Battery Bank Wire Size

[mitchk]

I plan on drawing 35A max DC and my inverter is 1500W with a 3000W surge. I am sizing the cable necessary to connect the batteries right now. I plan on connecting my 3 138Ah lithium batteries in parallel using two stud junction posts (one for positive, one for negative). That would mean the cable length would be 1.5ft max. According to an online calculator at 1.5ft I need 4AWG for 160Amps. This accounts for 35A of general DC current and 125A for the inverter. Do I need to size the cable for the 3000W surge even if it is only for a couple of seconds? I am positive I would never have a surge like that and honestly will probably only ever pull 100A from the inverter. Does 4AWG sound big enough to you guys?

[TheArgobus]

It sounds right but what are you powering with your inverter? 160 amps DC makes me nervous

[mitchk]

Originally Posted by TheArgobus

It sounds right but what are you powering with your inverter? 160 amps DC makes me nervous

So the 160 number was a quick max I tried to calculate. That is assuming I am using the following appliances and that they are at ramp up amperage:

Maxxair fan: 3 max
Shurflo Water pump: 7.5 max
LED Lights: 2A max
Fridge: 6A max
Diesel Heater: 9A max
12V receptacles: 10A max

My biggest inverter draw would be my instant pot at 700W or 58A DC pull from the battery bank. Even if I was using my computer and monitor and other misc 120V items I can't imagine I would ever be above 100A DC going to the inverter.

The 12V DC loads are all maxs and realistically they would all be lower, especially the heater.

I just said 160 as a max in order to try and calculate worst case. Maybe I will get a small microwave or something that will use more of the inverter capability.

What do you mean 160A DC makes you nervous? I was under the impression that people pull more then that with a similar battery bank to mine but I definitely am not an expert and I appreciate your input!

[mitchk]

And now that I'm thinking about it maybe I should just get a 1000W inverter if I never really plan on going over 900W anyways.

[dzl_]

Here is my understanding of how the process of sizing wire / circuit protection should go:



Step one,

Calculate the max continuous amperage you will draw at any given time (you've already done this):

160A


Step two,
Multiply by 1.25, this will give you your ballpark fuse/breaker size:
200A


Step three,
Select a wire size that has an ampacity rating higher than 200A (such as 2 AWG if you use marine grade copper wire, if you use automotive wire look up the ampacities). And choose a wire size that meets your voltage drop target (3% is the standard for marine) remembering that this is calculated over the whole length of a circuit and uses round trip distance.

Do I need to size the cable for the 3000W surge even if it is only for a couple of seconds? I am positive I would never have a surge like that and honestly will probably only ever pull 100A from the inverter.

Don't worry about the surge capacity here, like you said it'll only be milliseconds, not enough time for the wires to heat up or most fuses to trip. Size for the max possible continuous output, and just make damn sure your wire is sized above your fuse, and choose a fuse or breaker. An MRBF fuse is a good option for a main battery fuse.


Here is a good resource for wire sizing


And a good wire size calculator

[mitchk]

Originally Posted by dzl_

Here is my understanding of how the process of sizing wire / circuit protection should go:



Step one,

Calculate the max continuous amperage you will draw at any given time (you've already done this):

160A


Step two,
Multiply by 1.25, this will give you your ballpark fuse/breaker size:
200A


Step three,
Select a wire size that has an ampacity rating higher than 200A (such as 2 AWG if you use marine grade copper wire, if you use automotive wire look up the ampacities). And choose a wire size that meets your voltage drop target (3% is the standard for marine) remembering that this is calculated over the whole length of a circuit and uses round trip distance.





Don't worry about the surge capacity here, like you said it'll only be milliseconds, not enough time for the wires to heat up or most fuses to trip. Size for the max possible continuous output, and just make damn sure your wire is sized above your fuse, and choose a fuse or breaker. An MRBF fuse is a good option for a main battery fuse.


Here is a good resource for wire sizing


And a good wire size calculator

Thanks for the info. This is very helpful. I was wondering about cable size for different components. All three batteries will go to a common stud. A cable will go from the stud to the inverter and a cable will go from the stud to the DC fusebox/ bus bar.

