Voltage rating of a fuse simply needs to be higher than your system voltage. If you have a 12V system, a 32V rated fuse is fine. The current rating is what really matters for fuse selection. Keep in mind the current protects the wiring, not the load. So you select a fuse based on the wiring that feeds the load. The simplest approach: look up the ampacity of the wire based on its gauge and insulation (and bundling), and select a fuse with a current rating that is at or below the ampacity. The fuse should be as close to the power source as possible. If the circuit branches to smaller wire, you need to fuse those wires appropriately as well.

 

This is the correct and valid answer - though complicated at times. The voltage rating of a fuse or breaker is a maximum - as long as you're under it, you're just dealing with the current / amperage.

In bullet points:

• load / current dictates wire ampacity required
• wire ampacity dictates fuse / breaker size required
• rule of thumb is fuse 30% higher than ampacity

And the complication factor:

• distance / length of wire (full run) creates voltage loss, often demanding increasing the wire gauge
• when wire gauge increases to reduce voltage loss, see above

(IOW, technically, fuse / breaker goes UP with wire gauge, not load)

Power-source potential (battery size) doesn't affect load calculations, thus not wire-gauge or protection. IOW, a massive battery bank has nothing to do with the loads.

Sounds simple enough, but it is SO often confused by discussion of the load equating to fuse / breaker. As @natecostello says, fuses protect wires, not loads. But this gets confused easily.

So... you have nothing in there about your loads. The maximum possible output of the battery is not the factor. And the amp-hours (quantity of storage) in the battery (or batteries) has nothing to do with the output (other than safe limits of said output and input).

What will you be running? An inverter? What size? An air-conditioner? Other loads?


(To pre-empt: I'm /trying/ to be functionally useful... though there are more detailed explanations that are more accurate. Feel free to do the search-stuff to validate the above.)

 

Are you talking main system fuse or per battery?

 

Main fuse. 4X100AX12V, thinking a 600 A fuse, <7" from battery bank, if everything I have read is correct (and my gosh, I had looked at ExploristLife, Ross Lukeman and have read all kinds of stuff, including Blue Sea calculator).

 

You should start rereading all that stuff and identify your worst case load or in this case loads being that it is a main fuse. Then you can determine the wire size and fusing needed. The size of your battery bank, in not really part of the computation, other than the short circuit current rating for determining the AIC rating.

 

When using the Blue Seas Calculator, you can insert info regarding battery bank size, and the calculator will give you circuit protection options, both type of "fuse" and amp rating. Each type has limitations based on AIC. When amperage gets in the higher ranges, Class T fuses come into play.

I've managed to weld things together with a single lead acid 12 volt, by accident. Wish the internet was around back then.

 

This is the correct and valid answer - though complicated at times. The voltage rating of a fuse or breaker is a maximum - as long as you're under it, you're just dealing with the current / amperage.

In bullet points:

• load / current dictates wire ampacity required
• wire ampacity dictates fuse / breaker size required
• rule of thumb is fuse 30% higher than ampacity

And the complication factor:

• distance / length of wire (full run) creates voltage loss, often demanding increasing the wire gauge
• when wire gauge increases to reduce voltage loss, see above

(IOW, technically, fuse / breaker goes UP with wire gauge, not load)

Power-source potential (battery size) doesn't affect load calculations, thus not wire-gauge or protection. IOW, a massive battery bank has nothing to do with the loads.

Sounds simple enough, but it is SO often confused by discussion of the load equating to fuse / breaker. As @natecostello says, fuses protect wires, not loads. But this gets confused easily.

So... you have nothing in there about your loads. The maximum possible output of the battery is not the factor. And the amp-hours (quantity of storage) in the battery (or batteries) has nothing to do with the output (other than safe limits of said output and input).

What will you be running? An inverter? What size? An air-conditioner? Other loads?


(To pre-empt: I'm /trying/ to be functionally useful... though there are more detailed explanations that are more accurate. Feel free to do the search-stuff to validate the above.)

So I believe that you are a little incorrect here. We certainly use load as part of calculation for fusing. The load value would be the minimum value that we would choose for a fuse or breaker. Obviously that would not be a great choice, as we would probably get some unwanted trips. The ampacity of our wire should cover the load value. So we would like to choose a fuse/breaker value that is between the minimum load and maximum ampacity of the wire. It is true that voltage drop calculations will increase our wire size and thus the upper ampacity value of the fuse/breaker choice. My general choice is that the fuse value is 125% of the load value. I believe this is where you got your 30% increase . To protect the wire you generally need to have the fuse/breaker to be a lower value than the ampacity of the wire. So yes I agree the fuse/breaker protects the wire not the load, but load is certainly part of the equation.

