Conductor sizing for 48v 100ah batteries in parallel

[manfromidaho]

I'm brand new to this forum so bear with me. I have a small off-grid system with following:

System: 5000 watt inverter/charge controller (41.7 max current output), (8) 410 watt solar panels, (1) 48v 100ah LiFePO4 battery. I'm looking to add a second battery in parallel with the present battery, giving me a 48v, 200ah setup. I currently use a 30 amp master circuit breaker on the AC subpanel for loads. My one battery is connected using 2awg battery cables through a DC disconnect to the inverter. I use this system to power a garage minisplit, freezer, water softener and recirc pump and a second minisplit inside the house.

My question: with adding another battery in parallel, do I need to be looking at upgrading my battery cabling? I'm currently using 2 awg copper which is rated to 95 amps (4560 watts at 48v). Maximum 120v AC load with my 30 amp AC breaker is 3600 watts. I'm not making any changes on the load side. I therefore see no need to change the cabling from the present 2awg which is more than adequate for my 30 amp 120v subpanel. I will be adding bus bars to connect the 2 batteries with the DC disconnect, but planning to make those connections using 2 awg cables. (Note: Vendor recommended 4 awg in their documentation but I upgraded the cabling to 2 awg.) If I were to max out the inverter and change my 120v AC breaker to 40 amp, I would need to be able to handle 100 amps DC, and at that point I believe I'd need to upgrade my cabling to 1awg based on the standard ampacity charts.

 

[Daddy Tanuki]

no your 2AWG should be fine, your max load will not change, just your overall capacity.

Just make sure your bussbars, or cables if used are clean, antioxident coated and torqued properly. biggest cause of fires in the up in smoke forum are 99.9 connection issues usually not torqued, or poorly prepared. make them same length and ensure that the BMS's in the batteries you use are happy running in a parallel setup (should not be an issue these days but you never know).

 

[ricardocello]

welcome to the forum!

as Daddy Tanuki said, no need to upgrade cables if the load stays the same.

But there should probably be a 100A or 125A fuse to protect the 2 AWG cables.

What are you doing for battery fuses or circuit breakers when you add the busbars?

 

[Daddy Tanuki]

welcome to the forum!

as Daddy Tanuki said, no need to upgrade cables if the load stays the same.

But there should probably be a 100A or 125A fuse to protect the 2 AWG cables.

What are you doing for battery fuses or circuit breakers when you add the busbars?

good catch... just bite the bullet and buy some T class fuses and get the painful part over with. I am the odd bird on this one, but I run 80 amp class T between each DIY battery and the bussbar... I also keep them within 8" of the positive terminal so that if they should catastrophically fail they cannot short out somewhere. YMMV

 

[ricardocello]

good catch... just bite the bullet and buy some T class fuses and get the painful part over with. I am the odd bird on this one, but I run 80 amp class T between each DIY battery and the bussbar... I also keep them within 8" of the positive terminal so that if they should catastrophically fail they cannot short out somewhere. YMMV

If I did my own DIY batteries, I would certainly do the same.
There are too many fire videos and stories out there.

 

[manfromidaho]

no your 2AWG should be fine, your max load will not change, just your overall capacity.

Just make sure your bussbars, or cables if used are clean, antioxident coated and torqued properly. biggest cause of fires in the up in smoke forum are 99.9 connection issues usually not torqued, or poorly prepared. make them same length and ensure that the BMS's in the batteries you use are happy running in a parallel setup (should not be an issue these days but you never know).

Thanks, appreciate the advice on the connection issues. You mention an antioxident--our air here in AZ is very dry, but sounds like a good idea in any case. Any recommendation on a specific product?

 

[manfromidaho]

welcome to the forum!

as Daddy Tanuki said, no need to upgrade cables if the load stays the same.

But there should probably be a 100A or 125A fuse to protect the 2 AWG cables.

What are you doing for battery fuses or circuit breakers when you add the busbars?

The batteries have a 125A breaker built in. That's all I would have between the batteries and the busbars with my current plan. So you're saying I should add a fuse between each battery and the busbar as an extra precaution to protect the conductors
?

 

[ricardocello]

The batteries have a 125A breaker built in. That's all I would have between the batteries and the busbars with my current plan. So you're saying I should add a fuse between each battery and the busbar as an extra precaution to protect the conductors
?

No, those breakers should be fine. That’s how they sell them.
Some people here don’t trust them, but in my opinion, as long as you have some protection for the cables from each battery, that’s good.

You might consider a single Class T fuse from the + busbar to the inverter however.

 

[manfromidaho]

No, those breakers should be fine. That’s how they sell them.
Some people here don’t trust them, but in my opinion, as long as you have some protection for the cables from each battery, that’s good.

You might consider a single Class T fuse from the + busbar to the inverter however.

