Fuse / Wire Gauge / Inverter Question

[chadmichael]

I have a 2200 Watt 12v Pure Sine Wave inverter. I went with 4/0 AWG wire, 9 inches in length, to connect my inverter to my battery array (2x170A LiPo in parallel). I know this is overkill, but I'm trying to minimize loss because I hope to run an induction cooktop. Also, Will generally recommends as large a wire as you can here.

The math for fusing the inverter line to the battery seems to vary but it appears that 250 or 300 will work. Those numbers are based on anticipated load. However, I've also read that you should calculate fuse size to protect the wire, and that unnecessarily small fuses introduce resistance. In that case, this fuse could be huge ... 9 inches of 4/0 surely carries more amps that my system will ever need.

Do I need to worry that the 250 or 300 Amp fuse is going to introduce problematic resistance? What would be the consequences of a 400A fuse? What would be the consequences of unnecessary resisteance with the 250 A fuse?

 

[NVS]

If you do the the math your 2200watt inverter can pull 183.3333 amps (2200watts ÷ 12v= 183.333) fuse should be 125% of load so 183.33 ×1.25 = 229amps. So a 250amp should be ok.

 

[HRTKD]

Chad, I would still put in a fuse.

 

[Zil]

The 4/0 marine grade cable, whatever length, can only carry 445 amperes. The largest Class-T fuse I have is 400A. That is what I would fuse the battery positive terminals. Each battery fused to a BusBar. Then 250 or 300 ampere fuse to inverter with 2/0 cable.

 

[chrisski]

Fusing protects wiring…

A 250 amp fuse would be fine. 200 amp fuse should be good, but should be can get expensive if I’m wrong.

Nothing wrong with 4/0 and a 250 amp or 200 amp fuse. Just be sure you can manipulate the wire. 4/0 is as thick as my thumb. A class T fuse holder is 6” and ANL is 4“. You may need more than 9” to get the wiring where it needs to go.

 

[NVS]

Just remember you are protecting the equipment not the wire. So keep that in mind. 250amp fuse is correct for this application.

 

[Zil]

No No NO Fuses protect wire! Even devices with built-in fuses have them to protect the internal wires from short circuit.
As to using smaller fuses than the cable gauge ampacity, it introduces more voltage drop than a fuse sized to the ampacity. The other risk is selecting a too small of fuse and suffering nuisance failures.

 

[chadmichael]

No No NO Fuses protect wire! Even devices with built-in fuses have them to protect the internal wires from short circuit.
As to using smaller fuses than the cable gauge ampacity, it introduces more voltage drop than a fuse sized to the ampacity. The other risk is selecting a too small of fuse and suffering nuisance failures.

Seems like you understand what I'm actually asking ... so is there a downside to having a oversized wire and then having a fuse based on that ampacity? I imagine for instance, with less than foot or two of 4/0 I could have an even higher rated fuse than 300Amp. Sounds like in terms of efficiency this is all good. Any draw backs to that approach?

 

[John Frum]

Seems like you understand what I'm actually asking ... so is there a downside to having a oversized wire and then having a fuse based on that ampacity? I imagine for instance, with less than foot or two of 4/0 I could have an even higher rated fuse than 300Amp. Sounds like in terms of efficiency this is all good. Any draw backs to that approach?

Assuming pure copper wire with 105C insulation...
6 awg gets 100 amp fuse
4 awg gets 150 amp fuse
2 awg gets 200 amp fuse
2/0 awg gets 300 amp fuse
4/0 awg gets 400 amp fuse

The max continuous current you should draw on each of those wire sizes is 80% of the fuse rating.

 

[John Frum]

Check each appliance to see if it has built in over-current protection.
If it doesn't have it, either design to accommodate or better yet, get an appliance that does have its own over-current protection.
Passive in-line components are considered to be part of the wire.

In-line component examples: busbars, shunts, disconnect switches.

Appliance examples: inverter, solar charge controller, dc2dc converter.

 

[740GLE]

Any inverter worth buying will have internal overload protection.

Buying a larger fuse will save you from buying another fuse if you source a larger inverter in a year or so.

As for internal resistance/voltage drop of say a 200a va 400a fuse. You’ll never notice a difference.

What BMS do the batteries have? What are they rated for?

 

[Supervstech]

My solar trailer build has two 6500W inverters, I wired everything with 2/0 and 4/0 to the main switch...
The class T fuses on each leg are 400A to cover any catastrophic failure.
I'm putting DC 1000V 165A breakers on each inverter...
Fuse for wire protection, breakers for equipment overload protection.

 

[Zil]

There is a row of answers! No, I don't see any electrical downside to using oversize cables with a large fuse to protect that cable. Lower voltage drop is always a good thing with battery dc.
Except; If you have LFP battery with a BMS, I would prefer not drawing current over the rated amperes of the BMS. I'm not sure the fuse is the way to do that. I think system design is how that is accomplished. My opinion, the BMS should never open except in a last trench emergency.