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Old 09-04-2017, 10:33   #31
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Re: DC Wiring Question - Fuse Placement

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Originally Posted by KISS View Post
Precisely



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?
False.

The main feed between the battery and buss will have a max current carrying capacity X, depending on the size of wire and buss capacity. Each branch circuit will have a mac current carrying capacity, depending on the size of wire. Branch circuit capacity is usually significantly less than the main feed. The advantage of branch over current protection, is that the branch fuse blows in the event of a fault, leaving the other branches fully operational. This is why it is important to have all critical equipment on independent branches. (So you lose the lowest possible amount of critical equipment, in the event of a fault.)
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Old 09-04-2017, 10:55   #32
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Re: DC Wiring Question - Fuse Placement

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False.

The main feed between the battery and buss will have a max current carrying capacity X, depending on the size of wire and buss capacity. Each branch circuit will have a mac current carrying capacity, depending on the size of wire. Branch circuit capacity is usually significantly less than the main feed. The advantage of branch over current protection, is that the branch fuse blows in the event of a fault, leaving the other branches fully operational. This is why it is important to have all critical equipment on independent branches. (So you lose the lowest possible amount of critical equipment, in the event of a fault.)
A bus (i.e. bus bar) has no real life effective current limit. Manufacture ratings are very subjective and use of differebt rated bys bars has no appreciative difference in system performance.

Branch circuit isolation is important to target in a system design but only achieved if you have what it called "electrical coordination". This requires "upstream" circuits closer to the power source to be of larger gauge and/or have larger fuse values. Since boats have a typical min wire gauge for power circuits for physical strength reasons, often people will fuse for the wire and not less than that resulting in sub branch circuits with the same fuse value as the feed. In that situation it would not be clear which fuse (feed or branch ) would blow first during an overload event.
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Old 09-04-2017, 12:41   #33
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Re: DC Wiring Question - Fuse Placement

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The feed is on the bottom via a 250 amp class T from the battery switch. All other smaller circuits feeding from the bus are sized/ fused based on load requirements.


This should be done with a circuit breaker panel, not fuses.
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Old 09-04-2017, 12:59   #34
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Re: DC Wiring Question - Fuse Placement

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This should be done with a circuit breaker panel, not fuses.
//scratching head//

why?
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Old 09-04-2017, 13:29   #35
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Re: DC Wiring Question - Fuse Placement

PV does not have the big picture. There are 30+ circuit breakers (5 to 20 amps) and probably a dozen fuses downstream of what is visible in that photo.
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Old 09-04-2017, 13:45   #36
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Re: DC Wiring Question - Fuse Placement

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PV does not have the big picture. There are 30+ circuit breakers (5 to 20 amps) and probably a dozen fuses downstream of what is visible in that photo.
Sorry Frankly, I stand corrected, although if you have circuit breakers, why do you also need fuses? Either one will protect the wire against overloading; to use both adds additional connections and complexity.

Circuit breakers are more expensive and require an enclosure and panel but can act as an on/off switch and can be reset.

I was taught that you use OPBs (circuit breakers) to protect the circuit (wire) and fuses to protect the device. Many devices are serviced by wires with much higher ampacity then their rated load.

Fuses are simpler, easier to install, don't wear out, but are subject to error (wrong size when replacing), and can be difficult to replace.

This discussion, in general, is a bit over-done as all these wiring subjects are well-covered by ABYC and many wiring books (e.g. Nigel Calder).
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Old 09-04-2017, 14:05   #37
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Re: DC Wiring Question - Fuse Placement

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Sorry Frankly, I stand corrected, although if you have circuit breakers, why do you also need fuses? Either one will protect the wire against overloading; to use both adds additional connections and complexity.

Circuit breakers are more expensive and require an enclosure and panel but can act as an on/off switch and can be reset.

I was taught that you use OPBs (circuit breakers) to protect the circuit (wire) and fuses to protect the device. Many devices are serviced by wires with much higher ampacity then their rated load.

Fuses are simpler, easier to install, don't wear out, but are subject to error (wrong size when replacing), and can be difficult to replace.

This discussion, in general, is a bit over-done as all these wiring subjects are well-covered by ABYC and many wiring books (e.g. Nigel Calder).
For protecting, either works fine. Breakers offer combined switching but at much higher total cost. A well designed system will not require a breaker to be reset or a fuse to be replaced but load devices can fail and wires can chaffe on grounded edges. Those should both be rare and require more than a quick reset of a breaker to solve.

