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?

[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

[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 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.

[Chuck Hawley]

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?

I think you've given enough information. I think that, ideally, you'd fuse (or use a circuit breaker) on every wire leading to loads from the bus bar. The idea is to protect the wire leading to and from the load from over-current. Let's say that you've run a 6 gauge wire to the main distribution panel. That wire has a certain ampacity (I think 100A, but I am not entirely sure), so you need to protect the wire from loads over 100A.

I have a 100A fuse on the terminal of each of three Gr 27 batteries in parallel, to protect one of them from going all "China Syndrome" on me if a cell fails. But together they have the ability to conduct 300A of current, which is way too high for auxiliary circuits. So I have to fuse those circuits (panel feed wire, electric winch, inverter).

Cheers,

Chuck

[rwidman]

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?

Of course it's necessary to have fuses in the circuits leaving the bus bar. Each circuit needs a fuse sized for the current carrying capacity of the wire.

There needs to be a fuse at the beginning of the wire that supplies power to the buss bar.

If you don't understand the purpose of fuses in circuits (circuit breakers serve the same function), you shouldn't be doing electrical work on your boat or anywhere else.

[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?

[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.

[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.

[Golden Echo]

I don't know if this will help. Let's say the main from Battery to Buss is a 1/0 cable. ABYC best practices states this should be fused within so many inches from the battery. So a 1/0 cable is rated for (outside of engine spaces) 285 amps. "The size of the fuse or breaker is determined by the sum of all loads on the circuit or by the current-carrying capacity of the smallest wire." (Ref: Sailboat Electrics Simplified; Don Casey). So let's say you fused that 1/0 cable at 200 amps. Then you connect it to your buss bar and connect a bilge pump to it. The wire you use for the bilge pump is 16 AWG. You decide not to fuse it because, Hey it's already fused. Max amp rating for 16 AWG is 25 amps. Your bilge pump is only using 15 amps so you think it's ok. Next thing you know there is some sort of short on the 16 gauge wire and the wire gets red hot and starts to melt and setting things on fire because the battery fuse allows the full 200 amps from the battery through the wire.

For how cheap in-line fuses are it's just great peace of mind to add one on the buss bar.

The amp ratings for the wire above is for BC5W2 wire.