested at least 10 items in a batch to ensure they would blow at 1A!

[Guest Close Guest]

Thanks Leigh. Noted.

[RanJ Avid PP Poster! Find all posts View Profile Close RanJ]

One should keep in mind overloading of the cabling. In the event of a short circuit
having a 150A fuse is ok as it will most likley blow, however if you overload the cable
to say 225A then the fuse is unlikely to rupture before everthing else goes up in smoke.

In the event of an overload condition 50A cable is going to be compromised if you have 100A
flowing through it for some time.

This is the reason it is a good idea to fuse the cable at its maxium rated current, this does not
mean a 50A fuse for 50A cable more likely a 30A would be more appropriate. Keep in mind fuses
are primitive devices, it takes around 150% - 200% times the fuses rated current flowing for 10
seconds or so to rupture the fuse, a 50A fuse could maintain 75A for around 100 seconds or so
depending on its design characteristcs. I have tested many fuses that have maintained 250% of
their rated current continuosly for 30 minutes and have not ruptured!

If I wanted to protect a circuit for a maximum of 1A for example I wouldn't use anything higher
then a 500ma high percision fuse and I would still have doubts as to wether it would blow at 1A
unless I had tested at least 10 items in a batch to ensure they would blow at 1A!

For sure... and his overload protection is the 50A breaker at the other end of the cable. I wouldn't suggest a 50A fuse at one end and then another at the other end. You could just whack the 50A breaker in the engine bay and be done with it. But if you want the breaker at the rear then you must have some form of protection down at the source.

Lastly, there is no question about the 150A blowing in the event of a short circuit... It wouldn't stand a chance. In high powered car audio, we often run amplifiers that have no fuse at the amp, only on the supply cable. During a SPL competition we often destroy a subwoofer voice coil and the coil becomes a dead short. When this happens, we instantly blow both 300A fuses. No countdown.... just a quick flash from the fuse block and then gone. Inspection of the fuse shows there was no meltdown... just vaporised lol

[Guest Close Guest]

I was told a breaker/fuse at both batteries.

[Symo Advanced Member Find all posts View Profile Close Symo]

I have several fuses on my setup:

From starter battery >150A fuse then 1.2m charge cable to isolator > then 150A from isolator to aux batt > then the 6 B&S to rear which has two 20A Cig plugs starts at the aux batt with a 75A Breaker > then 20A fuse at each plug > then each device plugged in is fused with 5-10A fuses.

My thinking is that high currents can be tolerated and I won't melt the rear plugs in a short, or if the main power run to the rear shorts (rubs through) the breaker with pop before the cable melts. Plus any short in the charging circuit is protected on each cable.

[joel231 Find all posts View Profile Close joel231]

I was told a breaker/fuse at both batteries.

yup your right deffinately fuse at both ends, if you get a short anywhere in the cable it can backfeed from the unfused battery

Cheers Joel

[RanJ Avid PP Poster! Find all posts View Profile Close RanJ]

I was told a breaker/fuse at both batteries.

There is two... 150s at one end and 50A at the other end.

Reality is, there are many ways to skin a cat.

When I look at all three suggestions (amts, LeighW and myself) I really can't find fault with any of them. They'll all work sufficiently and all have their pros and cons.

So...Dermis and feline can be divorced by manifold methods.

Whatever works best for you and keeps your butt covered.

[shmick82 Junior Member Find all posts View Profile Close shmick82]

Thanks for all the info guys. So my take is pretty much leave the 100A (with its 25s) up front and have the circuit breaker in the back (with or without additional fuse). I suppose 100A fuse up front wont be too big for 6B&S cable.
Regarding the 4B&S cable I dont want to use - there was opinions that a 6B&S cable would be sufficient not to have too much voltage drop and much easier to run and use than 4B&S.

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi RanJ, sorry mate but a 150 amp fuse in a dual battery circuit using 6B&S cable, is way to big.

The maximum safe rating for 6B&S cable is 60 amps, and it is set at that rate for exactly what Leigh posted about.

In a dual battery setup, you are far more likely to suffer a system overload problem than you are from a dead short.

The most common cause of overloading is when one of the batteries in the system, drops a cell and shorts internally. This is not a dead short but it can cause huge current draws. This can easily push the cable currents above the continuos safe load for a given cable size.

So again, the maximum current rating for 6B&S ( 13.5mm2 ) being used in a dual battery system is 60 amps, if the cable is 8B&S ( 7.9mm2 ) then it is 40 amps, and 6mm AUTO ( 4.5mm2 ) is 25 amps.

[RanJ Avid PP Poster! Find all posts View Profile Close RanJ]

Hi RanJ, sorry mate but a 150 amp fuse in a dual battery circuit using 6B&S cable, is way to big.

The maximum safe rating fore 6B&S cable is 60 amps, and it is set at that rate for exactly what Leigh posted about.

In a dual battery setup, you are far more likely to suffer a system overload problem than you are from a short circuit.

The most common cause of overloading is when one of the batteries in the system, drops a cell and shorts internally. This is not a dead short but it can cause huge current draws. This will push the cable currents above the continuos safe load for a given cable size.

So again, the maximum current rating for 6B&S ( 13.5mm2 ) being used in a dual battery system is 60 amps, if the cable is 8B&S ( 7.9mm2 ) then it is 40 amps, and 6mm AUTO ( 4.5mm2 ) is 25 amps.

150A would be way too big if it was the only line of protection in the circuit.

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi RanJ, the size of the cable determines the maximum safe size of the protection device.

With your suggested set up, only the cable between the 50 amp circuit breaker at the Thumper and the battery in the thumper would be properly protected.

I understand what you are stating but as the maximum continuos load for 6B&S cable is 100 amp, the 150 amp CB gives no genuine safety protection.

Why not just fit the correct safety device in the first place?