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

[R Ray Walters]

First, I agree with you, that a 48 v system can run a single string of smaller batteries.
Usually 48 v systems are running larger battery banks already.
As far as undersizing the jumpers that carry less current, a 2/0 cable has a free air rating of 300 amps (table 310.17 for THHN at room temp).
Battery jumpers can use the fee air rating, just not the main cable when it goes into conduit.
Also, like I and the Exeltech rep stated: the battery post to lug connection is the weak link, not the cable. 
Finally, if the battery bank is so bad, as to have multiple cell failures, once again, if the customer tries and runs the inverter at full surge capacity, it's not going to be a cable that fails,
It will be a corroded connection, another battery cell, or low voltage shut down.
I do high amp testing with sealed batteries in electric vehicles, you're right, they do have some sustained surge capability, but they also have very small surfaces to bolt to. 

Bottom line, just use the fee air rating for your jumpers, and the 'in conduit' rating (table 310.16) for the main. 
You end up with 4/0 cable for the main, and the parallels, and series jumpers can be 2/0 (assuming an inverter that requires a 250 A disco)
All NEC compliant.
As far as Real Safety issues in the battery area, cable sizing isn't much of an issue at all, it's all those exposed, unfused battery lugs, waiting for your wrench to drop.
I'm still advocating for separate OCP on each series string, BTW.

R. Walters
ray at solarray.com
Solar Engineer




On Feb 24, 2010, at 3:25 PM, Michael Welch wrote:

> 
> From: jfh at greenhousepc.com
> 
> 
> Solar Ray writes:
> 
>>> These are grid tie with battery backup systems. All inspected.
>>> 
>>> Personally, I think that we should have OCP on each series string, but that's a whole nother can o' worms.
>>> Most sealed batteries (and golf cart batteries) are fairly current limited. You need at least 2 parallel strings
>>> to even get close to meeting the inverters surge requirements. (especially in 24 v systems)
>> 
>> This is true for 24 volt systems with small banks of small batteries. 
>> It is absolutely not true for 48
>> volt systems where Vdrop is 1/4th what it is for a 24 volt system, and
>> 1/16th what it is for a 12 volt system.
>> T-105s and similar sizes are pretty small, but there are sealed
>> batteries with the capacity and plate
>> surface area to deliver substantially more current.
>> 
>> It's also a matter of where the battery is operating at the time you try
>> testing Psurge.  All batteries have a
>> problem with "coup de fouet" / "the crack of the whip" (the sharp drop
>> after an abrupt increase in load from 
>> a low or no-load condition), but most will recover significantly as the
>> battery adjusts to the load.  Based on
>> that, the worst-case scenario is a steadily increasing load as the
>> voltage will fall much more slowly than
>> with a single large load applied at once.  I've achieved C/1 in
>> discharge on AGMs during tests, and if I'd had
>> larger shunts, I would have tried higher rates of discharge.  This was
>> for far longer than the 5 seconds you
>> mentioned elsewhere.
>> 
>> I have a test coming up for a client where I'm going to try and get a
>> pair of GVFX3648's to sell 7.2KW.
>> I'll try to remember to capture some voltage plots and post them
>> somewhere.  The reduction in voltage drop
>> is significant as the load is increased, compared to all at once.  I had
>> those inverters at 2/3rds capacity
>> and still above 46 volts.  I was able, just now, to run both at 100%
>> (and then the chargers turned on due to
>> low voltage, because I'd not turned them off ...) and keep the voltage
>> above 46.4V -- here's the plot:
>> 
>> http://www.greenhousepc.com/GVFX-Sell-All.gif
>> 
>> Here's the numerical meter info --
>> 
>> STATUS/FX/METERS-P01
>> inverter     0.0 aac
>> current     31.0 max
>> DOWN  UP   TOP  PORT
>> 
>> STATUS/FX/METERS-P02
>> inverter     0.0 aac
>> current     30.0 max
>> DOWN  UP   TOP  PORT
>> 
>> STATUS/DC/BATT---P04
>> battery min today
>>         46.6 vdc
>> DOWN  UP  RESET  TOP
>> 
>> This is compared to clients' systems were they've had more transient
>> loads (in the range of 4.5KW for a
>> pair of VFX3648) and reached lower voltages (in the 44's and 45's) with
>> larger battery banks.
>> 
>>> Main cables and breaker are sized as always, (4/0 with a 250 A breaker) but parallel string jumpers are
>>> reduced to 2/0 as they are carrying 1/2 to 1/3 of main current.
>> 
>> Having seen all but one string of batteries fail in a multi-string
>> system, this is not a safe assumption.
>> You cannot assume all of the strings are working properly, or even that
>> only one string fails.
>> --
>> Julie Haugh
>> Senior Design Engineer
>> greenHouse Computers, LLC // jfh at greenhousepc.com // greenHousePC on
>> Skype
> 
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