The pic I see shows upstream higher amp fuses likely feeding branch panels. Personally I have no issues keeping some spare fuses and managing the unlikely chance of a blow and would not hesitate to have only fuses on my boat. I understand the penchant for 'total control' of everything that a huge breaker panel provides but also appreciate the elegance of a well designed and installed system that doesn't expect the operator to switch on and off dozens of control switches thoughout the day and warn guests/kids if they get too close to the panel. To each their own.
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Old 09-04-2017, 17:18   #38
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Re: DC Wiring Question - Fuse Placement

What is visible in that picture is the higher amperage end of the power distribution system. You can probably read the feeds if you stand on your head in front of the computer. Those ANL fuses range from 25 amps (HF Radio) to 200 Amps for the Yanmar starter. Just no real need for any on/off function.

The BS ATO fuse block tapped from that bus bar feeds things like the Balmar regulator, Panel Voltmeter, Emeter reference. Again things that just need no switching function (switch would be a detriment in MO). There is another bigger ATO fuse block behind the IP Factory Panel that feeds the stereo, TV, TV antenna Preamp, 4 cig lighter type charging receptacles (2 are dual USB charging stations), and a bunch of other low amperage stuff. Fuses are less expensive, more compact, and in many ways more reliable. There are 4 other circuit breaker panels around the ship feeding nearby loads and the original 14 circuit Factory IP panel.

I came to this arrangement after 44 years doing this type of stuff for the US Navy and in the commercial sector. Lots of different ways to skin this cat, this is just my way.

The OP asked the question about why needing fuses at the bus bar when you have a main fuse. My first reaction is if you are that ignorant about electricity, probably got no business even being on a boat. But if a few minutes of my time can help educate someone then glad to pitch in.

Electrical things have a way of being beaten to death on these forums but just way things are. Know what they say about opinions being like as*holes. Everybody has one.
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Old 09-04-2017, 19:41   #39
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Re: DC Wiring Question - Fuse Placement

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https://www.bluesea.com/support/arti...uit_Protection

The above article from Blue Sea points out the issues with very slow blow ANL fuses. The problem is more pronounced in the smaller rated fuses but is an issue with the entire line. I try to minimize this with under sizing the fuses and have not had any nuisance issues to date.

In the same article they also visit the issue of overload vice fault protection. In my mind with the exception of motor circuits the fault protection is the most concerning, and this not only the wiring protection but protection of the batteries. Not interested in running the experiment on what happens when you subject a 600 to 1000 AH battery bank to high amp fault conditions. That is a place where a fast acting class T might "make/ save your day".
The same issues you point out above make ANL fuses excellent for starting circuits or any other high load circuit such as a windlass or bow thruster. Nuisance blows should never happen if the wire size is large enough. As long as the correct wire size is chosen overload isn't much of an issue. With that being the case the fuse is there primarily for short circuit protection. The issue of ANL fuses being slow to blow is for overload, not short circuit situations. They blow very quickly in the case of a short.

I use ANL or MRBF fuses for main battery and heavy load short circuit protection. I can't remember the last time I used one of these rated less than 200 amps.
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Old 10-04-2017, 07:29   #40
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Re: DC Wiring Question - Fuse Placement

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Originally Posted by camel2012 View Post
A bus (i.e. bus bar) has no real life effective current limit. Manufacture ratings are very subjective and use of differebt rated bys bars has no appreciative difference in system performance.

Branch circuit isolation is important to target in a system design but only achieved if you have what it called "electrical coordination". This requires "upstream" circuits closer to the power source to be of larger gauge and/or have larger fuse values. Since boats have a typical min wire gauge for power circuits for physical strength reasons, often people will fuse for the wire and not less than that resulting in sub branch circuits with the same fuse value as the feed. In that situation it would not be clear which fuse (feed or branch ) would blow first during an overload event.
Commercially available marine bus bars have a max current rating.

Reference: Blue Sea

https://www.bluesea.com/products/category/18/BusBars

A properly designed and wired marine electrical distribution system will always have a feed current rated greater than the highest branch current.

