DC Wiring Question - Fuse Placement

[Frankly]

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.

[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.

[JOHNMARDALL]

Originally Posted by KISS

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?

Sorry, but not right. Ycu could have three sub-circuits each wired for 15 amps, with the supply from the battery to the bus bar wire and fused for 50 amps. If sub A was carrying 25 amps, sub B 10 amps and sub C 10 amps, sub A would be overloaded but the bus supply would only be carrying 45 smps and therefore not overloaded. WIth only a main fuse, that fuse would not blow but the wiring in Sub circuit A would be damaged.

All the best
John Mardall
Vetus Maxwell Group

[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

[Adelie]

The advice I read in CWing's or NCalder's books was to:
A] fuse the positive battery cable near the battery so that:
1] If you accidentally shorted in the breaker panel you didn't start a fire or fry the battery.
2] You didn't overload the battery cable if the sum of the sub-circuit capacities exceeded the capacity of the battery cable.
B] Breaker or fuse each sub-circuit at the panel for the capacity of the line.
C] Fuse some of the sub-circuits (electronics, ?) near the load to prevent power surges that would damage the item but wouldn't exceed the line capacity. This would be on common circuits that carried multiple loads like instruments.

I would only do C] for electronics, but there might be additional things if I thought about it.

[ayates]

Does anybody have views on Class T vs ANL fuses for house battery bank fusing?


Allan.

[Frankly]

While most of the focus on fuse/ wire discussions tends to be on overload conditions an equally likely problem is short circuit or ground fault problems. This is where you have a low resistance short with currents measured in hundreds or thousands of amps. For modest size battery banks then type ANL fuse are considered suitable, but for big battery banks and some of the newest chemistry batteries then the available fault current may exceed the interrupt rating of ANL fuses and require class T.

Because the class T fuses are more expensive, it often works out to just use a class T for the main fuse and then ANL for the smaller feeds. Now under high fault conditions they will both open and safely protect the batteries and wiring. Only issue with this is now you have lost your entire electrical system.

Life is full of all kind of trade offs and power distribution is no different.

[Stu Jackson]

Originally Posted by ayates

Does anybody have views on Class T vs ANL fuses for house battery bank fusing?


Allan.

Yes, folks may have "views" on it, but I suggest going to the BlueSeas website and finding out from experts what their conclusions, not views, are. They know better.

[old frog]

With regard to over-current placement. Screen shots of NFPA 302 are below (to me, they are an easier read than ABYC which is much the same. I am not attempting to start a dueling standards debate, simply to provide source data for the questioned over-current placement):

[Lepke]

I can't remember any issues with too many fuses, but have seen some beauties where fuses were not used.

[camel2012]

ANL trip times and current levels make them a bad thermal overcurrent device, and can be dangerous if paired with a thermoplastic fuse block that could melt before the fuse blows. Relying on them as a short circuit interrupter they would be fine. I would stuck with MEGA/AMG or Class T. Higher value MEGA fuses are also only rated as circuit interrupters. ANN fuses (look like ANL ) are also thermal overload devices, but less widely available.

More fuses can be a problem if not thought out well. (1) Fuses create heat through resistance that mostly needs to be dissipated through conduction out the connected cables/wires combined with convection from those wires to the ambient air... It takes about 3 to 4 feet of cable in free air before and after a fuse to dissipate the heat generated... placing multiple fuses in series in close proximity will increase and localize heat and remove valuable heat shedding condictor... further, cables that are bundled together after fusing or just in general are no longer a single cable in free air and will not dissipate their own internal heat generation let alone the heat from the attached fuses. (2) Fuses are resistors by design so that they can eventually fo their job by heating up so much that they blow the thin metal conductor. A negative outcome of this is voltage drop. For a average lighting circuit 40 mV of fuse v drop may not be an issue, but fir a starting circuit where the battery voltage may naturally drop... one fuse let alone more than one could detrimental affect the ability to drive the starter in as wide of conditions as possible.

[camel2012]

Originally Posted by ayates

Does anybody have views on Class T vs ANL fuses for house battery bank fusing?


Allan.

I would use a MEGA for house. Class T are very fast acting, so unless your fuse and feed cable are heavily oversized the chances for nuisance tripping from motor or other high inrush loads are high. Class T fuses made their way into marine from Inverter manufacturers that demanded their own device protection to be very quick and though the curcuit protector could do both jobs. Your branch house loads will probably be fused as well (hydraulic/magnetic breakers ?)

[mitiempo]

I most often use either ANL or MRBF for main bank fusing - whether start or house. The AIC rating of a fuse is important when the bank is large and ANL is 6000. MRBF is 10000@ 14 volts.

AMG fuses at only 2000 AIC are not as suitable for fusing a bank of any appreciable size.

[camel2012]

Originally Posted by mitiempo

I most often use either ANL or MRBF for main bank fusing - whether start or house. The AIC rating of a fuse is important when the bank is large and ANL is 6000. MRBF is 10000@ 14 volts.

AMG fuses at only 2000 AIC are not as suitable for fusing a bank of any appreciable size.

Again consider if you want an overload fuse or a short circuit interrupter before you select an ANL... They will typically not blow under 200 percent of rated current value..

The MRBF is a good fuse, but like many other issues there are finer points to examine. The MRBF is essentially an AMG (same as MEGA) folded in half. Same fuse element...The only differences are geometry and testing... The later point is important. The MRBF was specifically tested /certified at 14V as well as for ignition protection. This does not mean that the AMG / MEGA would not meet those same levels, just that they were not tested. The marine electrical fuse protection revenue opportunities are fairly limited to justify certifying every automotive fuse for 12V applications . Also important to note is where the fuse is located , and whether there is a secondary cover on the fuse. An interrupt rating less than ideal does not mean the fuse will not blow, it means the fuse case will not contain the event. Secondary protection in my opinion is even more critical for typical very large marine house banks because none of the commercially pushed fuses are rated for a dead short on the main house cable from 4 8Ds or 8 G31s. The ABYC recommended circuit breaker ratings (note they do not reference fuses for a reason ) only go up to 1,100 A combined cranking current. A single Lifeline G31 will deliver 880 CCA at 68 deg... One doesn't need to do much computing to extrapolate the challenge of having 7,000 A+ of CCA.

The problem of insufficient fuse and circuit breaker short circuit interrupt ratings is not limited to high amp circuits. Look at how many 24 hr low amp circuits are either not protected or protected with an ATO style fuse. The small gauge wire of course greatly helps to reduce likely let through current, but that depends on the location of the fault from the batteries and with very large battery banks, the ATO fuses are not guaranteed to not blow.

Given above, I would always recommend fuse covers before fretting over the published fuse interrupt ratings. The MRBF holder product solutions currently emphasized do not make secondary covers an easy thing to implement, which is one reason I would stay away from them. There are other design related issues which I also think are concerning, but won't touch on that here.

[Frankly]

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".