Assuming I go with an 1200W inverter the max amps I will push to the inverter is 100A. I believe my max DC would be 40A. So couldn't my cable from the common battery stud to the DC bus bar be a smaller gauge then the cable going to the inverter? Along those lines about a third of the total amperage will be coming from each battery so the cable from each battery to the common stud would actually be sized for a third of the total amperage leaving the battery bank stud. Am I thinking about this correctly?

On another note if I size the cable for the max wattage as opposed to the surge wattage and I want to add a fuse in between the battery and the inverter wouldn't the fuse have to be rated above the cable in order to account for the surge amperage?

[dzl_]

Originally Posted by mitchk

I was wondering about cable size for different components. All three batteries will go to a common stud. A cable will go from the stud to the inverter and a cable will go from the stud to the DC fusebox/ bus bar.

Your on the right track, wire size for the main battery + and - needs to be able to handle the total current (in one direction -- charge or discharge not both together) of your system. But each branch circuit only needs to be sized for the current that will flow through. But you do have to keep in mind that voltage drop is cumulative over the whole path current will take to an appliance and back.

Assuming I go with an 1200W inverter the max amps I will push to the inverter is 100A.

One thing I've been pondering is if you have a 1200W inverter and that rating is the power it can output. Should you take into account your inverters efficiency (ballpark of ~85%, lets say 80% to be safe). If your inverter is 80% efficient and can output 1200W, then to output 1200W its really drawing 1500W or 125A.

The easy answer to this is just follow the fuse size recommendation in your inverter manual, and make sure the wire gauge is larger than the fuse.

So couldn't my cable from the common battery stud to the DC bus bar be a smaller gauge then the cable going to the inverter?

Exactly correct, each branch of the system only needs to be rated/protected for the current that could flow through it. BUT remember, each time you step down in wire size, you need a smaller fuse/breaker near the start of that branch to protect the wire.

Along those lines about a third of the total amperage will be coming from each battery so the cable from each battery to the common stud would actually be sized for a third of the total amperage leaving the battery bank stud. Am I thinking about this correctly?

Its beyond my comfort zone / knowledge to give input on this. I do have some thoughts though, but take em with a grain of salt.

1st, I see what you are getting at, 3 batteries in parallel should each be supplying 1/3 of the current in theory. I think you understand it right on a theoretical level.

2nd, but I'm wondering about failure modes in the real world. Would slightly different resistances of the wire, crimps, connections, etc mean that in practice one battery might supply more than its theoretical share of the current. Or if there was an issue with one battery, you would now have two batteries supplying half the current each with wire only rated for a 1/3 of the current. If you had fuses on each of the three wires rated for 1/3 the total current, this would not be a safety issue. but if you only had one fuse after the common stud, this would be a potential safety risk

Maybe these concerns are exaggerated or solveable, I don't know, these are just my initial thoughts on a topic I don't have much knowledge of.

[dzl_]

wouldn't the fuse have to be rated above the cable in order to account for the surge amperage?

This is a good question, I asked a similar question not too long ago, turns out the answer is usually no.

Inverter surges are measured in milliseconds or at most a couple seconds (for expensive low frequency inverters). Fuses and breakers have what is called a 'trip curve.' Essentially they don't immediately trip the second their rating is exceeded, the amount of time it takes to trip is proportional to the extent that the rating is exceeded. Her is an example of the trip curve of a popular circuit breaker

In this example the red band is the area where the circuit breaker would be expected to trip. From this we can see that a 300% surge for 1 second or a 200% surge for 5 seconds shouldn't trip the breaker, Every type of fuse and breaker will have a distinct 'trip curve' but the same general principle applies. There are 'fast acting' fuses and 'slow blow' fuses and breakers which allow you to tailor your protection device to what its protecting, but the principle is the same, the curve will just look different. Often your inverter manufacturer will recommend a specific type fuse in the manual.


Though there are no 'trip curves' charts for wire as far as I know, the same principle applies, a wire won't fail immediately unless its a massive surge. wire fails by overheating, melting the insulation, shorting or causing fire. The rating of the wire is a continuous rating (and its worth noting that the rating is based on the temperature rating of the insulation, not all wire of the same gauge will be rated the same), a short surge won't cause it to fail unless its a huge surge.