ABYC E11 AC & DC Electrical Systems on Vessels, Section 11.8 identifies how to calculate the load represented by a panel board (DC distribution panel) to size the feeder wire and 11.9 for overcurrent protection.

Basically, the distribution panel and feeder wire must be capable of handling the sum of the continuous loads (irrespective of branch circuit wiring size) plus 10% of the sum of the intermittent loads (or the highest intermittent load, whichever is greater).

Only if all loads were miniscule, could the feeder wire the same size as the branch wires be valid.

Therefore, it is possible for the feeder wire to be correctly sized the same as the branch circuits, but in practice, extremely rare, and in most cases, would likely be a wiring error.

An exception that comes to mind is a 4 or 6 circuit fuse block to low power instruments from an "instrument" breaker. However, in that case, the fuse block fuses all have to be rated lower than the "instrument" breaker feeding the fuse block.

Anything else is a wiring error.

Two overcurrent protection devices of the same rating in a single series circuit, (or a load fuse larger than a branch fuse) is a wiring error.

Individual overcurrent protection devices, rated to the component they are attached, being less than the branch circuit over-protection, is valid, and should be done to prevent other parallel circuits of the branch from being disabled in the event of a single device fault.
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Old 10-04-2017, 08:06   #41
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Re: DC Wiring Question - Fuse Placement

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I was taught that you use OPBs (circuit breakers) to protect the circuit (wire) and fuses to protect the device. )
What is your training? (Ie. Taught by whom and to what standard?)

Quote:
Originally Posted by pacific_voyager View Post
This discussion, in general, is a bit over-done as all these wiring subjects are well-covered by ABYC and many wiring books (e.g. Nigel Calder).
Please show me where ABYC, or any marine electrical "wiring book", specifies "breakers for branches vs fuses for loads".
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Old 10-04-2017, 08:21   #42
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Re: DC Wiring Question - Fuse Placement

I don't think you'll find any.


Fuses are "preferred" from a safety perspective, breakers are for convenience.


Why? Fuses fault (fail) in open position...breakers _usually_ fault (fail) closed.


A lot of people think that circuit breakers (whether fuses or breakers) are to protect equipment. This isn't true, it's to protect your wires to prevent wires from overheating and potentially starting a fire.


If you have a 100 amp main lead to a bar, and you have 5x 20 amp branches coming from that, you want to protect your 20 amp branches from _potentially_ carrying up to 100amps due to a short somewhere.


Remember, the breaker protects the wire, not your equipment...it is wrong to think of a breaker being used to protect equipment.... yes it can depending on what the wire, fuse and equipment is rated for, but that's not why we use them. If you think in terms of protecting your wire from getting hot, you will never make a mistake in choosing where to put what rating of breakers.


If you have a piece of equipment you want to isolate with surge protection or shorting, then isolate it AFTER installing fuses and breakers on the branches and give it a fuse that will break before the surge will hurt the equipment, but then you will have a fuse at the bar to protect the wire, AND you will have a different fuse at the equipment to protect the equipment. The fuse at the equipment will blow to protect the equipment, but if there is still a short somewhere in the branch circuit, you still need the fuse to protect the branch from overheating and catching fire.
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Old 10-04-2017, 08:53   #43
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Re: DC Wiring Question - Fuse Placement

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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.
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Old 10-04-2017, 09:03   #44
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Re: DC Wiring Question - Fuse Placement

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What is your training? (Ie. Taught by whom and to what standard?) ..............................
That is a good question and one that should be asked of many folks providing "electrical advice" on boating forums.
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Old 10-04-2017, 09:12   #45
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Re: DC Wiring Question - Fuse Placement

System coordination (IE over current device closest to the load opening) is not easy in commercial power distribution systems and the device selection is probably a hundred times greater than the very limited number of devices that are used in the small craft marine field (breaking DC Voltages doesn't help the situation either).

Just because I have a 100 amp main breaker and a 20 amp equipment breaker and the equipment develops a short (note short vice an overload) does not guarantee that both breakers or fuses will not open. Coordination is desirable but may not always work out. Often I have seen fuses partially melt only to open the next time they are stressed.

But as a general rule, a bigger ratio will provide highest probability of the desired outcome (the fault problem will be cleared and the remainder of the system still left operational). Trip curves (for both fuses and breakers) can help but still the choice is limited in the boat business.
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