Math question

[mj52]

Trojan T1275 12v flooded batteries

http://www.trojanbattery.com/pdf/datasheets/T1275_Trojan_Data_Sheets.pdf

I've run into a mental wall. What is the math used for sizing battery cables?

I understand the math for series and parallel from panels to controller. The runs are less than 8ft (system will be built on an industrial sized garden wagon and I'm just going to use #8 for panels to controller and controller to batteries. I believe thats way over sized but for the extra $30 I'm all set for quite some time and even a good deal of expansion. Building 200W but plan on expanding to 400W using Renogy 100W panels

But I'm lost for sizing battery interconnects.

The batteries will be spaced side by side and any inverter will also be as close as practical.

I'm looking for both 12V and 24V info

Thanks, Mike

[inetdog]

Trojan T1275 12v flooded batteries

http://www.trojanbattery.com/pdf/datasheets/T1275_Trojan_Data_Sheets.pdf

I've run into a mental wall. What is the math used for sizing battery cables?

I understand the math for series and parallel from panels to controller. The runs are less than 8ft (system will be built on an industrial sized garden wagon and I'm just going to use #8 for panels to controller and controller to batteries. I believe thats way over sized but for the extra $30 I'm all set for quite some time and even a good deal of expansion. Building 200W but plan on expanding to 400W using Renogy 100W panels

But I'm lost for sizing battery interconnects.

The batteries will be spaced side by side and any inverter will also be as close as practical.

I'm looking for both 12V and 24V info

Thanks, Mike

The math is pretty simple if all you are concerned about is the safety of the cables.
All of the interconnects in a series string carry the same current as that string.
Each string, of N strings, carries 1/N of the current of the inverter or CC wiring to the bank.
That said, it is often a good idea to size the interconnects somewhat larger than strictly required, to minimize the heating effect on the battery terminals and also reduce the extent to which differences in interconnect wire length can affect the total resistance difference between strings.

The amount of current that a wire can safely carry can be found in tables in the National Electrical Code (NEC), among other places.

[mj52]

Thank You but I'm still confused.

This is what I think you said.

1000W inverter being fed by 24V would need cables = to 1000W/24V or 41.6A (50A)
1000W .............................12V.................. ............... 1000W/12V or 83.3A (100A)
300W................................24V........... ..................... 300W/24V or 12.5A
300W................................12V........... ..................... 300W/12V or 25.0A


Look up for 50 and 100A cables and just keep using #8 (since all other cabling is #8) for 12.5 and 25A

BUT FOR BATTERY INTERCONNECT I'd use cabling equal to the highest draw. In this case a 1000W.

[Sunking]

Mike there is no real correct answer. It all depends on how much current you are going to run, distance, and the amount of power loss you can afford.

For example if you charge at 50 amps, then you know you need at least #8 AWG. If your load is say a 1000 amp inverter at 12 volts you are going to kneed at least #4 AWG for the interconnects and wiring to the Inverter.

There is no safety, performance, or code issues with over sizing conductors. Under size and you got a lot of problems with both performance and safety. Morale of the story, error on the side of over size within budget.

[Sunking]

BUT FOR BATTERY INTERCONNECT I'd use cabling equal to the highest draw. In this case a 1000W.

Inter-cell and Inter-Battery use the largest current demand requirement.

Example if you charge at 50 amps but only have a 12 volt 200 watt inverter means you need at least #8 from the controller to battery and inter-connects. From the battery to Inverter you can use AS SMALL AS #12 awg. Battery inter-connects always use the larger of the two being the source or load.

Understand?

[mj52]

Thank You, This seems to make sense.

"For example if you charge at 50 amps, then you know you need at least #8 AWG. If your load is say a 1000 amp inverter at 12 volts you are going to kneed at least #4 AWG for the interconnects and wiring to the Inverter."


Size wire for the appropriate task, but bigger is more betterer no matter what the task.

40A charging batteries uses #8 between CC and battery/bank. I think #8 is good for 55A, I'd have to look that up to be sure though.

Size cables used within the bank and/or feeding load to the largest load expected (for example: 1000w inverter) and use same for lighter loads (300W inverter). Of course if larger cable is physically too large to fit on a smaller inverter resize ONLY cables feeding the smaller inverter. BUT leave cabling within battery bank alone

I think I get it now.

Thanks, Mike

[Sunking]

You are welcome.

[Amy@altE]

There's a good chance the inverter manual will also recommend what size cable to use. Don't forget to have a fuse or breaker between the inverter and the battery bank.

[stevo]

How are cables ends attached? Would a swaging" tool be used?

[Sunking]

How are cables ends attached? Would a swaging" tool be used?

Depends on the size of the cable and if you care about safety. A UL listed compression connector will indicate the right tooling and procedures. Small gauge wires up to #6 AWgg can use a hand ratcheting Compression Tool like this but it takes more than 1 compression.



Up to 2/0 takes a lot more leverage to create 6 to 8 tons of force like this. About 3 feet in length.They have color coded dies to emboss a die code for inspection.



Once you get over 4/0 you use either a hand pump or electric hydraulic 15 ton tooling. They also require color coded dies for the correct wire size



If you don't care, DIY, or hack you use these tools:

[stevo]

So the cable ends are swagged, I have welding lead that's 5/16 or 3/8, I'll check at our local welding shop and compare prices with "tailor made" cables. Thank you

[Sunking]

So the cable ends are swagged, I have welding lead that's 5/16 or 3/8, I'll check at our local welding shop and compare prices with "tailor made" cables. Thank you

Must be regional slang because I do not know what you mean by Swagged.

But you are on the right path. You do not have to to buy the tooling, it is expensive. Lots of options like finding a local electrician with the tooling to make you up cables in his spare time. Golf Cart shops can usually handle up to #2 AWG, cable and motor shops, and it sounds like you live in an area with a Marine Industry and those guys should have lots of tooling for batteries and cables. Sounds like you might need Marine Grade materials.

[stevo]

What are those tools called that you posted?

[Sunking]

Electrical Compression Tools for Compression Connectors. Note example Die code number and Color Code

[inetdog]

Must be regional slang because I do not know what you mean by Swagged.

He means swaged (pronounced swedged). It is another name for a compression forming process as used in compression fittings.

http://en.wikipedia.org/wiki/Swaging