[RE-wrenches] single string of batts: must the interconnects be inverter size cables?

[R Ray Walters]

Hi Allan;

I with you, I normally downsize the interconnects in parallel strings to 2/0, but keep them the same as the main cables on anything that carries full current, as Phil made clear.
On golf cart batteries for instance, two 4/0 parallel lugs won't fit under one battery post, now that most manus shortened their posts. Also 4/0 cables can really strain those posts; I saw a couple of systems with 4/0 THHN used (NEC compliant) but the lead post ripped off the battery after several years from the bend put on the cable (creating a much more dangerous and unfused problem BTW)
Some of the sealed battery manus (B&B for instance) give max current output vs time charts, and that not only gives you something for cable sizing, but also is an eye opener, when expecting a single string of two 12 volt batteries to run a 4 Kw inverter. A battery that can only produce 250 amps for less than 5 sec, doesn't deserve a cable sized for 24 hr continuous at that output.

R. Walters
ray at solarray.com
Solar Engineer




On Feb 19, 2010, at 10:03 PM, Allan Sindelar wrote:

> 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
> Allan at positiveenergysolar.com
> NABCEP Certified Photovoltaic Installer
> EE98J Journeyman Electrician
> Positive Energy, Inc.
> 3201 Calle Marie
> Santa Fe, New Mexico 87507
> 505 424-1112
> www.positiveenergysolar.com
> 
> 
> 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.  
>> 
>> I'd use 4/0.
>> 
>> Phil Undercuffler
>> Conergy
>> 
>> 
>> 
>> 
>> 
>> On Fri, Feb 19, 2010 at 12:01 PM, Mick Abraham <mick at abrahamsolar.com> wrote:
>> Example: 12 two volt battery cells in one 24 volt string. 4kW inverter/charger has 250 amp DC breaker size and 4/0 cables...as the inverter folks would want.
>> 
>> So...do the "cell to cell" interconnect cables have to also be 4/0? In the example above, 2/0 interconnects would suffice from the standpoint of basic safety...and even from the voltage drop standpoint...right?
>> 
>> Inverter company people may be best qualified to answer this, but any replies will be appreciated. While I'm at it: do the inverter folks request oversize cables because they want more capacitance on the DC input...or what?
>> 
>> Jolliness,
>> 
>> Mick Abraham, Proprietor
>> www.abrahamsolar.com
>> 
>> Voice: 970-731-4675
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