Which end of circuit to add fuse. Battery, Charger or Both

[gpsatsea]

I am upgrading my charging circuit on my 20 year old Hunter. Replacing the charger, adding an FET Isolator and two new On/Off Siwtches. I am also adding several additional fuses (it only had 1 that I could find). I think I have them sized them correctly. (Thank you Blue Seas Circuit Wizard).

My last question is where do I place the fuse that protects the wire between the Charger and the Bus Bar from the Battery Bank.

It is obviously better (and probably to code) to place a fuse close to the "source" of the power. So connecting a battery to a starter the fuse is located close to the battery. But the battery to charger circuit actually has two "sources". It is conceivable that a wire coming loose at either end could create a short.

Do I really need to have a fuse at both ends???

My diagram show the question as "A" near the Battery Bus or"B: near the Charger. For simplicity I removed all of the other circuits from the diagram.

[Frankly]

Absolutely on the battery end, and on the charger end if the charger is not equipped with some output current limiting device the charger needs one.


Frankly

[Wotname]

To expand on what Frankly posted - always fuse close to the source(s) wherever they occur in the circuit unless the source is current limited to a value below that of the wire ampacity.

[Manateeman]

Frankly is correct. I read something about a FET fire but cannot find it.
If it’s Vectron, you have a good unit.
Manatee

[Reefmagnet]

Actually, you can have it at both ends. The battery should have a main fuse protecting the entire electrical circuit already. If not, add one pronto!


Then you fuse the charger with a lighter duty fuse at it's end and connected on the output side of the battery main fuse. At the end of the day you have two fuses in circuit, but performing separate roles so it's not really a case of a wire with a fuse at each end.



So the theoretical answer to your question is you fuse at the charger end because the battery will (should!) already be fused.

[Scott Berg]

In most cases you'll need a fuse at each end to be code compliant because they are both sources of power. Some chargers have fuses or other current limiting built in but unless they say so you'll need both.

[ozsailer]

Gday Gpsatsea,
you will have to excuse my ignorance but when looking at your wiring diagram why do you have the busbar wired in series between the Charger and the batteries. Should not the charger (fused) go directy to the battery then a seperate fused line from the battery to the bus bar.

Greg H
Lagoon 440
Oz

[gpsatsea]

Thanks for all the feedback.

I definitely will have a 200 amp MRBF fuse directly on the battery to protect the larger circuit. I complicated the fusing requirement by going from the battery though a BUS in addition to accepting the charge from the BC, the BUS will also accept the alternator charge via the Isolator and feed the House Panel. All with different size wires.
From what I am hearing I should have a fuse The same size at both the charger end and the BUS end. I guess if it was only a foot or two and well secured I could get by with just. one

Another option that may be cheaper for the short runs is to upsize the wires to match the battery cable. Then all down stream wires are protected by the main MRBF.

[s/v Jedi]

Use this schematic as a general guideline:

[gpsatsea]

Thanks Jedi but I have studied that great diagram and used it effectively. However, it does not show a fuse coming out of the charger. Maybe because that charger has an internal fuse on the output already
I need to check my new Pronautic
Thanks.

[CharlieJ]

There is no OCPD required at the charger as it is self-limiting; i.e., it cannot put out anymore current than it was designed for. A battery, on the other hand, has an enormous amount of short circuit current capability so the OCPD is required within 7" of the conductor's attachment. (There are exceptions to the 7" rule.)

[gpsatsea]

Thank You Charlie.

That makes sense to me and glad to know that it not required.

I guess this is the same reasoning of not requiring OCPD from an Alternator to an FET Isolator? I may add one to my 35 amp alternator just because it runs through the engine compartment and under my bed!

Thanks again.

[skipmac]

Originally Posted by CharlieJ

There is no OCPD required at the charger as it is self-limiting; i.e., it cannot put out anymore current than it was designed for. A battery, on the other hand, has an enormous amount of short circuit current capability so the OCPD is required within 7" of the conductor's attachment. (There are exceptions to the 7" rule.)

Second this recommendation. All the charging sources I'm familiar with are current limited meaning they can produce the amount of power they are designed and rated to produce. This applies to battery chargers, solar panels, alternators, wind, etc. Short a 250 Watt solar panel and it will not dump more than 250 W into the short. Short a 100 amp alternator and it can only dump 100 amps into the short.

So as long as the cable from any charging source to the battery is large enough to handle the max output of the source then there is not a risk of melting the wires and starting a fire from the charging source.

The risk is from the battery side which is not current limited. Short a battery and it will dump all the power it has into that wire. So put the fuses on the battery side.

DISCLAIMER: There may be some type of charging source that violates this rule but none that I am immediately aware of. However I'm sure someone will point it out if there is one.

[smac999]

Battery end.

A charger can never output more then it’s rating so no damage can be done if shorted. Normally they are fused inside anyways.

[David B]

Originally Posted by gpsatsea

I am upgrading my charging circuit on my 20 year old Hunter. Replacing the charger, adding an FET Isolator and two new On/Off Siwtches. I am also adding several additional fuses (it only had 1 that I could find). I think I have them sized them correctly. (Thank you Blue Seas Circuit Wizard).

My last question is where do I place the fuse that protects the wire between the Charger and the Bus Bar from the Battery Bank.

It is obviously better (and probably to code) to place a fuse close to the "source" of the power. So connecting a battery to a starter the fuse is located close to the battery. But the battery to charger circuit actually has two "sources". It is conceivable that a wire coming loose at either end could create a short.

Do I really need to have a fuse at both ends???

My diagram show the question as "A" near the Battery Bus or"B: near the Charger. For simplicity I removed all of the other circuits from the diagram.

Fuses (or MCB's) are there to protect the wiring - the fuse melts instead of the cable. A large fuse should be right at the battery, then when that main cable feeds off from a distrubution bus, each branching cable needs to be fused in line with the cable size and so on (close to the bus).

Having said all of that, the charging circuit should be separate, and go straight to the battery, bypassing the main cabling, and bypassing the main isolators. That way, when all isolators are off, the batteries can still be charged. So again, a fuse right at the battery in line with the wire size you are using from the charger. Also at the charger end, there should be a protection mechanism, and if not, then a fuse there also, although most modern chargers will not output if they don't see battery volts.