Thanks, really appreciate the input here. The busbars connect into the DC breaker, which then connects to the inverter in my current scheme. So a Class T fuse either between the + busbar and the DC breaker or between the DC breaker and the inverter? I'm not familiar with the Class T fuses--would you mind pointing me to one?

 

[ricardocello]

Thanks, really appreciate the input here. The busbars connect into the DC breaker, which then connects to the inverter in my current scheme. So a Class T fuse either between the + busbar and the DC breaker or between the DC breaker and the inverter? I'm not familiar with the Class T fuses--would you mind pointing me to one?

If you could draw a quick wiring diagram and post it for us to look at, we can give better advice.
Can be paper and pencil. Please label what breakers are connected to each component.

My interpretation on what you've said far is:

Battery 1 (125A breaker built-in) ->
Battery 2 (125A breaker built-in) -> Busbars -> Breaker (?) or Class T fuse -> Inverter
Ideally the busbars would be located close to the batteries.

Amazon.com : class t

Most people here recommend the Blue Sea Systems Class T fuse block.

The purpose of the Class T is to prevent a short on the busbars or inverter from melting everything and catching fire.
The more LiFePO4 batteries you put in parallel, the more dangerous the instantaneous short circuit current becomes.

With one battery, its built-in breaker alone was probably sufficient.
With two or more, you've got to add layers of safety.

 

[manfromidaho]

Thanks, I'll put together a diagram; will have to be a bit later today but will post out here.

 

[Daddy Tanuki]

this is probably one of the most common/popular but any would work... remember use a wire brush to scrub it into the piece youa re coating. it removes any oxidation while coating it at the same time to prevent new oxidation from forming. aluminum forms in tenths of a second thats why it looks like it does not "rust" but it does, quickly forms a hard coating that prevents more oxygen but also is a barrier to a good connection .

Ox-Gard Anti-Oxidant Compound

Ox-Gard Anti-Oxidant Compound contains prime grade zinc and is formulated with metallic zinc to create a compound that enhances electrical and thermal conductivity which is used on high power crimp connections and stud bolt mountings. When the connection is tightened, the grease is displaced...

www.gardnerbender.com

 

[manfromidaho]

uploaded a poorly drawn diagram to give an idea of what I've got. Sounds like I need to add a fuse, just need to figure out where.
I'll have to procure some more cables. I may go with 1AWG cables for the additional cables so I can increase load later with fewer cable changeouts. The new battery will connect with 1awg and the current battery with 2awg. I don't see a downside as long as I'm consistent with each battery in terms of length and gauge.

 

[Daddy Tanuki]

uploaded a poorly drawn diagram to give an idea of what I've got. Sounds like I need to add a fuse, just need to figure out where.
I'll have to procure some more cables. I may go with 1AWG cables for the additional cables so I can increase load later with fewer cable changeouts. The new battery will connect with 1awg and the current battery with 2awg. I don't see a downside as long as I'm consistent with each battery in terms of length and gauge.

looks like you have a breaker in between the bussbars and the inverter. got a model number or specs for it? reason I ask is that for a DC circuit breaker to handle the grunt of a 5kw inverter its usually pretty large, and usually single pole single throw on the positive side. not both.

 

[manfromidaho]

Yes it's a beast: Suntree, looks lie the model is SM8-250HPV/2.

 

[Daddy Tanuki]

Yes it's a beast: Suntree, looks lie the model is SM8-250HPV/2.

that looks pretty beefy I have never seen one thats dual pole so thats interesting in itself.

 

[manfromidaho]

The recommendation in the manual was for a 200A dual pole but went with the 160A. With that in the mix wondering if I still need the fuse in there between the Busbar and the breaker

 

[ricardocello]

The recommendation in the manual was for a 200A dual pole but went with the 160A. With that in the mix wondering if I still need the fuse in there between the Busbar and the breaker

Honestly, that beast is probably sufficient without a class T fuse. It has a 20kA interrupt rating, same as class T.
And I never thought I would say something like that.
Others here may disagree.

My only concern is the 5 ft of cable from the batteries to the busbars.
I would put the busbars as close as possible to the batteries.

 

[manfromidaho]

OK, thanks. Makes sense. Let me see what I can do about that.

 

[manfromidaho]

I can get the 5 footers down to 3 ft from the batteries to the busbars, then 3 ft to the DC breaker, then 2 ft to the inverter. This is using cables available in 1 ft increments. I could have some custom trimming and terminating done and further reduce the lengths by a few inches here and there but not sure that is really worth it. I don't have the specialty equipment to build my own cables but I can see that might be advantageous if a person did a lot of reconfiguring of his setup.

 

[wilmeek]

uploaded a poorly drawn diagram to give an idea of what I've got. Sounds like I need to add a fuse, just need to figure out where.
I'll have to procure some more cables. I may go with 1AWG cables for the additional cables so I can increase load later with fewer cable changeouts. The new battery will connect with 1awg and the current battery with 2awg. I don't see a downside as long as I'm consistent with each battery in terms of length and gauge.

