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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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<p class="MsoNormal" style=""><span class="il"><b style="">Allan</b></span><b
style=""> 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 style="">Positive Energy, Inc.</b><br>
3201 Calle Marie<br>
Santa Fe, New Mexico 87507<br>
<b style="">505 424-1112</b><br>
<a href="http://www.positiveenergysolar.com/" target="_blank">www.positiveenergysolar.com</a></span></p>
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<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 moz-do-not-send="true"
href="mailto:mick@abrahamsolar.com">mick@abrahamsolar.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote"
style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">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 moz-do-not-send="true" href="http://www.abrahamsolar.com"
target="_blank">www.abrahamsolar.com</a><br>
<br>
Voice: 970-731-4675<br>
<br>
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