Are you building a battery bank for an alternative energy system? Or are you just wondering what size wire to connect 12 volt batteries?
You may be trying to determine the correct size interconnect cables between the batteries and the charger/inverter.
How long should the cables be? Size? More exactly, what caliber (AWG)?
Read on... or go directly to the chart (AWG vs. Amps) at the bottom of the page.
The short answer (regardless of your application) is to know the maximum potential current (amps) that will flow through the interconnect circuit, based on your schematic. So there is no quick and correct answer. That depends…
I recommend buying a clamp meter that measures DC current! This will help you check what's really going through your strands! The clamp multimeter below will measure up to 400 amps.
Klein Tools Digital Amplifier Clamp
(see on amazon)
the battery bank
A battery bank for an off-grid solar powered alternative energy system will consist of several batteries and their interconnecting terminal cables.
Batteries will be connected in various series parallel configurations based on their schematic layout to achieve the desired voltage and capacity to work best with the inverter (and charger).
The batteries will serve as an energy storage center to provide power during periods when the solar panels are dark or not working properly due to weather conditions.
I will not go into recommendations for connecting batteries in parallel in series (another article). However, I'll cite my own current setup as an example:
My system as an example...
The 24 batteries are rated at 12 volts and 100 AH. There are 6 strings in series (4 each) that are all connected in parallel to form a 48 volt 600 AH battery bank. That's a total capacity of 28.8 KwH, although 14.4 KwH can safely be used (50% maximum). That said, my rule of thumb is to avoid depleting more than the top 30% of lead-acid batteries... so that gives me about 10 KwH to play with.
[ Learn:How to Calculate Ah (Amp Hours) and Wh (Watt Hours) of a Battery Bank]
That, combined with nearly 4,000 watts of photovoltaic panels, was enough to provide basic power and essential power needs for the systems within my modest home. For starters, my electrical needs are low, but I have to take care of them and reduce loads, especially during the fall and winter months. Unless your system is huge (and extremely expensive), you'll have to keep an eye on things anyway...
I built in the ability to balance (select) my individual loads to come from "grid" or "solar/batteries/inverter". I installed two 10-circuit transfer switch boxes (Reliance Controls) for 20 circuits on my power panel. This makes the system totally 100% flexible to balance loads and demands throughout the year.
My Trusted Controls A510Ctransfer keys shown below. 10 circuits each. I installed two of them. This gives me great control over balancing seasonal load differences and PV system variations due to weather and seasons.
DC wire size between connections
How long should the cables be?
First, you will need to calculate the maximum current that can flow through the various interconnect cables before choosing the proper cable size.
Cables must be sized to support the maximum load the system can deliver. Undersized wires will result in overheating and even melted connections. Even worse, it can become a potential fire hazard.
There are three maximum DC current specifications to discover:
1. DC charger charging current (battery bank charger).
2. Inverter DC load demand (inverter battery bank).
3. DC current flow at battery bank interconnects.
Battery charger: maximum current
My solar battery charger is the 'Midnite Solar Classic 200'.
According to its specifications, the maximum load you can put on the 48 volt battery bank is 74 amps (~3500 watts). Use the table below to choose your wire size. Give yourself a good head start!
Inverter: maximum battery current
My inverter is an Outback VFX 3648.
According to their specifications, the maximum peak current ampacity is 175 amps and they require an OBDD-175 circuit breaker on the DC side. Of course, there is built-in surge handling capability on the DC side at 175 amps.
The inverter only supplies 3600 watts maximum, which would be about 75 amps from the 48 volt battery bank for normal peak operation. But given the inverter's overcurrent rating, I had to choose this wire size for 175 amps.
So I chose 00 (2/0) AWG for this part of the overall circuit.
Maximum current within the battery bank
You will need to determine how many amps will pass through the battery bank interconnect cables. This involves a basic understanding of series-parallel wiring and how it affects volts and amps. I assume you have this basic knowledge... (e.g. Ohm's Law)
First, we know that in my example there could be up to 175 amps drawn from the battery bank (worst case). However, I have 6 strings of batteries in series, all in parallel, so only one-sixth of the maximum will flow through any given string, as the strings share the charge. This results in about 30 amps (worst case maximum overcurrent) flowing through each of the six strings.
That said, I try not to live on the edge,and always oversized with a margin.
In fact, for my current installation, I sized all of my DC cables to meet the worst case 175 amp rating. Because I had the cable. Therefore, all interconnecting wires are 00 (2/0) AWG per the table below. However, he didn't need them to be that big. keep reading…
My interconnect cables inside my battery bank... I ended up making my own after buying some pre-made ones. It saved some money. I needed 00 (2/0) AWG for the main trunk, but I overdid it a LOT with my serial string interconnects. I used the same cable (because I bought it in bulk).Instead you could have used something like 6 AWG, even with a margin of 50%. Well... at least there are no worries with 2/0!
Note: Again, the wire size in the photo above is too "excessive" for my series/parallel wiring of my 48 volt battery bank consisting of 24 12 volt batteries. I had the wire, and it sure makes the resistance of the wire extremely low (smile)...
Wire Size Ampacity Table
the follow-upmaximumAmps vs. Wire Size (AWG) are from the NEC 2011 version. As far as I know, these values are current to this day. However, for more details, consult the National Electrical Code and your own zoning laws.
Note: I would NEVER design a patch cord close in size to your maximum amps! That's too hot for me! Heat loss equals efficiency loss! Give yourself a wide berth.
– Copper conductor ampacity based on 75°C (167°F) reference
– Tipos de cabos: RHW, THHW, THW, THWN, XHHW, USE, ZW
|Wire Size (AWG - Caliber)||Amplifiers - Ampacity|
|0 (1/0) AWG||150|
|00 (2/0) AWG||175|
|000 (3/0) AWG||200|
|0000 (4/0) CAE||230|
Make your own battery interconnect cables?
Eventually I decided to do it myself to make heavy duty cables for my battery bank. I bought cable in bulk (just choose your size). And a heavy-duty wire crimper (and associated wire terminals).
Temco bulk cable
(see on amazon)
[ Learn:Battery State of Charge Table]
[ Learn:The Four Essentials of Off-Grid Solar Power]