New to solar but long time electrician here. Your safety device needs to be as absolutely close to the battery terminals as possible. Its function is to stop a catastrophic short. The further away the protection device is, the greater the odds that the catastrophic failure will happen behind your protection. Your breaker is certainly up to the task. It is a beast. But if you want to be able to disconnect your inverter easily closer to the inverter (to limit idle consumption or whatever) maybe another less beefy DC breaker to act as a disconnect. Or a T class fuse close to the battery and leave your breaker where it is.
In my own system, I have a fuse on the negative set about double the surge capacity of all of my loads and a breaker on my positive set to 50% the surge capacity of my inverter and a disconnect right before my inverter just to be able to shut the system down. I also have DC breakers on every string because I like to be able to configure easily. Depending on if you have multiple panels in parallel code requires protection on the strings to prevent feeding a catastrophic short in a panel.

 

[manfromidaho]

New to solar but long time electrician here. Your safety device needs to be as absolutely close to the battery terminals as possible. Its function is to stop a catastrophic short. The further away the protection device is, the greater the odds that the catastrophic failure will happen behind your protection. Your breaker is certainly up to the task. It is a beast. But if you want to be able to disconnect your inverter easily closer to the inverter (to limit idle consumption or whatever) maybe another less beefy DC breaker to act as a disconnect. Or a T class fuse close to the battery and leave your breaker where it is.
In my own system, I have a fuse on the negative set about double the surge capacity of all of my loads and a breaker on my positive set to 50% the surge capacity of my inverter and a disconnect right before my inverter just to be able to shut the system down. I also have DC breakers on every string because I like to be able to configure easily. Depending on if you have multiple panels in parallel code requires protection on the strings to prevent feeding a catastrophic short in a panel.

Thanks. So, translating that, I'm thinking the simplest solution for me would be a fuse on the negative cable between the battery cable and the busbar. The "Max AC bypass current" on the inverter is 63A, so a 125A fuse seems about right. The question then is does that cause an issue with "balance" since the positive and negative would now have different resistance.

Regarding the panels, I have 8 panels wired in series with a 30A disconnect between the panels and the inverter.

 

[wilmeek]

Thanks. So, translating that, I'm thinking the simplest solution for me would be a fuse on the negative cable between the battery cable and the busbar. The "Max AC bypass current" on the inverter is 63A, so a 125A fuse seems about right. The question then is does that cause an issue with "balance" since the positive and negative would now have different resistance.

Regarding the panels, I have 8 panels wired in series with a 30A disconnect between the panels and the inverter.

First, I need to apologize, I forgot that you have batteries with built in protection. No need for the fuses. I saw the picture of the breaker and in my head, I made that the breaker for protection. Those built in breakers will be good enough.
"AC bypass" refers the grid power your all-in-one can put into your ac side bypassing the inverter. ie, it doesn't come out of the battery.
You will want to look at your inverter surge capacity. Most will either be double rated or 50% more. What inverter and batteries do you have?
Your 5K inverter will pull 100 amps at 5 kW. That is very near the continuous rating of most BMSs for a 100 Ah battery.
A surge could exceed it and shut the battery down if you put on that much load.
In general, I set fuses for the maximum capacity of the system (protection from catastrophic failure) and breakers for 50% above the maximum expected load (protection from overcurrent conditions)
But these concerns will evaporate with the second battery.
Also, a single string of panels isn't required to have protection. The other panels in series will limit current from a short. But a breaker is nice to have if you like to play around with configuring panels or want to do work downstream when the sun is out.

For your original question... Like others said, your wiring is fine. The DC current draw is determined by the inverter. You absolutely want your wiring rated to the capacity of the inverter rather than expected loads but 2 AWG is plenty good for that already.

 

[wilmeek]

Thanks. So, translating that, I'm thinking the simplest solution for me would be a fuse on the negative cable between the battery cable and the busbar. The "Max AC bypass current" on the inverter is 63A, so a 125A fuse seems about right. The question then is does that cause an issue with "balance" since the positive and negative would now have different resistance.

Regarding the panels, I have 8 panels wired in series with a 30A disconnect between the panels and the inverter.

One last thing. Make sure you are using pure copper wire. None of that CCA (copper clad aluminum) crap. The current charts for copper wire do not work for CCA. Inside wiring needs to be at least pure copper and outside should be specifically PV wire. It has extras like pure copper, extra abrasion resistance, and UV protection. I like to use marine grade PV wire (tin coated copper for greater protection from corrosion)

 

[Daddy Tanuki]

agreed with all.... make your cables as short as you can while routing them safely with tie downs of some sort. then remove any excess loose wires are trip/hook/pull hazards you either trip on them, hook them on something and then pull them or accidentally grab it for some stupid reason... as short as you can safely secured to the panel.