Fuse size? Please help before I catch fire.

[Jet]

My set up is a 200ah 24V battery made with 16 Frey (Fortune) 100ah cells). Between the battery box and the rest of the power system is a 32 in long chee fridge/freezer. Everything will be mounted fairly close together with room for air to circulate on a plywood sheet. The DC-DC charger will have a fan.

I have a Blue Sea 100 amp fuse box. Should I fuse it at 100 amps or 100 x 1.25 = 125A if the wire can handle it. I have a 100A attery protect between the distribution and DC fuse box. Should there be a fuse there too?

Is a 350 amp fuse too big for a 2/0 battery cable? The battery cable length should be about 9 ft. roundtrip but definitely under 10.5 ft. Blue Sea Wizard says 2/0 is good for 330A at up to 28 feet but 331A 1 ft. circut needs 3/0. It doesn't say if that's for voltage drop or safety. My victron Multiplus calls for 1/0 wire and a 300 amp fuse. According to that same calculator a 1 ft. length circuit with 1/0 can't carry more then 285 amps, and at 28 ft the limit is 261 amps yet the circuit needs a 300 amp fuse to be safe. The inverter and DC panel will be within 2 feet of the Lynx distributor. Most DC loads will be less then 5 feet if the DC panel. The exception being diesel heater pump and a water pump with a max amps of 2.4 amps (24V).

So confused. Please help.

 

[HOU_Castle]

What is your total load (at the inverter/load)? including power surge

 

[Jet]

What is your total load (at the inverter/load)? including power surge

• Cont. output power at 75 °F(VA): 3000
• Peak power (W): 6000

 

[Zil]

Rule Number One; Fuses protect wires.
There are two different calculations you need. (#2) Ampacity of the awg cable. Marine grade cable 2/0 is safe up to 330 amperes. (#1) Voltage drop from battery to load. For that you need round trip cable length and total ampere load of the device. The cable awg is selected for lowest voltage drop. The fuse is selected for ampacity of that cable awg.
You should always have a catastrophic fuse at the battery positive terminal.
Update; The 3000W inverter is too much draw for a 12 volt system. You will need 4/0 cable. I use 2000W inverters with 12 volts. Why do you need so much 120V power?

 

[HOU_Castle]

If your output is 115V, then you will have about 130Ah at your battery (24V). What is your battery BMS? 100A?150A?

 

[Jet]

Rule Number One; Fuses protect wires.
There are two different calculations you need. (#2) Ampacity of the awg cable. Marine grade cable 2/0 is safe up to 330 amperes. (#1) Voltage drop from battery to load. For that you need round trip cable length and total ampere load of the device. The cable awg is selected for lowest voltage drop. The fuse is selected for ampacity of that cable awg.
You should always have a catastrophic fuse at the battery positive terminal.
Update; The 3000W inverter is too much draw for a 12 volt system. You will need 4/0 cable. I use 2000W inverters with 12 volts. Why do you need so much 120V power?

It's a 24 volt system not 12. Why not 3000 watt Victron is nearly the same price as a 2000. I'll probably leave it off and power it on only need slot of power. I have a little 24v 300 watt inverter for small 120 stuff like charging my laptop.

 

[Jet]

Rule Number One; Fuses protect wires.
There are two different calculations you need. (#2) Ampacity of the awg cable. Marine grade cable 2/0 is safe up to 330 amperes. (#1) Voltage drop from battery to load. For that you need round trip cable length and total ampere load of the device. The cable awg is selected for lowest voltage drop. The fuse is selected for ampacity of that cable awg.
You should always have a catastrophic fuse at the battery positive terminal.
Update; The 3000W inverter is too much draw for a 12 volt system. You will need 4/0 cable. I use 2000W inverters with 12 volts. Why do you need so much 120V power?

It's a 24 volt system not 12. Why not 3000 watt Victron is nearly the same price as a 2000. I'll probably leave it off and power it on only need alot of power to cook or run the dehumidifier or vacuum. I have a little 24v 300 watt inverter for small 120 stuff like charging my laptop.

 

[Jet]

If your output is 115V, then you will have about 130Ah at your battery (24V). What is your battery BMS? 100A?150A?

My BMS is isn't fet based. I have a 100 amp relay cut off for DC panel. The 300 watt inverter goes through there. BMS can turn off the Multiplus, Orion and solar controller too. Battery continuous discharge rate is 2 C but will probably never be much above 0.5 C.

