Series fuses/combiner boxes [RE-wrenches]

Tue Nov 2 09:47:47 PST 2004

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Doug / Wrenches:

When we are discussing when we need or do not need fuses the NEC says
we have to consider the fault current contribution form all sources.
Consider a system with 2 parallel strings (A and B) feeding one
inverter: 

If you have a fault in string A the only possible sources of fault
current are String B and the inverter backfeeding the fault.
Assume the inverter does not contribute backfeed current so the falt
current from this potential source is zero.
String A does not contribute backfeed current to a fault in its own
string.
The only other source of fault current remaining is String B.
Per NEC the fault current form String B is 1.56 * Isc.
This is equal to to minimum ampacity of the wire and the minimum size
fuse that can be used in that string.
Since the maximum fault current is less that the minimum required
ampacity of the fuses or wire nothing will happen.
The wire will not melt and the fuse will not open.
This is why the NEC has the exception that allows us to eliminate fuses
all the wire is designed to handle the worst case fault current.

Best Regards,

John Berdner

>>> dwells at solar-works.com 11/2/2004 07:21:34 AM >>>

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Wrenches,
I noticed a case recently where even 2 strings in parallel was in
question.
I was designing a system with Sharp 185's and noticed that the Isc of
two
modules exceeds the 10 amp fuse rating. Interesting.
Doug Wells
Solar Works Inc.

-----Original Message-----
From: Gary Higbee [mailto:gary at windstreamsolar.com] 
Sent: Sunday, October 31, 2004 2:00 AM
To: RE-wrenches at topica.com 
Subject: Series fuses/combiner boxes [RE-wrenches]

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Wrenches,

I'd appreciate some help in determining when we need fuses or breakers
for
parallel strings. For some reason I thought that two strings in
parallel are
OK, but that for three and above we need to use the appropriate
combiner box
with breakers or fuses. This feels too vague--especially as I am now
being
asked to review and approve systems (for the Energy Trust of Oregon)
with
more than two parallel circuits.

Please shed some photons on this--when do we not need and when do we
need
parallel string fuses or breakers, and where does this correspond to in
the
code--or make common sense?

Thanks so much!

Gary

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ Windstream Solar ~
Gary Higbee (gary at windstreamsolar.com )
(541 ) 607-1818 (Eugene)
(541) 954-3881 (Cell)
Solar, wind, and hydro site analysis and system design
Components dealer and installation assistance
Energy Trust of Oregon contracted system inspector
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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