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Phil, Given the scenario that Mick presented, your advice is sound, and I'd do as you suggest. But by changing the scenario just a bit, it brings up a separate but related issue. Mick's scenario describes two 12V batteries in series, or a single string. In that case all current flows through the one string, so a series interconnect would need to be sized equal to the battery cables, in this case 4/0. But if there were two (or more) parallel strings, would the same size requirement apply? That is, if a battery bank had two strings, in theory each would carry 180 max amps (your example, 8,000/22, shared by 2 strings), which is within the ampacity of 2/0 interconnects. In reality, this only applies if current is equally spread among strings. Is it? It seems to me that a properly wired and torqued set will split current paths equally until a cell fails, either prematurely or when the set approaches end of life. If there are three strings, then failure of one cell/string in this example would still not exceed ampacity on the remaining 2/0 strings. As with many issues, the goal is to find a balance between safety and performance versus cost and worst-case accommodation. I will always use 4/0 with one cell string and a 250A breaker, but I have never seen a real-world problem using 2/0 interconnects, both series and parallel, with 2 or more strings. Phil, what do you say? Others? Allan Sindelar Phil Undercuffler wrote: The interconnects are part of the circuit. Typical flexible cables used with batteries are THW rated (75C column), so in free air 2/0 interconnects would be good for 265 amps, before any temperature derates. Therefore, you'd be safe from a NEC point of view. However, if the inverter manufacturer has recommended 4/0 cables, then that recommendation would apply to all cables in the circuit. Battery based inverters are asked to surge many times their rated capacity -- a 24V 4kW inverter surging to 2x capacity (pretty normal circumstances) is going to draw over 360 amps (8,000 / 22), even if only for a short period of time. You don't want your interconnects to be the weak link in the system, causing shutdowns due to undervoltage unnecessarily. |
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