DC Wiring Question - Fuse Placement

[KISS]

I've read that you should put a fuse after the positive bus bar, because it's considered a power source.

...I don't understand this. How is the buss bar a power source?

If you already have a fuse after the positive terminal of the battery, why have another down the line?

Any insight?

[toddster8]

Presumably, branch circuits leading away from the busbar are of lower gauge or longer length, and require additional protection. Even if not, I like to have breakers to isolate each branch without powering down the whole boat.

[rwidman]

Originally Posted by KISS

I've read that you should put a fuse after the positive bus bar, because it's considered a power source.

...I don't understand this. How is the buss bar a power source?

If you already have a fuse after the positive terminal of the battery, why have another down the line?

Any insight?

I don't understand the question. You need to provide a lot more information if you want a meaningful answer. You might do best to buy a good book on boat electrical wiring.

[KISS]

Originally Posted by rwidman

I don't understand the question. You need to provide a lot more information if you want a meaningful answer. You might do best to buy a good book on boat electrical wiring.

The question is whether it's necessary to have fuses on the wires exiting the positive bus bar, if you already have a fuse on the wire exiting the positive terminal of the battery.

What other information would you like?

[old frog]

Understanding that you are describing a situation where the the battery feeds a buss bar and that circuit has over current protection (fused). From that positive buss are sub (branch) circuits and your question is why over-current protection is needed on the sub-circuits.

The primary over-current (fuse) protects the circuit from the battery to & including the buss. The sub-circuit needs it's own over-current protection as, typically, the sub-circuit conductors are smaller gage & loads and due not have the capacity of the larger buss feed circuit.

The over-current protection (fuse) in the buss feed circuit should be sized & protect according to that entire circuit, conductor & loads. The sub-circuit, needs to be sized and over-current protected for it's run and loads.

Hypothetical Example: A 00 battery cable feeds a buss bar. The buss bar has 3 sub-circuits & each of these sub-circuits has different size conductors, (i.e., 1 - 12 ga, 1 - 14 ga. & 1 - 16 ga), has of different length and loads. Each has different requirements.

In response to your more specific question: How is the buss the power source? The battery is the power source for the primary circuit. The buss is the power source for the sub-circuits.

ABYC & NFPA are the standards. There is within them, something referred to as the 7', 40" & 72" rule within which the over-current protection needs to be present.

[KISS]

Originally Posted by old frog

Understanding that you are describing a situation where the the battery feeds a buss bar and that circuit has over current protection (fused). From that positive buss are sub (branch) circuits and your question is why over-current protection is needed on the sub-circuits.

Precisely

The primary over-current (fuse) protects the circuit from the battery to & including the buss. The sub-circuit needs it's own over-current protection as, typically, the sub-circuit conductors are smaller gage & loads and due not have the capacity of the larger buss feed circuit.

The over-current protection (fuse) in the buss feed circuit should be sized & protect according to that entire circuit, conductor & loads. The sub-circuit, needs to be sized and over-current protected for it's run and loads.

Hypothetical Example: A 00 battery cable feeds a buss bar. The buss bar has 3 sub-circuits & each of these sub-circuits has different size conductors, (i.e., 1 - 12 ga, 1 - 14 ga. & 1 - 16 ga), has of different length and loads. Each has different requirements.

In response to your more specific question: How is the buss the power source? The battery is the power source for the primary circuit. The buss is the power source for the sub-circuits.

ABYC & NFPA are the standards. There is within them, something referred to as the 7', 40" & 72" rule within which the over-current protection needs to be present.

If that's the practice, no problem, it's not like fuses are expensive, but I still don't quite understand. The sub-circuits can't be overloaded unless the main circuit is overloaded, right? And if the main circuit is overloaded, then the fuse on the main circuit has already blown, and sub-circuits have zero load, no?

[smac999]

It depends how big the cables are, what the load size is. How big the feed cable is. And how big the main fuse is


If you have a 4/0 feed cable with a 400a anl feeding the bus. And four 6 gauges comming off. Obviously they all need to be fused again at 100a or less. The max of a 6awg. Otherwise you can pull say 300a down one 6awg and the main won't blow. And that 6awg is now melting and burning

If you a 6awg feeding a bus with a 100a main fuse. And four 6awg leaving. Then the 4 leaving wires are fused ok from the main one. However if something happens in one circuit and blows the main fuse you just lost power on the whole boat. Vs if you fused each branch at say 75. Only one branch would blow. And everything else will still be working.

So there is the safety side of having wires fused correctly . But also reliablity if something happens. You can't have one bad device take down the whole boat

[old frog]

Assume a 200A fuse in the primary 00 battery circuit to the buss. But, feeding from the buss is the sub-circuit with 16 ga. conductor. At maximum, the 16 ga. conductor has a capacity of 10A. Appropriate protection is needed.