[Kubla]

100 amp is 2 gauge, do that stuff at work

[mitchk]

Originally Posted by dzl_

Your on the right track, wire size for the main battery + and - needs to be able to handle the total current (in one direction -- charge or discharge not both together) of your system. But each branch circuit only needs to be sized for the current that will flow through. But you do have to keep in mind that voltage drop is cumulative over the whole path current will take to an appliance and back.

Wow thanks for all your input. This has been very helpful. I read somewhere that you should only attach 3 wire lugs to a stud so I guess I will get a positive and negative bus bar and have one or two cables per stud on the bus bar. I will probably just us 2AWG for all the battery connections just to be safe. I think I could use 4AWG for the cables going to the inverter and the DC bus bar but maybe I will do it all in 2AWG just to be safe. Although 4 does seem to be pretty safe... I still have to wrap my head around fuses/ breakers and where I need to put them.

[Iceni John]

I'm glad that you're giving some serious thought to cable size etc, but all too often folk overlook the cable's lugs, and more specifically how they're attached to the cable. There's not much point in having the correct gauge of cable if it's terminated with poor quality lugs, especially if they're attached with one of those godawful hammer crimpers. Spend the extra (and it needn't be that much more overall) to use good closed-end tinned lugs, and use a proper circumferential crimper such as the long-handle FTZ 94284 that will easily and consistently make a gas-tight connection that essentially cold-fuses the cable and lug together. Even the cheapo Harbor Fright hydraulic crimper makes decent hexagonal crimps on smaller lugs, but the big FTZ makes the best crimps overall. I've cut such lugs apart to see how well they are crimped, and the cable's strands and lug have become one indivisible solid mass of copper. Very impressive! If you finish off the cable ends with adhesive heatshrink to prevent any moisture from penetrating inside, then you'll have cables that will never give you any problems even years down the road.

John

[dzl_]

Originally Posted by mitchk

I still have to wrap my head around fuses/ breakers and where I need to put them.

Yeah it's sometimes quite confusing. For the major components they often recommend a particular fuse size and placement, or give an example diagram in the manual.


For me I've found it helpful to remember the saying 'the fuse protects the wire' which conveys that the (1) sized, and (2) positioned in a way that protects the wiring, not just the device.



I found these two resources super helpful when trying to wrap my head around fuses and circuit protection:

1. Video on fuses and circuit protection
2. Fuse/breaker sizing and positioning and accompanying forum post

Regarding busbars, I think you have the right idea. Not only is it better from a mechanical point of view, using busbars can help keep things cleaner/less messy/easier to trace. You can also use something like this to kill two birds with one stone (busbar + circuit protection).



John makes a good point about proper crimps and connections and it sounds like he really knows his stuff! If you just have a few large gauge crimps to make and don't want to buy a crimper, companies like baymarinesupply.com and genuindealz.com will make custom length cables for you and make the connections for you.

[Phatman]

Mitch - if you use DLO cable (locomotive cable) the ampacity of #2 is 190 amps. Don't know if they do it where you are but here in the Houston area can rent a hydraulic crimper from Tool Mart. You can go to an electrical contractor and explain what you need and they may be able to help you out if you have all your material. Find a commercial/ industrial contractor not a residential. Good luck.

[mitchk]

Originally Posted by dzl_

Yeah it's sometimes quite confusing. For the major components they often recommend a particular fuse size and placement, or give an example diagram in the manual.


For me I've found it helpful to remember the saying 'the fuse protects the wire' which conveys that the (1) sized, and (2) positioned in a way that protects the wiring, not just the device.



I found these two resources super helpful when trying to wrap my head around fuses and circuit protection:

1. Video on fuses and circuit protection
2. Fuse/breaker sizing and positioning and accompanying forum post

Regarding busbars, I think you have the right idea. Not only is it better from a mechanical point of view, using busbars can help keep things cleaner/less messy/easier to trace. You can also use something like this to kill two birds with one stone (busbar + circuit protection).



John makes a good point about proper crimps and connections and it sounds like he really knows his stuff! If you just have a few large gauge crimps to make and don't want to buy a crimper, companies like baymarinesupply.com and genuindealz.com will make custom length cables for you and make the connections for you.

