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<DIV><FONT size=2 face=Calibri>We usually figure that (for example) in a
2-string bank, that one conductor could carry 2/3 of the current if the
batteries are unbalanced for some reason. In other words, rather than cutting
the amperage rating in half, we cut it by about 1/3. For 3 banks would cut it by
1/2, etc.</FONT></DIV>
<DIV><FONT size=2 face=Calibri></FONT> </DIV>
<DIV><FONT size=2 face=Calibri>Probably not ideal in theory, as in a worst case
scenario you could have one totally failed bank, but on the other hand selling
the customer on a full array of expensive 4/0 jumpers is not always an easy
thing to do. In fact if someone is using that much battery, they should probably
be using a 24 or 48 volt system.</FONT></DIV>
<DIV><FONT size=2 face=Calibri></FONT> </DIV>
<DIV>..................................................................................................<BR>Northern
Arizona Wind & Sun - Electricity From The Sun Since 1979<BR>Solar Discussion
Forum: <A
href="http://www.wind-sun.com/ForumVB/">http://www.wind-sun.com/ForumVB/</A><BR>..................................................................................................</DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=allan@positiveenergysolar.com
href="mailto:allan@positiveenergysolar.com">Allan Sindelar</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=re-wrenches@lists.re-wrenches.org
href="mailto:re-wrenches@lists.re-wrenches.org">RE-wrenches</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Friday, February 19, 2010 10:03
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [RE-wrenches] single string
of batts: must the interconnects be inverter size cables?</DIV>
<DIV><BR></DIV>Phil,<BR>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. <BR><BR>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? <BR><BR>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.<BR><BR>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?<BR><BR>
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<DIV class=Section1>
<P class=MsoNormal><SPAN class=il><B>Allan</B></SPAN><B> Sindelar</B><SPAN
style="FONT-SIZE: 10pt"><BR><A href="mailto:Allan@positiveenergysolar.com"
target=_blank><SPAN
class=il>Allan</SPAN>@positiveenergysolar.com</A><BR>NABCEP Certified
Photovoltaic Installer<BR>EE98J Journeyman Electrician<BR><B>Positive Energy,
Inc.</B><BR>3201 Calle Marie<BR>Santa Fe, New Mexico 87507<BR><B>505
424-1112</B><BR><A href="http://www.positiveenergysolar.com/"
target=_blank>www.positiveenergysolar.com</A></SPAN></P></DIV></DIV><BR><BR>Phil
Undercuffler wrote:
<BLOCKQUOTE
cite=mid:8a40c54d1002191128u3a3b199au1625e30c09aeb2a@mail.gmail.com
type="cite">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.
<BR><BR>I'd use 4/0.<BR><BR clear=all>Phil
Undercuffler<BR>Conergy<BR><BR><BR><BR><BR><BR>
<DIV class=gmail_quote>On Fri, Feb 19, 2010 at 12:01 PM, Mick Abraham <SPAN
dir=ltr><<A href="mailto:mick@abrahamsolar.com"
moz-do-not-send="true">mick@abrahamsolar.com</A>></SPAN> wrote:<BR>
<BLOCKQUOTE
style="BORDER-LEFT: rgb(204,204,204) 1px solid; MARGIN: 0pt 0pt 0pt 0.8ex; PADDING-LEFT: 1ex"
class=gmail_quote>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.<BR><BR>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?<BR><BR>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?<BR><BR>Jolliness,<BR><BR clear=all>Mick Abraham, Proprietor<BR><A
href="http://www.abrahamsolar.com" target=_blank
moz-do-not-send="true">www.abrahamsolar.com</A><BR><BR>Voice:
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