The 200A primary fuse does not adequately protect the smaller circuit.

[Frankly]

The battery feed is on the bottom and is protected with a 250 amp class T fuse. Other fuses are IAW wire size and load.

For some reason the image is not flying.

[Maine Sail]

Originally Posted by KISS

The question is whether it's necessary to have fuses on the wires exiting the positive bus bar, if you already have a fuse on the wire exiting the positive terminal of the battery.

What other information would you like?

Let's say you have a 2/0 wire from the battery and a house bank fused at 300A. To keep the battery terminals clean and uncluttered you make a 12" 2/0 wire jumper over to a busbar, a very common practice. You now have a busbar with:

6GA battery charger feed
4GA alternator feed
10GA bilge pump load
2/0 wire to battery switch

The only wire that does not need an additional fuse is the 2/0 wire going to the battery switch.

You can't adequately protect the 6GA wire, the 4GA wire & the 10GA wire with a 300A fuse.

Any time you step down in wire gauge, the new wires will need over-current protection based on that wires safe ampacity. Usually the biggest violator of this is battery switches but busbars & battery posts are another..

If the fuse at the battery is sized for the smallest wire for charging or loads off the bank, then you're okay. In this case you'd need to fuse the house bank based on the 10GA bilge pump wire. Of course if you ever need to start a motor or have a load larger than the 10GA fuse, the fuse would just go pop if not sized for the vessels largest load.

[KISS]

Originally Posted by old frog

Assume a 200A fuse in the primary 00 battery circuit to the buss. But, feeding from the buss is the sub-circuit with 16 ga. conductor. At maximum, the 16 ga. conductor has a capacity of 10A. Appropriate protection is needed.

The 200A primary fuse does not adequately protect the smaller circuit.

Now I think I get it - a load that fries the sub-circuits may not be high enough to blow the fuse on the main circuit. It all depends on the ratings of the main circuit compared to the rating of the sub-circuits (bigger the gap, more likely you need fuses for the sub-circuits). I suspect the small system I'm envisioning (fuse on the main circuit would probably be only about 45amps) won't require fuses on the sub-circuits, but I'll have to crunch the numbers.

[KISS]

Originally Posted by Maine Sail

Let's say you have a 2/0 wire from the battery and a house bank fused at 300A. To keep the battery terminals clean and uncluttered you make a 12" 2/0 wire jumper over to a busbar, a very common practice. You now have a busbar with:

6GA battery charger feed
4GA alternator feed
10GA bilge pump load
2/0 wire to battery switch

The only wire that does not need an additional fuse is the 2/0 wire going to the battery switch.

You can't adequately protect the 6GA wire, the 4GA wire & the 10GA wire with a 300A fuse.

Any time you step down in wire gauge, the new wires will need over-current protection based on that wires safe ampacity. Usually the biggest violator of this is battery switches but busbars & battery posts are another..

If the fuse at the battery is sized for the smallest wire for charging or loads off the bank, then you're okay. In this case you'd need to fuse the house bank based on the 10GA bilge pump wire. Of course if you ever need to start a motor or have a load larger than the 10GA fuse, the fuse would just go pop if not sized for the vessels largest load.

Thanks

I think I understand it now.

See my response above to old frog.

[old frog]

In the event you have not come across it yet, Blue Seas has an easy to use wire size calculator: Circuit Wizard - Blue Sea Systems

Remember, on a DC load, the conductor length is from power source to load & return (out & back).

Pay special attention to DC motor circuits that might be susceptible to locked rotor conditions (ie. bilge pump). Proper over current protection sizing is critical to protect both the circuit and the load).

[KISS]

Originally Posted by old frog

In the event you have not come across it yet, Blue Seas has an easy to use wire size calculator: Circuit Wizard - Blue Sea Systems

I just recently discovered that, as it happens.

Very useful indeed

Remember, on a DC load, the conductor length is from power source to load & return (out & back).

Yep

Pay special attention to DC motor circuits that might be susceptible to locked rotor conditions (ie. bilge pump). Proper over current protection sizing is critical to protect both the circuit and the load).

Will do, thanks

At some point I'll probably post a full diagram for the forum experts to review.

[Stu Jackson]

Wire Sizing Charts from West Marine http://www.westmarine.com/WestAdviso...e-And-Ampacity www.bluesea.com also has good material on this and fusing, but it's a PDF download and doesn't link. Here's the link to the page, scroll down for the PDF download of the Fuse & Wire Sizing Tables: https://www.bluesea.com/support/reference

The power source is the battery.