Sizing inverter cables and fuse for milk crate build

[StinkyT]

Howdy from Texas! It hot so what better time to start a solar power project and work outside!

Jokes aside, I've got a question regarding this initial startup project I have going. This is my first foray into solar and I believe I have a good start to this milk crate/small power box unit it seems I am putting together. I've been running all sorts of numbers (even before finding FilterGuy's energy audit spreadsheet here...cheers for that!) and think I've got it laid out. I'll post a scribbled on modified copy of Will's schematic here so I can show what I am looking at.



The inverter I've selected is the following: Krieger 1100 W Pure Sine

This unit is advertised to work down to 10.5 V, comes with its own cables (think I saw 2 AWG but cannot confirm with the 1100 W unit) and a fuse (again, I saw in a Q&A that its a 110 A). My conundrum is the interaction between the inverter and battery and sizing the wires and main fuse. I've seen a number of threads here with good information but they seem to disagree in the size requirements. For the most part, bigger is better, but it causes problems for weight and finances. I've been back and forth multiple times through charts and explanations and at this point my head is spinning. I'm really just wanting to know if I have the wires and fuse sized correctly. Been back and forth between 2 and 2/0 AWG and 125, 150, 175 and 200 A for these items so now I'm just confusing myself with differing requirements...and the BMS doesn't help clarify that issue with the 150 A overcurrent protection.

This system was originally sized a bit larger (1500 W inverter with corresponding increases in wires/fuse) but given its use case, the battery (12 V, 100 Ah) and BMS became the limiting factor, so I needed to scale it down some. I intend on buying cells and a BMS and assembling my own battery too, and that's another issue but one that I think is more straightforward and fairly well documented.

This has shaped up to be a cool but really nerve-wracking project. Just want to make sure my first shot scores a hit!

 

[rmaddy]

1100W on a 12V battery can be 110A pulled from the battery (1100W / 12V / 85% efficiency = 110A). For a LiFePO₄ battery getting down to 12.0V is just under 10% SOC. 2AWG would be the minimum size. If you plan to fully push 1100W regularly I would go with 1AWG. If you were getting closer to 175A then 1/0AWG or 200A then 2/0AWG.

If you go with the 2AWG (or larger) then a 150A fuse would be a good size. 125A might work but it's a bit close to the 110A load.

If you have a 150A BMS then you could use a 1500W inverter since that would max out at 150A from the battery. If you do that then go with 1AWG (1/0AWG would be better if you will make full use of the 1500W regularly). Then use a 200A fuse.

Unrelated but you probably don't need 8AWG PV wires unless your panels are going to be really far from the charge controller. You can probably use 10AWG, maybe even 12AWG depending on the details.

 

[73powerstroke]

Howdy from Texas! It hot so what better time to start a solar power project and work outside!

Jokes aside, I've got a question regarding this initial startup project I have going. This is my first foray into solar and I believe I have a good start to this milk crate/small power box unit it seems I am putting together. I've been running all sorts of numbers (even before finding FilterGuy's energy audit spreadsheet here...cheers for that!) and think I've got it laid out. I'll post a scribbled on modified copy of Will's schematic here so I can show what I am looking at.

View attachment 101689

The inverter I've selected is the following: Krieger 1100 W Pure Sine

This unit is advertised to work down to 10.5 V, comes with its own cables (think I saw 2 AWG but cannot confirm with the 1100 W unit) and a fuse (again, I saw in a Q&A that its a 110 A). My conundrum is the interaction between the inverter and battery and sizing the wires and main fuse. I've seen a number of threads here with good information but they seem to disagree in the size requirements. For the most part, bigger is better, but it causes problems for weight and finances. I've been back and forth multiple times through charts and explanations and at this point my head is spinning. I'm really just wanting to know if I have the wires and fuse sized correctly. Been back and forth between 2 and 2/0 AWG and 125, 150, 175 and 200 A for these items so now I'm just confusing myself with differing requirements...and the BMS doesn't help clarify that issue with the 150 A overcurrent protection.

