Fuse sizing (again)

navillus

New Member
Joined
Feb 11, 2021
Messages
55
I have a question that has probably been asked many times. I am using a 206ah battery, 3000 watt inverter in my van electrical build. I am using positive and negative bus bars. From the bus bars to the different devices in the system I have fuses, appropriate to the load of this devices. I am using a 350 amp ANL fuse between my inverter and the + bus bar. I am not sure exactly how to size the fuse between the battery and the positive bus bar. Should the fuse between the battery and positive bus bar match the inverter size (350amp)? In addition, can I assume that I can size wire from the bus bar to each device based on its power draw, and not the largest draw attached to the bus bar? Appreciate advice on this. Thanks
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
1,210
Is this a SOK 206Ah battery? Do you just have one for 12V, two in parallel for 12V, or two in series for 24V? All specific answers on fuse and wire size depend on the answer to those questions. The following are more general answers for now.

Wire size depends on the amount of current (and voltage drop) for that wire, not the bus bar it's connected to. It's quite common for a bus bar to have large wires and small wires connected to it. They do not all need to be the same. Just appropriate for its use. Then each fuse is sized to protect the wire.

The size of the fuse between the battery and positive bus bar is based on the amount of current the battery will provide. The inverter is a large percentage usually but the battery may need to also power DC device while the inverter is on. So in many cases the battery fuse will be a bit bigger than the inverter fuse.
 

navillus

New Member
Joined
Feb 11, 2021
Messages
55
Is this a SOK 206Ah battery? Do you just have one for 12V, two in parallel for 12V, or two in series for 24V? All specific answers on fuse and wire size depend on the answer to those questions. The following are more general answers for now.

Wire size depends on the amount of current (and voltage drop) for that wire, not the bus bar it's connected to. It's quite common for a bus bar to have large wires and small wires connected to it. They do not all need to be the same. Just appropriate for its use. Then each fuse is sized to protect the wire.

The size of the fuse between the battery and positive bus bar is based on the amount of current the battery will provide. The inverter is a large percentage usually but the battery may need to also power DC device while the inverter is on. So in many cases the battery fuse will be a bit bigger than the inverter fuse.
Thank you for the informative response. I have a single 206Ah battery at this time (plan to purchase another later). The inverter called for a 300 amp fuse minimum, so I have a 350 amp fuse. Would a 350 amp be sufficient for the fuse between the battery and bus bar. BTW I appreciate the information about differing wire sizing connected to bus bar. Thanks for your time.
 

chrisski

Photon Sorcerer
Joined
Aug 14, 2020
Messages
2,343
If you have sized a fuse for 350 amps, you really need to consider going to a higher voltage system. Start looking at the amps city of wire, When I checked, 4/0 goes to 260 amps. THat’s pretty thick wire to work with.

I am not a fan of 3000 watt inverters on 12 volt systems Because of how thick the wire needs to be, FOr systems that really gets used, 12 volts is good to 1000 watts, except for a four or five excursions a day to 2000 watts to power a microwave a couple minutes at a time. Many will disagree with that.
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
1,210
Sorry to say but you can't use one SOK 12V battery with a 3000W inverter. If you look at the specifications data for the battery, the max continuous discharge current is 100A. A 3000W inverter at 12V needs 250A. Even if you add a second battery, you can't run a 3000W inverter. With two 12V SOK batteries in parallel you can support a discharge of 200A. 12V @ 200A is 2400W, obviously still less than 3000W. Even if you wired the two batteries in series for a 24V system, you are back down to 100A max discharge and a 3000W inverter at 24V needs 125A. The SOK 12V batteries can support 200A discharge but only for 3 seconds.

You could get away with using the 3000W inverter with 2 SOK 12V batteries if you never attempt to use more than 2400W.

You either need 3 SOK batteries in parallel to safely use the full 3000W or get a 2000W inverter with 2 SOK batteries (which is my setup).
 

navillus

New Member
Joined
Feb 11, 2021
Messages
55
I see. If I am not using the full 3000 watts of the inverters capacity, can I still use it with 1 battery? Will it only cause problems if I use an obscene amount of power from the inverter. I purchased the larger inverter so I could scale up my battery bank down the road.
 

chrisski

Photon Sorcerer
Joined
Aug 14, 2020
Messages
2,343
Your money. You can find out. I have three people I share my RV with. Those three don’t really believe in limits when I’m away. If you’re alone, you can lean more towards a yes.