Also when choosing a Main or branch fuse or breaker, the size of the battery bank does come into the calculation of the minimum AIC

 

As the system is currently set up, I really have no idea what the loads are, as it's a bit challenging determining what will be running at the same time probably at one time-lights, fridge, water pump, and AC would be worse case, although my wife knowing her will want to plug a hair drier or curling iron in and will need to be stopped. I do have an 12 V air conditioner, which I believe, if memory serves draws 60 Amps when running at highest setting (not sure what the surge cold is) thought I would fuse that with a 90 amp fuse, have a 3000W inverter-250 amp fuse, those are the big draws, the rest will be controlled via a distribution box and includes the water pump, gray water heaters, diesel heater, a 12 volt TV, puck lights, 12 volt fridge, 12 volt water heater, USB phone chargers.

 

@gregoryx, I apologize if I came off harsh in my response. Not Intended , but as you say it can be a complicated subject. To be clear the part I had the most contention with is saying that the fuse would be +30% of the ampacity of the wire. That would put your fuse over the rated ampacity of the wire. But if you applied the +30% to the load value and that value was below the ampacity of the wire you would be correct.

I would say that the low end of the fuse range is based on load and the high end is based on the ampacity of the wire. So our fuse should be between the load value and the ampacity value of the wire. We would like to be at least 125% of load but still under the ampacity value of the wire. If couldn't do that, it would another reason to go to a larger ampacity wire ( just like voltage drop)

if we look at the circuit wizard I put in some basic data .

This gives us the results of AWG 4 wire with a ampacity of 160 Amps. Our minimum fuse rating is 100 amps, with a suggested rating of 126A or 125% of the load rating. It also states a Minimum AIC of 3000. This based off of the 1000 battery CCA that input. Now we can further look at the choices for fuses that would meet are requirements.

You are totally correct the protection is for the wire not the source or battery. Yes, the AIC thing can be confusing addition. It would be where we need to know the size of the battery bank or CCA rating. Usually with Lithium I look for the short circuit rating.

Hopefully I made it a little clearer and not more confusing.

 

...
Hopefully I made it a little clearer and not more confusing.

Did for me. Thank you.

Especially helps me with the 125% thing relating to the load, not the wire. That had confused me; but I can see now that I was probably crossing the two with each other and ending up with an invalid mix.

Imma try to find a way to state that more effectively for future reference / helpfulness.


So... how can we give a useful answer to @Magic Bus on the original question - and after including the follow-up data posted by @Magic Bus above?


@Magic Bus, what batteries are you considering? And are you considering fusing each individually before they go into the Lynx? And why Lynx versus just a basic bus bar? Do you intend to do Victron batteries and BMS? or use the Victron Lynx Shunt as well?

I ask that because there's the question of how best to connect multiple batteries. I think the fuse-per-battery is ideal (and a switch-per-battery as well - especially in a parallel setup); but that's easy for me to say since the batteries we chose have fuses and switches built in.

 

I prefer the MRBF fuse on each battery post as it protects the battery cable (as long as the fuse is sized properly for the battery cable used). While the MRBF fuse doesn’t provide as much AIC protection as a Class T fuse, it has been judged adequate by the ABYC I believe.

The Lynx box (intended to combine batteries) does have fuses, but it leaves the battery cable unprotected, and the Maxifuses it uses have less AIC protection than the MRBF fuses.

The Lynx distributor box can then protect wires distributed to each major draw (inverter, DC distribution Panel, Air C, and the like) with an appropriately sized Maxifuse.

While the Lynx Distributor is large physically, IMHO, it provides a good solution for remote monitoring (coupled with a Cerbo), and is actually more compact than separate fuses for each major consumer mounted on a bulkhead using traditional single fuse blocks.

I think the best approach for combining multiple parallel batteries is a traditional bus bar with a switch for each battery. That gets cumbersome if one uses more than two batteries in parallel. Individual switches may not be required if using a battery like @gregoryx with a physical switch built-into each battery. I was bummed, and amazed, that the high-dollar Victron Smart Lithium did not have a way using Bluetooth to turn off each battery. They depend on the LynxBMS contacter to remove the battery connection from the system. So, common sense suggests a physical switch to really disconnect the batteries for system service / trouble-shooting.

In my system currently under construction with only two batteries, I’m using a blue sea battery selector switch with four positions : 1,2,1+2, and off as a battery combiner / system battery disconnect bolted to the input of the Lynx BMS I’m using with one of the Explorist.Life short bus bar cut for this exact purpose.

 

For lithium I would go with a class T fuse, not an ANL. The AIC of an ANL is too low for lithium. I think the AIC of ANL is 5k ish amps and the AIC of class T is 20k, IIRC. With the low internal resistance of lithium batteries the current can be enormous. I believe the ABYC is working on adopting class T as correct for lithium.

