Can I tap into a 12v battery bank from multiple points at the same time?

[hlev00]

Most of the time I just start off of my house bank. I do occasionally use the actual start battery to be sure it is still up to the task and to send some charge to it. My start battery works as an emergency source in the event my house bank is run down too far, which hasn't happened yet. In that case, I would shut down all of my DC loads, switch the start battery on, and start the engine. My house bank is two 12V 4D FLA.

I haven't seen anything suggesting this is a problem for the deep cycles. They eventually get charged up again though not as fast as the starter battery would.

[hlev00]

In the case of AC in your house, neutral is to be at earth ground. For example, the ground prong in a plug provides another path to ground. If you fuse the neutral and it trips, there is still current flowing between the hot leg and the earth ground. In a two wire AC circuit it technically wouldn't matter which is fused, just as in a DC circuit technically doesn't matter. Either will break the circuit.

[svlamorocha]

Originally Posted by Nature Boy

svlamorocha.
New guy just guy just joined the other day.
Please allow me to ask why you would fuse the postive side of a D.C. circuit as this is after the load. Electricity comes out of the negative side of the battery. Would it not be better to fuse on the negative side before the load there by protecting it from a over current event?

Welcome to CF.

If you have only one fuse then it is NOT better to have it on the negative.

Your reason "for" fusing the negative is not valid. The fuse blows by current going through it in either direction and the absolute value of the current is the same at both terminals, hence positive and negative is the same for this purpose.

One reason to put the single fuse in the positive side is that under most circumstances folks want to have "unbroken" negative wires because they also act as a safety device to complete the AC circuit in the event a hot wire (black in North American boats) touches or is connected by mistake to a negative wire (black in most boats) that has continuity to an exposed part of the metal chassis of a DC device (say a car-style radio). To make this work the black negative wires are connected to the AC green that is then connected to the AC neutral at the AC source. If you were to insert a fuse in that DC negative then you would be "breaking" that device when the fuse blows.. Another story is that many people (me included) believe that with RCD devices found in proper 2015 installations it is not that necessary to link DC negative to AC green.

The one reason to fuse negatives is that the fuse does not need to be insulated (to protect from a wrench making contact with it and DC-grounded engine block or negative terminal, which is customarily not insulated, but the powers that be decided that this is less important than preserving the DC-negative-to-AC-green connection.

The practical principle is never break DC negative wire or AC safety green wire with a fuse or breaker. I know in some cases this would not do any harm but the powers that be decided, for a reason, to make this a hard rule.

[svlamorocha]

Originally Posted by hlev00

In the case of AC in your house, neutral is to be at earth ground. For example, the ground prong in a plug provides another path to ground. If you fuse the neutral and it trips, there is still current flowing between the hot leg and the earth ground. In a two wire AC circuit it technically wouldn't matter which is fused, just as in a DC circuit technically doesn't matter. Either will break the circuit.

I do not understand why you say that it does not matter which AC leg is fused.

In the less developed countries that still use one-pole breakers/fuses in AC wiring it is ***very important**** to put the breaker/fuse in the hot leg. That way, in addition to having overcurrent protection (too much current between hot and neutral) you can rely on the breaker to trip when there is an unintended contact between hot and metal things that are not supposed to be energized (aka "masses", such as the chassis of a washer-dryer or fridge) and are supposed to be grounded. In addition, when you flip off the breaker you know that the wiring will not zap you. If you break only the neutral you do not get those "additional" protections.

[jwcolby54]

Originally Posted by senormechanico

OK, someone's gotta say it...
Three posts and you fuse the neg because the juice comes out there???
I guess I had better completely rewire my boat !

LOL. Actually electrical engineers are taught that holes flow, not electrons. Thus engineers see the world backwards from everyone else. In the end it matters not unless you are designing semiconductors, in which case it apparently does matter.

[RaymondR]

We have to wire our batteries differently to you northern hemispherer's because they are upside down.

[valhalla360]

I would be inclined to have a single connection to the battery bank. That way you can set it up so a single switch kills all 12v power. You can branch out after that.

I would be hesitant to fuse the negative side (even if you are in the southern hemisphere) as anyone who works your electrical system is going to assume the fusing is on the positive side and that could result in them inadvertantly bypassing the fuse.

[Privilege]

Another reason why a fuse should be on the positive side of a DC system is this...The primary purpose of a fuse is not to protect the item using the power whether it be a pump, electronics or a bulb, but to protect the wire providing the current from overheating and causing a fire. If a wire gets damaged and exposed anywhere on the run, or there is a fault with the pump/electronics/bulb, the current can find a way to ground; the flow is unrestricted and the wire overheats.

If the fuse is on the negative side and the fault is anywhere between the fuse and the positive terminal, the fuse will not see this excess flow of current and it will not blow. Therefore, the fuse should always be on the positive side. (Preferably as close to the battery as practical). The positive feed from the battery should be at one terminal
with a single fuse protecting the entire system. Further fuses, at the breaker panel for example, will protect the individual runs.

[hlev00]

The current is the same on both sides of the load. The fuse protects both the positive and negative wires equally.

[Privilege]

Originally Posted by hlev00

The current is the same on both sides of the load. The fuse protects both the positive and negative wires equally.

...yes, but only if the current is flowing between the positive terminal and the negative terminal. If that circuit is broken, say by a cut/damaged wire sitting in a place where it can get to ground, there will be no current flowing on the negative side. All the current will flow from the positive terminal to its newly acquired ground.

[hlev00]

Ah, I think I understand where you are coming from. You are assuming that it is more likely that the load will find some other path to ground than a path to another hot circuit. That is definitely why in three wire AC systems, the hot is always fused. I suppose that is the case in our DC systems as well especially around the engine.

[jwcolby54]

So the OP asked if he could do it, not whether it was a good thing. He can do it, and it is not a good thing.

Some poster correctly posted that the fuse protects the WIRE itself, which if not protected can become hot enough to cause fires anywhere along its length.

In fact a fuse (or breaker) also may protect the item(s) at the end of the wire from catching fire.

In the 80s I worked for a super computer manufacturer. The computers were fed by these mongo power supplies capable of providing enormous currents. One day we came in to work to discover a machine shut down. On investigation we discovered a board inside the machine where the ground plane and +5V plane had shorted. Because the power supply had to be able to supply so much current, it did not even hesitate in supplying enough current to feed this short. The short melted the board itself (and set the fiberglass on fire in the process), creating a bigger and bigger circular short around the ever growing hole in the board. Eventually a smoke detector went off and turned off the power to the whole room.

I have to admit I personally had never before seen a circuit board with a board burned to a crisp by a power supply that refused to current limit. We eventually installed a smoke detector inside of the computer's cabinet to detect such an occurrence and went about our business.

The battery bank should have a single ground lead and a single hot lead brought out to a distribution panel. There should be a fuse between that panel and the battery. After that there should be individual circuits, with a hot and ground lead going off to things like lights, electronics etc. Each of those circuits should be fused at levels appropriate to the intended load.


And yes, a fuse should be in the hot lead. Fuses should always be sized for the wire that the fuse is protecting. Wires have current ratings and the fuse (IIRC) should be determined by that rating.