If you did that, I’d put a 100 amp fuse for the inverter and battery. Maybe a little more, but not much more. Something to so the fuse blows before the battery.

I have a 2000 watt 12 volt inverter. THat draws 1 amp every hour its on. I’m sure the 3000 watt is more. How will that cut into your battery storage. I shut mine off at night sometimes to save 15 amp hours of power. THat and a coupe other items I shut off saves me close to 30 amp hours of power, or a little more than 10% of my usable power limit on. My lead acid battery.
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
1,210
If you never use more than 1200W at the inverter then the battery won't pull more than 100A. If you forget and try to use 3000W with one battery then the BMS in the battery might shut it down before damage is done. Or it won't and you will kill an expensive battery or at least shorten its life.

Wire the battery and inverter like it can fully support the full 3000W. Then as @chrisski stated, you could fuse the battery low so the fuse blows before the battery fries. As you add battery you can update that fuse.

Besides all of that, 3000W on 12V is pushing the limits even with batteries that can handle the higher discharge current. You need 4/0 wire (heavy and expensive and hard to work with). Do you really have that much need for AC power? Consider 24V but it depends on other aspects of your system.
 

navillus

New Member
Joined
Feb 11, 2021
Messages
55
Your money. You can find out. I have three people I share my RV with. Those three don’t really believe in limits when I’m away. If you’re alone, you can lean more towards a yes.

If you did that, I’d put a 100 amp fuse for the inverter and battery. Maybe a little more, but not much more. Something to so the fuse blows before the battery.

I have a 2000 watt 12 volt inverter. THat draws 1 amp every hour its on. I’m sure the 3000 watt is more. How will that cut into your battery storage. I shut mine off at night sometimes to save 15 amp hours of power. THat and a coupe other items I shut off saves me close to 30 amp hours of power, or a little more than 10% of my usable power limit on. My lead acid battery.
I appreciate the insight. I have a switch that turns the inverter on and off, to keep it from drawing power. Just to clarify the fuse between the inverter and positive bus should be 100 amps? What about the fuse from battery to positive bus bar? Thanks again.
 

chrisski

Photon Sorcerer
Joined
Aug 14, 2020
Messages
2,343
I appreciate the insight. I have a switch that turns the inverter on and off, to keep it from drawing power. Just to clarify the fuse between the inverter and positive bus should be 100 amps? What about the fuse from battery to positive bus bar? Thanks again.
That gets a little complicated. If this were my money, I would use 100 amps, but I don't want you thinking I'm saying yes, so I recommend you do it.

I think its more important to figure out the draw off the battery in amps prior to connecting the device to the BMS. This would be the wattage you got off the killawatt meter / your inverter low voltage cutoff. My low voltage cutoff is 10.5 volts and mulitply that by 1.15 to account for inverter losses. Whatever that is will probably come out to about 1000 watts. That is also the most I would put on that inverter. There's some slop built into this, like its unlikely your voltage will sag that low, but you don't know until you plug it in and try it. If it doesn't sag that low, you still want to not change the low voltage cutoff because if you turn that on when the battery is in a low state of charge, it may then.

Class T fuses will not blow at its rated rating, its a combo of how long the current flows at how strong:

1626222553794.png
So with this chart, if a 125 amps are flowing across the fuse for 500 seconds will blow it, but still can run 200 amps through it for about a minute before it blows.

I think the limitation in your SOK is no more than 100 amps discharge. There's probably a BMS inside that shuts the battery down at that rate. One BMS I looked at had 110% the rated discharge rate for 5 seconds. I don't know what SOK uses. I also don't know how bad it is for the SOK battery to discharge at the 100 amp rate tripping this device. Maybe its not just the BMS tripping, but things inside melting. When I've had questions like that, I email whatever company tech department, and they are good at answering, and if no response, then I call.

I have a battery monitor, and I can watch how many amps my device pulls. I do not worry about amperage on my 12 volt system at all except for a couple of items. First, the 500 watt K-Cup coffee maker at 55 amps. Second, the Vacuum (800 watts?) at 75 amps. Those two would be fine to run off the battery one at a time. The last one is the 1200 watt microwave, that actually pulls 2100 DC watts at 155 amps. That would not be able to run off the battery because of the battery 100 amp limitation, but would run for at least a minute before the fuse blew. So the 100 amp fuse would not protect you from tripping the 100 amp battery cutoff. If you did a calculation fuse to blow quicker before the BMS kicks in, then it gets ridiculously small.