Dang, once again coming through with the clutch info! That video and power point was super helpful. I'm going to make a diagram next and hopefully get it checked here. After that I'll start buying components! Thanks for your helpful input!

[mitchk]

Originally Posted by Iceni John

I'm glad that you're giving some serious thought to cable size etc, but all too often folk overlook the cable's lugs, and more specifically how they're attached to the cable. There's not much point in having the correct gauge of cable if it's terminated with poor quality lugs, especially if they're attached with one of those godawful hammer crimpers. Spend the extra (and it needn't be that much more overall) to use good closed-end tinned lugs, and use a proper circumferential crimper such as the long-handle FTZ 94284 that will easily and consistently make a gas-tight connection that essentially cold-fuses the cable and lug together. Even the cheapo Harbor Fright hydraulic crimper makes decent hexagonal crimps on smaller lugs, but the big FTZ makes the best crimps overall. I've cut such lugs apart to see how well they are crimped, and the cable's strands and lug have become one indivisible solid mass of copper. Very impressive! If you finish off the cable ends with adhesive heatshrink to prevent any moisture from penetrating inside, then you'll have cables that will never give you any problems even years down the road.

John

Thanks for the info John. I was planning on using a hammer crimper but you have convinced me to go another direction.

[mitchk]

Originally Posted by Phatman

Mitch - if you use DLO cable (locomotive cable) the ampacity of #2 is 190 amps. Don't know if they do it where you are but here in the Houston area can rent a hydraulic crimper from Tool Mart. You can go to an electrical contractor and explain what you need and they may be able to help you out if you have all your material. Find a commercial/ industrial contractor not a residential. Good luck.

I'll look into that, thanks!

[mitchk]

Alright so I already have run into another question. I will probably use around 8AWG to wire my charge controller to battery bank. I am putting a fuse between the solar charge controller and the battery bus bar. I'm assuming I would want to put the fuse as close to the charge controller as possible since power will be starting in the charge controller and going to the battery bank. Or is their a failure scenario where the power would actually go backwards from the battery bank to the charge controller? If that could happen would I actually want the fuse closest to the battery bus bar?

Also I'm assuming I would want a master switch to turn off charging the batteries. I would want to put that switch between the solar panels and charge controller right? I know you never want the charge controller just plugged into the solar panels and not the batteries.

I also was thinking about the cable between the batteries and the bus bar. I plan on making it 2AWG which has a large safety factor built in. Do people usually put fuses in between the batteries and bus bar? I am wiring my 3 batteries in parallel directly to the bus bar and not with each other and then to the bus bar.

[sproutroot]

Are all of these components going into one cabinet? Is it sealed? Will any of your big cables be long?

My recommendation for sizing a 10' cable would be completely different than a 3' cable, and airflow also plays a huge part. Playing on the safe side with huge wires is nice if you're the Hulk, but in a small cabinet those thick wires are very difficult to deal with. If you don't have the muscle to truly shape the cables, they'll be putting a ton of strain on your crimps, which could spell disaster.

Along those lines about a third of the total amperage will be coming from each battery so the cable from each battery to the common stud would actually be sized for a third of the total amperage leaving the battery bank stud. Am I thinking about this correctly?

Not quite, but close. dzl_ was right that amperage is rarely consistent between 3 batteries. However, there is another way to calculate wire size.

Starting batteries have a CCA rating (cold cranking amps), and it's the max current they can discharge. Deep cycle batteries don't have this, but you can figure it out. I use a deep cycle as a starting battery for my boat, and I had to oversize it by 20% compared to the recommended starting battery for my application. (source: "Deep Cycle Battery as Starting Battery" - https://www.solar-electric.com/learn...ting%20battery)

That is a rule I've seen elsewhere as well, so:
(Deep Cycle Battery Amp Hours) * 0.8 = (Comparable Starting Battery Size)

Then you look up CCA for that size starting battery, and you've got a pretty good idea of how to size your cables (ALWAYS round up).

I'm assuming I would want to put the fuse as close to the charge controller as possible since power will be starting in the charge controller and going to the battery bank.

Again, the size of this installation is key here.

If all these components are in one cabinet, there isn't much to worry about. Put the fuse where you can get at it.

