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Old 04-03-2012, 08:55   #1
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General Electrical Design Cable Switch

You probably know, i am changing, redo ,the electical part of the boat
was i bit afraid of the spaghetty cable, when i bought the boat

What i have in board
2 engine
1 Bank battery
1 emergency Batery
1 breaker for the windlass
1 fuse for the Inverter
1 solar charger
1 batery monitor

I have remouved Few Cable from the batery to all the Swicth breaker (About 6 feet away)

I had
1 cable windlass to Swicth breaker then to heat breaker 1500W
1 Cable for the 2 Engine Switch 2*1000W
2 Cable House to the Switch
1 cable inverter to fuse to the inverter 1500W


So in total 5 cable Going to 4 Swicth breaker 1 on cable directly to the inverter
All the switch was conecter together too (input part only) all the output was made OK

As You don't start and use evrything at the same time
And all the cable was i think in double use

I have roumouved quite alot
What i have done

So i left all the swith conected
Just Keep two cable from the batery to all switch
the inverter was the bigest cable, now is going thought the switch
And i just keep another Cable Also going to the switch
So lets say i used all the Swicth as a bus Bar (perhaps i will make a coper plater instead of the cable conecting them together)

I have also remouved the switch from the windlass as they is just a breaker windlass next to him

I will use this switch plus another swith to connect the emergency baterry in paralell of the battery bank
So i can load or use it when i want (because i think this don't have to be in charge all the time)

I have mouved the inverter fuse next to the switch (About 6 feet away)
and just have read in the last threatd that the fuse must be very close from the batery
Mistake ??

I am planing to add 2 more fuse for the Alternator (55A alternator), as i think they never go this hight , i will put 50A ANL fuse
And i think i will add 2 more fuse on the two cable going to Switch (150 A)

For now it is working like this
And i know i will never start the two engine at the same time when the windlass is runing !!

What do you think
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Old 04-03-2012, 09:23   #2
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Re: General electrical design cacble Swicth

I would move your fuses to as close to the battery banks as possible. The ABYC says they should be within 7 inches, but that is almost impossible unless one uses the fuses that go right onto the batteries. Within one foot or so is about the best most of us can manage with heavy battery cables. I would also suggest the alternator fuse should be about 20% more than the maximum output of your alternator, so that should be around 70 amps or so. Make sure your cable from the alternator is big enough for at a minimum that rating, but better to go oversized. You don't want to take a chance that the alternator output will be cut off or you will surely burn out the alternator. I am not clear on how your wires go to and from your switches, but I will add that switches are not the best for serving as bus bars because inevitably the contactors corrode, the springs become looser, etc. Also, you need to have it set up so that is unlikely you will shut off the alternator output by accident.
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Old 04-03-2012, 09:49   #3
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Re: General electrical design cacble Swicth

i will try to put the fuse as close to the battery

the alternator cable out goes back directly to the battery not going thought any switch, only the starting engine cable goes thought a switch

I will also go for 20% more for the fuse alternator

philippe



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Originally Posted by Kettlewell View Post
I would move your fuses to as close to the battery banks as possible. The ABYC says they should be within 7 inches, but that is almost impossible unless one uses the fuses that go right onto the batteries. Within one foot or so is about the best most of us can manage with heavy battery cables. I would also suggest the alternator fuse should be about 20% more than the maximum output of your alternator, so that should be around 70 amps or so. Make sure your cable from the alternator is big enough for at a minimum that rating, but better to go oversized. You don't want to take a chance that the alternator output will be cut off or you will surely burn out the alternator. I am not clear on how your wires go to and from your switches, but I will add that switches are not the best for serving as bus bars because inevitably the contactors corrode, the springs become looser, etc. Also, you need to have it set up so that is unlikely you will shut off the alternator output by accident.
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Old 09-03-2012, 12:13   #4
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do not fuse the alternator

If the cable between the alternator and battery is sized sufficiently to carry maximum alternator current (as it should) do not fuse the alternator end. All alternators are current limited and, therefore, the battery end fuse is the only one needed in the case that the alternator end of the cable shorts to the negative.

I do not like any fuses between the alternator (when using proper installation techniques and cables) and battery yet realize that overzealous ABYC "recommendations" have become de-facto "requirements" for anyone carrying insurance.
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Old 09-03-2012, 12:27   #5
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Re: General electrical design cacble Swicth

A 50A fuse that installs on the battery post. The cable installs on the fuse. Very easy to do. Moderate price.
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Old 09-03-2012, 17:10   #6
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Re: General electrical design cacble Swicth

A conductor without a fuse or circuit breaker will become, what is called in the industry, a strip heater should a fault to ground occur.