That total 100 amps for the battery is also a bit hard to calculate. When I have my propane heater on, it pulls 9 amps form the battery. It comes on when it gets cold throughout the night. There's also draw of certain things like .5 amps for my stereo, .5 amps to run my propane fridge. That goes into the 100 amp limit, not just the one high wattage item.

Overall, I still feel by putting that 100 amp fuse in my system protects me from when I walk away from the RV, and someone decides to run the microwave for 2 minutes. If the battery doesn't shut off, then before the time is up on the microwave, the fuse will blow.

What I'm telling you some will say is way too much, but I bet there will be a lot less likelhood of having an electric fire.

EDIT: I do not have a fuse between the bus bar and the inverter, only between the the battery and the busbar. I'm not sure why you put one there unless your inverter manual says to do so.
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
1,210
I think the limitation in your SOK is no more than 100 amps discharge. There's probably a BMS inside that shuts the battery down at that rate. One BMS I looked at had 110% the rated discharge rate for 5 seconds. I don't know what SOK uses. I also don't know how bad it is for the SOK battery to discharge at the 100 amp rate tripping this device. Maybe its not just the BMS tripping, but things inside melting.
From the SOK spec sheet:

Maximum continuous discharge current: 100A
Peak discharge current: 200A (3s)
BMS discharge cut-off current: 330A +/- 10A (6-16ms)
 

chrisski

Photon Sorcerer
Joined
Aug 14, 2020
Messages
2,343
From the SOK spec sheet:

Maximum continuous discharge current: 100A
Peak discharge current: 200A (3s)
BMS discharge cut-off current: 330A +/- 10A (6-16ms)
That gets me more to a 100 amp fuse because that would not blow in 3 seconds by Blue Sea's specs, but would still think knowing the amperage of a high wattage load and making sure it doesn't exceed 100 amps on a constant load is important. Some loads at this 100 amps may still trip the BMS if they have a high draw, like an air conditioners locked rotor amp would exceed the 330 amp for 6 ms, but once spun up would be a little less than 100 amps.

That's just the way I look at it, but I am self taught leaning to the conservative side, as in less likelihood of fires.

Edit: I think the only reason to press forward with the 3000 watt inverter is because you already have it and its not returnable.
 

navillus

New Member
Joined
Feb 11, 2021
Messages
55
That gets a little complicated. If this were my money, I would use 100 amps, but I don't want you thinking I'm saying yes, so I recommend you do it.

I think its more important to figure out the draw off the battery in amps prior to connecting the device to the BMS. This would be the wattage you got off the killawatt meter / your inverter low voltage cutoff. My low voltage cutoff is 10.5 volts and mulitply that by 1.15 to account for inverter losses. Whatever that is will probably come out to about 1000 watts. That is also the most I would put on that inverter. There's some slop built into this, like its unlikely your voltage will sag that low, but you don't know until you plug it in and try it. If it doesn't sag that low, you still want to not change the low voltage cutoff because if you turn that on when the battery is in a low state of charge, it may then.

Class T fuses will not blow at its rated rating, its a combo of how long the current flows at how strong:

View attachment 56160
So with this chart, if a 125 amps are flowing across the fuse for 500 seconds will blow it, but still can run 200 amps through it for about a minute before it blows.

I think the limitation in your SOK is no more than 100 amps discharge. There's probably a BMS inside that shuts the battery down at that rate. One BMS I looked at had 110% the rated discharge rate for 5 seconds. I don't know what SOK uses. I also don't know how bad it is for the SOK battery to discharge at the 100 amp rate tripping this device. Maybe its not just the BMS tripping, but things inside melting. When I've had questions like that, I email whatever company tech department, and they are good at answering, and if no response, then I call.

I have a battery monitor, and I can watch how many amps my device pulls. I do not worry about amperage on my 12 volt system at all except for a couple of items. First, the 500 watt K-Cup coffee maker at 55 amps. Second, the Vacuum (800 watts?) at 75 amps. Those two would be fine to run off the battery one at a time. The last one is the 1200 watt microwave, that actually pulls 2100 DC watts at 155 amps. That would not be able to run off the battery because of the battery 100 amp limitation, but would run for at least a minute before the fuse blew. So the 100 amp fuse would not protect you from tripping the 100 amp battery cutoff. If you did a calculation fuse to blow quicker before the BMS kicks in, then it gets ridiculously small.