If this circuit goes through a bulkhead, or the cable's at risk of getting pinched or cut by something, put the fuse on the controller side of the "danger zone". You should also be using conduit and grommets to protect this type of circuit.

[mitchk]

Originally Posted by sproutroot

Are all of these components going into one cabinet? Is it sealed? Will any of your big cables be long?

My recommendation for sizing a 10' cable would be completely different than a 3' cable, and airflow also plays a huge part. Playing on the safe side with huge wires is nice if you're the Hulk, but in a small cabinet those thick wires are very difficult to deal with. If you don't have the muscle to truly shape the cables, they'll be putting a ton of strain on your crimps, which could spell disaster.



Not quite, but close. dzl_ was right that amperage is rarely consistent between 3 batteries. However, there is another way to calculate wire size.

Starting batteries have a CCA rating (cold cranking amps), and it's the max current they can discharge. Deep cycle batteries don't have this, but you can figure it out. I use a deep cycle as a starting battery for my boat, and I had to oversize it by 20% compared to the recommended starting battery for my application. (source: "Deep Cycle Battery as Starting Battery" - https://www.solar-electric.com/learn...ting%20battery)

That is a rule I've seen elsewhere as well, so:
(Deep Cycle Battery Amp Hours) * 0.8 = (Comparable Starting Battery Size)

Then you look up CCA for that size starting battery, and you've got a pretty good idea of how to size your cables (ALWAYS round up).



Again, the size of this installation is key here.

If all these components are in one cabinet, there isn't much to worry about. Put the fuse where you can get at it.

If this circuit goes through a bulkhead, or the cable's at risk of getting pinched or cut by something, put the fuse on the controller side of the "danger zone". You should also be using conduit and grommets to protect this type of circuit.

I am planning on putting everything underneath my couch. So it will be pretty close together but not crazy tight or anything. I believe that my longest length would be from my solar panels to my charge controller and the AC wall outlets from the inverter. I could add a vent fan to the outside in order to try and cool down the compartment.

Speaking of that I noticed that most 1000W inverters don't have an area for you to hardwired your wall outlets into. Is it as simple as plugging into the inverter outlets and then wiring that to your wall outlets?

It looks like the Valence 12XP batteries I am using have a max continuous discharge rate of 150A so that would be well within what I need since I am using 3 in parallel.

Also since we are going over everything wiring. Wouldn't it be bad if my main 200A circuit breaker coming from the battery tripped? This would mean that the charge controller would be disconnected from the batteries and would be fried by the solar panels right? I guess I could connect the charge controller to the battery side of the circuit breaker so it is always connected. Although then I don't have a master switch way of turning everything, including the charge controller off.

[sproutroot]

Originally Posted by mitchk

I believe that my longest length would be from my solar panels to my charge controller and the AC wall outlets from the inverter.

Neither of these are a concern. Your AC circuits are 110V, so their efficiency loss is negligible. They're sized and fused following standard wattage formulas, which you've already been over in this thread. I'm assuming your solar installation is a kit, which means the wires are undersized, but I wouldn't spend the money to replace them.

Originally Posted by mitchk

I could add a vent fan to the outside in order to try and cool down the compartment.

If you're going to vent it, do it outside. You really shouldn't need a fan. If I did a fan, I'd have it hooked to a thermostat - I don't like giving away wattage.

Originally Posted by mitchk

Speaking of that I noticed that most 1000W inverters don't have an area for you to hardwired your wall outlets into. Is it as simple as plugging into the inverter outlets and then wiring that to your wall outlets?

I only have experience with this insofar as how bad the practice can get if used in a farmhouse over several generations. Wall circuits plugged into wall outlets.
Seems silly to tell you not to do it in a bus, but I'd warn you to keep your wall outlets to an absolute minimum if these wires are inside the walls.

Originally Posted by mitchk

I guess I could connect the charge controller to the battery side of the circuit breaker so it is always connected. Although then I don't have a master switch way of turning everything, including the charge controller off.

This is how I have it on my boat. I use DC breakers instead of switches, but there's one between the batteries and the inverter/converter, another between the batteries and the alternator, and a third between the batteries and solar. There are ways to have it all happen automatically, but I like "operating" my equipment.