 

We selected class T fuses for our Victron 24v 200ah batteries as well, for that same reason. The maximum available fault current of modern lithium batteries doesn't seem to be well established, and any attempts to extract that data have led back to "the battery is internally protected" or similar unsatisfactory answers.

We have a 350a fuse on each battery, which is well below their actual output capabilities, but more than adequate for our system. Hypothetically if we had one battery isolated and ran the 24v AC and the inverter at full load simultaneously we might approach that value, but I'm pretty confident that'll never happen. And if it does, we'll be safe.

 

Here's my recipe:

1. Determine MAXIMUM POSSIBLE load
2. Size cable to handle that load
3. Confirm that voltage drop is acceptable
4. Size fuse to protect that cable
5. Use a Class T fuse for its high AIC rating (and ignore the cost)

 

Do not use fuses as they are slow to react and waste precious current/energy.
Only use Circuit breakers or thermally sensitive circuit breakers. Get a Circuit breaker box or two.
A improved method:
1. Determine MAXIMUM POSSIBLE load potential. Add up the wattage's of everything you will put in the van + 20%.
2. Size Copper cable to handle the maximum main or branch load, plus one gauge size thicker.
3. Place a Main breaker at the source, The battery. and connect that breaker to a distribution bus bar with smaller breakers for each branch. Better to have extra branches than too few. For example do not put the lighting and microwave on the same branch. Any branch drawing high current over 40 amps should be its own branch with one destination only. The smaller breakers when added together can not exceed the rating of the main breaker.
4. Determine the length of cable +10% for each branch. Generally 8-12 Gauge is most common for runs under 12ft.
5. Have battery monitoring devices for voltage and temperature.
6. Only use pure Copper wire, NOT CCA wire.
7. Voltage drop is acceptable if under 3%
8. Size main Breaker to protect battery at battery maximum Continuous capacity fuse to protect that cable.
9. Size branch breakers to MAXIMUM POSSIBLE branch load potential plus 30-50%.
10. For any critical circuits, you need redundant wiring of at least two circuit paths. Such as the vehicles starter.
The negative ground should be redundant.
11. Use proper copper terminals. Never use bare ended wires wrapped around a terminal.
12: All stranded wire ends must be Soldered /Tinned using 60/40 Rosin core or 4% silver solder.
13. Double insulate all connections with weather rated shrink tubing. Insulate all Battery terminals. All connections Must be water / weather proof.
14. Do not use the Chassis as a local load grounding point. Meaning do not draw current through the chassis. Batteries are grounded to the chassis only. Galvanic corrosion will occur if you draw current through the chassis. the chassis is to remain neutral.
15. 12VDC will not get the job done. Recommend 24-48v house system. You will be glad you did.
16. Label both ends of all your wires twice. Place labels 6-8 inches from end of wire.

Using these rules, you will create very reliable circuits.
Protection devices are there to protect the wiring from overheating. But the wires can handle more than their rated current for short periods of time. You want the breakers to be rated slightly higher than the recommended wire size because it will eliminate false trips from transient spikes and surge currents. Transient spikes are very common in vehicles.
It is NOT necessary to put, nor is their any advantage to placing breakers on both battery terminals.
I have wired up many vehicles successfully.
This method will work in all situations. Do not skimp on electrical connections.
Plus I am an electrical engineer (for decades).

Hope this helps.

 

Friend who had van built for her in Phenix just had wire fire in her van -
electric system never worked properly from day one -
she has 600AH Battleborn batteries - 750 watts of solar - $$$
system would not power 2 small fridges and a computer - led lights

I never got to look at system to see what was wrong with it - but something clearly defective -
about a month ago I get email saying her electrical system started on fire -
saw picture of wiring that showed Insulation burned off what appears to be several 2 gauge wire bundles -

I dont know if Fuses / Breakers did not trip - or not Sized Correctly or what happened -

at some point circuit was broken because copper in wires did not melt -

 

I would love to know what circuit breakers people recommend for higher (48+) voltages. That's the main thing keeping me away from a 48v build (well that and the higher voltage also needed for the solar panels).

 

Might be something @Van Gogh's build thread, or reach out to him. IIRC he does discuss some of the the challenges in sourcing 48V components. Now that I've tagged him, he might even chime in.

 

I would think the Midnite Solar DC breakers are probably appropriate:

Category Products - MidNite Solar's

MidNite Solar Category Products.

www.midnitesolar.com

 

I have used Midnite breakers on many solar installs. Excellent product, known supply chain and excellent technocal advice a phone call away.

 

Thanks! And woah. Certainly makes the T fuses seem cheap.

 

$200 Dollars for a used one, That is what I am using. It has been tested and recalibrated. I just posted that other link because it had some specs.