Are you putting together a battery bank for an alternative energy system?
If you are, then you might be thinking about the size of interconnecting cables between the batteries themselves and the charger / inverter.
How big should the cables be? What size? More accurately, what gauge (AWG)?
Read on… or go straight to the chart at bottom of page.
The Battery Bank
A battery bank for an Off grid solar powered alternative energy system will consist of a number of batteries and their interconnecting terminal cables.
The batteries will be connected together in various series-parallel configurations to achieve a desired voltage and capacity to work best with the inverter (and charger).
The batteries will serve as a energy storage center to deliver power during periods when the solar panels themselves are either in the dark or under-delivering due to weather conditions.
I’m not going to get into recommendations for series-parallel battery wiring (another article). However I will cite my own current configuration as an example:
All 24 batteries are 12 volt 100 AH rated. They are configured in groups of 4 in series, making 48 volt strings. 6 strings are connected in parallel to comprise a battery bank of 48 volts @ 600 AH. That’s a total capacity of 28.8 KwH, although 14.4 KwH is safely usable (50% max). That said, my rule of thumb is to avoid draining more than 30% off the top for lead acid batteries…so that gives me just about 10 KwH to play with.
This, combined with almost 4,000 watts of PV panels has been enough to supply basic power and energy needs within my modest home. My electrical needs are low to begin with, but I do have to watch it and reduce loads especially during the Fall and Winter months, but that’s part of the fun ;) Unless your system is enormous (and enormously expensive) you will have to keep an eye on things anyway…
I have integrated the ability to balance (select) my individual loads to be sourced from either ‘grid’ or ‘solar/batteries/inverter’. I’ve installed a few boxes (Reliance Controls) filled with transfer switches for each individual circuit breaker from the mains panel to accommodate this. It makes the system entirely 100% flexible for balancing loads and demands throughout the year.
DC Cable Size Between Connections
How big should the cables be?
First you will need to calculate the maximum current that may potentially flow through the various interconnecting cables before you choose the proper cable size.
Cables must be sized to carry the maximum expected load. Undersized cables may result in overheating, melted connections, and even become a potential fire hazard.
There are three maximum DC current specifications to figure out:
1. DC Charger charging current.
2. DC Load demand from the inverter.
3. DC current flow within the battery bank interconnects.
Battery Charger – Max Current
My solar battery charger is the ‘Midnite Solar Classic 200’.
According to its specifications, the maximum charge that it can put to the battery bank at 48 volts is 74 amps (~ 3500 watts).
Inverter – Max Battery Current
My inverter is an Outback VFX 3648.
According to the specifications, the maximum overcurrent ampacity is 175 amps and they call for a OBCD-175 breaker on the DC side. Evidently there’s a built-in surge handling capability on the DC side to 175 amps. Therefore the cables between the battery bank and inverter must be sized for at least 175 amps.
Max Current Within Battery Bank
You will need to determine how many amps will flow through interconnecting cables of the battery bank. This involves some basic understanding of series-parallel wiring and how it affects volts and amps. I will assume you have that basic understanding…
First, we know that in my example there may be up to 175 amps pulled from the battery bank (worst case scenario). However I have 6 strings of batteries in parallel, so only one sixth of the maximum will flow through any one string as the strings share the load. So that works out to be just about 30 amps flowing through each of the six strings.
So technically I only need at least 30 amp rated cable throughout the series strings of batteries (6 groups of 4 batteries in series).
That said, I try not to live on the edge, and always oversize with a wide margin where I can.
Actually, for my current installation I’ve sized all of my DC cables to meet the 175 amp worst case scenario. So all interconnecting cables are 2/0 based on the chart below.
Cable Size Ampacity Chart
The following maximum amps versus cable size (AWG) come from the NEC version 2011. As far as I know these values are valid as of today. For more detail though, check with the National Electrical Code as well as your own zoning laws.
– Copper Conductor Ampacity based on 75°C (167°F) reference
– Cable Types: RHW, THHW, THW, THWN, XHHW, USE, ZW
Size (AWG) — Ampacity
14 – 20
12 – 25
10 – 35
8 – 50
6 – 65
4 – 85
3 – 100
2 – 115
1 – 130
1/0 – 150
2/0 – 175
3/0 – 200
4/0 – 230