The ABYC Standards requiring said over current protection devices make good sense, IMHO.
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Old 09-03-2012, 17:16   #7
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Re: do not fuse the alternator

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If the cable between the alternator and battery is sized sufficiently to carry maximum alternator current (as it should) do not fuse the alternator end. All alternators are current limited and, therefore, the battery end fuse is the only one needed in the case that the alternator end of the cable shorts to the negative.....
+1, this is an often misunderstood point about alternators.
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Old 09-03-2012, 18:29   #8
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Re: General electrical design cacble Swicth

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If the cable between the alternator and battery is sized sufficiently to carry maximum alternator current (as it should) do not fuse the alternator end. All alternators are current limited and, therefore, the battery end fuse is the only one needed in the case that the alternator end of the cable shorts to the negative.
This can be true, but on many boats the alternator line runs direct to the starter and from there a big cable goes to the main battery switch. Some people fuse this start line but the ampacity of that fuse must be big enough to handle the starting loads for a few seconds, often meaning a fuse as big as 300-350 amps. That could very well be a lot bigger than the alternator line can carry. In that scenario you might very well want a fuse near the alternator or possibly a fusible link as is used on cars. This is one reason I like a separate line from the alternator to the battery. Also, watch out with 1,2, Both battery switch set ups that there isn't a possibility that starting current will have to travel through the fuse rated for your alternator output.
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Old 09-03-2012, 18:37   #9
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Re: General electrical design cacble Swicth

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Originally Posted by Kettlewell View Post
This can be true, but on many boats the alternator line runs direct to the starter and from there a big cable goes to the main battery switch. Some people fuse this start line but the ampacity of that fuse must be big enough to handle the starting loads for a few seconds, often meaning a fuse as big as 300-350 amps. That could very well be a lot bigger than the alternator line can carry. In that scenario you might very well want a fuse near the alternator or possibly a fusible link as is used on cars. This is one reason I like a separate line from the alternator to the battery. Also, watch out with 1,2, Both battery switch set ups that there isn't a possibility that starting current will have to travel through the fuse rated for your alternator output.
Fair point but the fuse should then be sited at the junction of the big starter cable and the smaller alternator cable and not at the alternator end.
I agree with the separate cables (far better) and your other points.
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Old 09-03-2012, 18:43   #10
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Re: General electrical design cacble Swicth

When the alternator line goes direct to the starter I don't like the idea of a fuse there. You really need an ANL fuse in a line with possible direct connect to more than 1000 CCA, and how can you site an ANL fuse on the starter so that it won't be subject to excess strain from the connected cables, vibration, etc. Most starter installations are pretty tight on the motor, meaning there is very little room for a fuse in close proximity. It might make more sense to be at the alternator end of things in this scenario, or use a fusible link. Car manufacturers seem to think this is a suitable way to do things and a lot of cars have fusible links near the alternator.
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Old 09-03-2012, 20:54   #11
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Re: General electrical design cacble Swicth

It is a mistake to assume the maximum current in the alternator's armature (after rectification) output is current limited therefore no over current protection needed. There is a condition that will create large currents in that wire such as a short circuit as mentioned earlier by CharlieJ. That wire should be protected from fault conditions or risk fire.

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Old 10-03-2012, 00:29   #12
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Re: General electrical design cacble Swicth

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Originally Posted by foggysail View Post
It is a mistake to assume the maximum current in the alternator's armature (after rectification) output is current limited therefore no over current protection needed. There is a condition that will create large currents in that wire such as a short circuit as mentioned earlier by CharlieJ. That wire should be protected from fault conditions or risk fire.

Foggy
Do you think so; just how much extra current can you drag out of say a 80 amp alternator? All the alternators I have seen won't supply much more than their rated output, some much less.

If the alternator wire is sized to safety carry its rated current (and there are no other unfused sources connected to it) then IMO it is safe. The primary function of any fuse is to protect the wiring.
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Old 10-03-2012, 05:39   #13
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Re: General Electrical Design Cable Swicth

Frankly, I suspect the most common dangerous situation with the alternator line is when it falls off the back of the alternator due to vibration, or the line gets chafed through somewhere. Bottom line is that you are safer having appropriate fuses as close to the batteries as possible, and if you run a separate line from the alternator direct to the battery that is easily doable at a modest cost.
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Old 10-03-2012, 07:13   #14
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Re: General Electrical Design Cable Swicth

Well, I thought I understood this, but now there is a creeping doubt!
OK - I have heavy cable running direct from alternator output, to a fuse at the battery post. If the cable vibrates off the alternator end, and shorts out, the fuse prevents my having an uncontrolled arc welder at play in the engine room. I always thought this was enough.
But, what happens if the cable comes loose at the battery fuse end? Now the alternator is the source of current for that cable, and it might be putting out 100+ amps. In this case, should there not also be a fuse at the alternator? How does the fact that alternator is current-limiting protect me in this instance?
Like previous poster, I am not really happy with the one fuse I have in that circuit, since it has been drilled into me over a lifetime that the alternator should never look into an open circuit. - even less happy having fuses at both ends of that alternator wire! So, what does happen if the alternator is running and the battery end of the cable comes off? Diode failure, yes! But will the alternator current do any real damage before the unit fails?
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Old 10-03-2012, 07:52   #15
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Re: General Electrical Design Cable Swicth

Your point is very valid and the solution is relatively painless and will not cost you a great deal of $.

Post #5 shows a Marine Rated Battery Fuse (MRBF) and its associated holder mounted on a battery post. These are packaged by Blue Sea Systems from 30A to 300A and have a 10,000A AIC at 14VDC.

As an example; assume a 100A alternator with the output circuit to the house battery of 15 feet. Designing for < 3% voltage drop would yield AWG 2 conductors. Inside the engine room AWG 2 has 178A ampacity so a 175A MRBF at the battery terminal or B+ bus bar protects the conductor against a fault to ground in the alternator and a 175A MRBF on the alternator B+ output post protects against the failure at the battery end of the conductor. And, since the max output of the example alternator is 100A and the OCPD is 175A, nuisance opening of the fuses and the possible destruction of the alternator diodes will not be an issue.

Belt and suspenders, but conservative and doable.

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