That total 100 amps for the battery is also a bit hard to calculate. When I have my propane heater on, it pulls 9 amps form the battery. It comes on when it gets cold throughout the night. There's also draw of certain things like .5 amps for my stereo, .5 amps to run my propane fridge. That goes into the 100 amp limit, not just the one high wattage item.

Overall, I still feel by putting that 100 amp fuse in my system protects me from when I walk away from the RV, and someone decides to run the microwave for 2 minutes. If the battery doesn't shut off, then before the time is up on the microwave, the fuse will blow.

What I'm telling you some will say is way too much, but I bet there will be a lot less likelhood of having an electric fire.

EDIT: I do not have a fuse between the bus bar and the inverter, only between the the battery and the busbar. I'm not sure why you put one there unless your inverter manual says to do so.
You guys are awesome, thanks for educating the new guy, I appreciate the time taken.
 

navillus

New Member
Joined
Feb 11, 2021
Messages
55
From the SOK spec sheet:

Maximum continuous discharge current: 100A
Peak discharge current: 200A (3s)
BMS discharge cut-off current: 330A +/- 10A (6-16ms)
Thank you for that information, I appreciate it!
 

rmaddy

Full-time Solar-powered Trailer Life
Joined
Nov 16, 2019
Messages
1,210
So with 2 batteries, are you fusing for the 130 amps per battery that the rep said is the overcurrent but off, or just 100 amps?
I don't know why the spec says 100A continuous while the rep says 130A continuous. They need to get their story straight. Meanwhile the battery's BMS will cutoff at 330A. Given your use of a 3000W inverter on these batteries which is too much for the 100A (or 130A?) recommended discharge current, it might be best to use a 125A fuse if you have one battery (or two in serial) or a 225A fuse if you have two in parallel. Then you are protecting the battery/batteries from over using the inverter and other loads.

If you are on Facebook you might ask SOK what is best. I'd be curious why the battery recommends 100A continuous but the BMS doesn't shutdown until 330A. That's a massive difference. I wonder what effects there are on the battery if you try to pull say 150A, 200A, 250A, or 300A continuous. The answer to that should help decide what fuse to use.
 

navillus

New Member
Joined
Feb 11, 2021
Messages
55
I don't know why the spec says 100A continuous while the rep says 130A continuous. They need to get their story straight. Meanwhile the battery's BMS will cutoff at 330A. Given your use of a 3000W inverter on these batteries which is too much for the 100A (or 130A?) recommended discharge current, it might be best to use a 125A fuse if you have one battery (or two in serial) or a 225A fuse if you have two in parallel. Then you are protecting the battery/batteries from over using the inverter and other loads.

If you are on Facebook you might ask SOK what is best. I'd be curious why the battery recommends 100A continuous but the BMS doesn't shutdown until 330A. That's a massive difference. I wonder what effects there are on the battery if you try to pull say 150A, 200A, 250A, or 300A continuous. The answer to that should help decide what fuse to use.
Thanks again. I don't have a Facebook account, however, I think I am going to fuse 2 batteries with a 250 amp fuse (2×130), I think that should cover me.
 

Just John

Photon Sorcerer
Joined
Aug 15, 2020
Messages
2,380
I have a question that has probably been asked many times. I am using a 206ah battery, 3000 watt inverter in my van electrical build. I am using positive and negative bus bars. From the bus bars to the different devices in the system I have fuses, appropriate to the load of this devices. I am using a 350 amp ANL fuse between my inverter and the + bus bar. I am not sure exactly how to size the fuse between the battery and the positive bus bar. Should the fuse between the battery and positive bus bar match the inverter size (350amp)? In addition, can I assume that I can size wire from the bus bar to each device based on its power draw, and not the largest draw attached to the bus bar? Appreciate advice on this. Thanks
Your fuse should be sized to protect the cable you are using (i.e. not above the rating for the wire/cable), then if lower, for the device.

The fuse is to prevent your wire from overheating and starting a fire. Individual loads should have fuses appropriate for their wire.
You can of course use smaller fuses, but never put a fuse on that is larger than what your wire can carry.
 
Top