This system was originally sized a bit larger (1500 W inverter with corresponding increases in wires/fuse) but given its use case, the battery (12 V, 100 Ah) and BMS became the limiting factor, so I needed to scale it down some. I intend on buying cells and a BMS and assembling my own battery too, and that's another issue but one that I think is more straightforward and fairly well documented.

This has shaped up to be a cool but really nerve-wracking project. Just want to make sure my first shot scores a hit!

I think depending on you want to run, that inverter is a bit small and definitely 2ga is fine for that small of one. 10ga is fine for the pv wire. I've run this exact panel configuration. You'll be somewhere around 10amps at 36v or so.
400w÷14.4v=27.7 amps available. The charge controller will handle that just fine. I always use 6ga thhn wire to my batteries but 8 should be fine. There's a chart showing the wire sizes and how many amps they can safely carry at what voltage. I'll see if I can find it. It's the only one you can rely on. Many other charts are for 120, 240v ac guidelines.
You need to know the max amps the inverter can draw under full load and size the breaker above that. The 50amp for a 40amp charge controller is fine. You probably will never even get close to 40amps out of the MPPT. Another good thing to have is a dc clamp ammeter and a shunt meter kit. You can install the shunt meter between the battery and the inverter and see and monitor your actual draw.
Here's a brief video I did of how the clamp meter is helpful and the shunt meter is shown in this clip.
I've since switched to 24v system and 24v inverter because I don't like high amps. But this should demonstrate how to see what you really need under load. And fuse according.

 

[rmaddy]

You can install the shunt meter between the battery and the inverter and see and monitor your actual draw.

The shunt needs to be between the battery and everything else (the SCC in this case), not just the inverter.

 

[73powerstroke]

The shunt needs to be between the battery and everything else (the SCC in this case), not just the inverter.

I think installing 3 is best . The one I have shows some pretty cool arrangements in the instructions. It's one of those peacefair Amazon deals but I got it from santan solar. As I recall you can use 2 meters on the same shunt to read charge and discharge. I got a bigger 300a one yesterday.
Here's the pictures from the smaller one and ways to use it.

 

[rmaddy]

I think installing 3 is best .

I guess it depends. I only have and need 1 shunt. My SCC provides its own details. My inverter/charger provides its own details. The shunt provides all of the details related to the battery and power going in and out. And I can see all of the relevant details of all of the devices on one nice screen. Granted, I paid more for it.

 

[73powerstroke]

I guess it depends. I only have and need 1 shunt. My SCC provides its own details. My inverter/charger provides its own details. The shunt provides all of the details related to the battery and power going in and out. And I can see all of the relevant details of all of the devices on one nice screen. Granted, I paid more for it.

The reason that I would put another one there is I am so sick and tired of toggling through the menus on my charge controllers

 

[StinkyT]

1100W on a 12V battery can be 110A pulled from the battery (1100W / 12V / 85% efficiency = 110A). For a LiFePO₄ battery getting down to 12.0V is just under 10% SOC. 2AWG would be the minimum size. If you plan to fully push 1100W regularly I would go with 1AWG. If you were getting closer to 175A then 1/0AWG or 200A then 2/0AWG.

If you go with the 2AWG (or larger) then a 150A fuse would be a good size. 125A might work but it's a bit close to the 110A load.

If you have a 150A BMS then you could use a 1500W inverter since that would max out at 150A from the battery. If you do that then go with 1AWG (1/0AWG would be better if you will make full use of the 1500W regularly). Then use a 200A fuse.

Unrelated but you probably don't need 8AWG PV wires unless your panels are going to be really far from the charge controller. You can probably use 10AWG, maybe even 12AWG depending on the details.

Thanks for the reply and the information. This lets me know I was on the right path but gives me some more to chew on in this iterative process.