 

[HOU_Castle]

My BMS is isn't fet based. I have a 100 amp relay cut off for DC panel. The 300 watt inverter goes through there. BMS can turn off the Multiplus, Orion and solar controller too. Battery continuous discharge rate is 2 C but will probably never be much above 0.5 C.

what is the total wire length from battery to inverter and inverter to battery?

 

[Jet]

what is the total wire length from battery to inverter and inverter to battery?

I don't know exactly but I'm estimating it will be 10 feet.

 

[Jet]

Should I just go with a 300 amp fuse to be safe. I doubt I would even blow a 250 amp fuse. I just don't want to have to get a bigger fuse later on if I add more stuff. Honesty I feel uncomfortable with the 300 amp fuse on the inverter. Do you think a 250 amp inverter fuse would do. Victron recommends only a 400A fuse for the 12V version of the same inverter but a 300A fuse for the 24V.

 

[HOU_Castle]

24V battery, roundtrip wire is 10ft, user max continuous load is 100Ah (avg cont. load is 40Ah) . You can use 100A fuse with 2/0 marine cable. I prefer to use bigger cable is to anticipate future load expansion and lots of safety margin. For fuse, you can always replace the fuse when your future load is greater than 100A. Just like Zill said, fuse is to protect wire/cable, so fuse needs to be smaller. But, cheap fuse may not always be the case.

 

[Jet]

24V battery, roundtrip wire is 10ft, user max continuous load is 100Ah (avg cont. load is 40Ah) . You can use 100A fuse with 2/0 marine cable. I prefer to use bigger cable is to anticipate future load expansion and lots of safety margin. For fuse, you can always replace the fuse when your future load is greater than 100A. Just like Zill said, fuse is to protect wire/cable, so fuse needs to be smaller. But, cheap fuse may not always be the case.

What do you mean the avg cont. load is 40AH? The 2/0 cable going from the battery will have the 100 amp DC circut and the 125 amp AC circut to the inverter. 125 x 1.5 = 187.5 + 100 a = 287.5 amp. I was thinking a 300 amp fuse for the battery fuse would be good. The circut for the inverter with the 300A fuse that Victron recommends (mega fuse in Lynx distributer) runs through the 2/0 wire that goes from the battery to the Lynx. A 250 amp fuse seems appropriate to me for the 1/0 (0 awg) wire from the Lynx to battery to Lynx. (The 12V version with twice the current uses only a 400 amp fuse). Does 100A mega fuse for Lynx ➞ Victron Battery Protect ➞ DC fuse box. 2 awg wire for Lynx ➞ BP and (2x) 4 awg wires for BP ➞ Fuse Box. 1/0 and 250A mega fuse for the inverter circuit. 2/0 wire and 300 amp T fuse for the battery cable. 2/0 wire for ground. I assume I should ground from the negative bus bar rather then the negative battery terminal?

 

[HOU_Castle]

Sorry, I miss-understood your earlier narrative/requirements. 125A @24V battery if the load is 3000W -120V. You said mostly you will consume 0.2c (40A) and hardly reach 0.5c (100A). Is this 0.5c an additional to the inverter load (125A)?

 

[Jet]

Sorry, I miss-understood your earlier narrative/requirements. 125A @24V battery if the load is 3000W -120V. You said mostly you will consume 0.2c (40A) and hardly reach 0.5c (100A). Is this 0.5c an additional to the inverter load (125A)?

I said I'll probably will probably never be much above 0.5 C. I don't know what my usage would be. What I was saying was the batteries are rated for 2 C discharge (400 amps). I said on average I would probably use 0.5 (100A) on average. I'l said that because will recommend a 0.5 discharge rate for very long life with LiPO4. I was considering a 300ah battery but right now I only have space for 200ah. I'm not sure what you mean by average. Average when I'm using the inverter or average through out the day. If I'm cooking something, using lights, fan, charging laptop and listening to music it could be higher. If the sun is out solar could be offsetting some of the load. Most of the time during the day i'll probably be using only a few amps (fan, light, phone/laptop charger).

 

[Zil]

Sorry, jet. i mis-read that 24.
But the concept is the same. Voltage drop to size cable awg. Ampacity to chose fuses. I fuse at the top end of ampacity in dc circuits to help keep voltage drop low and avoid nuisance fuse failure.