 

[StinkyT]

I think depending on you want to run, that inverter is a bit small and definitely 2ga is fine for that small of one. 10ga is fine for the pv wire. I've run this exact panel configuration. You'll be somewhere around 10amps at 36v or so.
400w÷14.4v=27.7 amps available. The charge controller will handle that just fine. I always use 6ga thhn wire to my batteries but 8 should be fine. There's a chart showing the wire sizes and how many amps they can safely carry at what voltage. I'll see if I can find it. It's the only one you can rely on. Many other charts are for 120, 240v ac guidelines.
You need to know the max amps the inverter can draw under full load and size the breaker above that. The 50amp for a 40amp charge controller is fine. You probably will never even get close to 40amps out of the MPPT. Another good thing to have is a dc clamp ammeter and a shunt meter kit. You can install the shunt meter between the battery and the inverter and see and monitor your actual draw.
Here's a brief video I did of how the clamp meter is helpful and the shunt meter is shown in this clip.
I've since switched to 24v system and 24v inverter because I don't like high amps. But this should demonstrate how to see what you really need under load. And fuse according.

The inverter is a bit small to do anything substantial with, but I'm working on a budget and just need something for emergencies; fans, cell phones, laptop, lights, etc... I've looked into some small appliances like a toaster oven and coffee maker but those require larger reserves than a single 12 V, 100 Ah battery can supply and I've come to learn that the batteries are the expensive part of these projects.

 

[73powerstroke]

I guess it depends. I only have and need 1 shunt. My SCC provides its own details. My inverter/charger provides its own details. The shunt provides all of the details related to the battery and power going in and out. And I can see all of the relevant details of all of the devices on one nice screen. Granted, I paid more for it.

Epever has a new cc that shows everything with out a scrolling or toggle screen and has higher pv voltage
Would have made my life better.

 

[73powerstroke]

The inverter is a bit small to do anything substantial with, but I'm working on a budget and just need something for emergencies; fans, cell phones, laptop, lights, etc... I've looked into some small appliances like a toaster oven and coffee maker but those require larger reserves than a single 12 V, 100 Ah battery can supply and I've come to learn that the batteries are the expensive part of these projects.

A 2500w reliable is about $300 from santan solar in az. But since you're in Texas you can get one on Amazon for similar price. Also had I know what I know now I would have went right to 24v or even 48v battery system. The current demand on 12v system is crazy. I'm running ac units however. I actually ran a window ac off a 2000w harbor freight inverter $179 for over a month. So it's possible to cheap out even more. But buy the protection plan lol ?

 

[StinkyT]

So here’s my next question; does the inverter power rating reflect the absolute max load that can be hooked up or is the max load actually below the power rating? For example, in a 1100 W inverter, is the max load 1100 W? Or is it around 900 W given the 85% efficiency rating? If an 1100 W load can be applied, even with the efficiency loss, what happens to the current draw from the battery?

 

[rmaddy]

An 1100W inverter can provide up to 1100W of continuous AC power (though cheap inverters might not be happy about doing so for too long).

Due to the inefficiency of the DC-AC conversion, the battery must provide roughly 1290W of power to the inverter for the inverter to output those 1100W. When the battery voltage is at 12.0V then those 1290W will be at 107.5A. When the battery voltage is at 14.4V then those 1290W will be at 89.5A.

Note how my answer in post #2 states the inverter can pull up to 110A to provide the 1100W. This matches (with some slight rounding differences) what I just showed.

Also note that some people will actually use 10.0V instead of 12.0V in their calculations. Meaning 1100W / 10.0V / 85% = 130A. 10.0V is 0% SOC for a LiFePO₄ battery. 12.0V is about 9.5% SOC for a LiFePO₄ battery. You certainly do not ever want your LiFePO₄ battery to get down to 10.0V. 12.0V is plenty low enough. However, most cheaper inverters have a pathetically low LVD (low voltage disconnect) meaning that the inverter won't shut itself off until the battery is basically near 0%. Keep this in mind depending on the inverter you have and your intended use. If you plan to run an 1100W load constantly and until the battery is dead (not a good idea) then you should probably go one size bigger on the wire.