Module Wiring RE LAB [RE-wrenches]
Bob Ellison, Alternative Energy Systems
ellison at gisco.net
Sat May 6 18:26:29 EDT 2000
Your right I NEVER would have expected this answer.
Now I am wondering how hot it has to get to reach the point where "cheese
cloth singes".
I wonder where this test came from?
If you feed it 20 amps can you cook on it?
Bob Ellison
-----Original Message-----
From: Drake Chamberlin, Quicksilver Electrical
[mailto:solar at eagle-access.net]
Sent: Saturday, May 06, 2000 1:33 PM
To: RE-wrenches at topica.com
Subject: Re: Module Wiring RE LAB [RE-wrenches]
<< File: ATT00002.htm >> << File: ATT00005.htm >> Pat,
It looks like we have the official word form Siemens concerning the
fusing of series sets of modules. The answer may have been a bit different
from what anyone suspected.
According to Arthur Rudin, we can parallel 2 sets of Siemens SP 75s, and
run the series - parallel set to a 15 amp fuse. Below is his reply.
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***************
Hi Drake,
You are asking the right questions.
The maximum series fuse rating indicated on the module rating label is a
result of testing and not a calculation. The rating indicates the modules
ability to tolerate reverse current without causing a fire (per UL).
Current is fed into the module until the temperature reaches a point where
cheese cloth singes. It is the maximum size fuse you can use. A smaller
size is acceptable.
For example, our SP 75 modules have a maximum series fuse rating of 15 amps.
You can parallel two strings with a single fuse. This is acceptable per UL.
Your suggested wiring configuration will work well.
I hope this information helps.
Best regards,
Arthur
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***************
My next task is to find more about the test. What voltage did it take
to force 15 amps backwards through the modules? Can the open circuit
voltage from a nominal 24 volt series set really push enough reverse current
through the modules to cook them?
I seem to remember an old "Code Corner" in Home Power where Mr. Wiles
indicated that it required a ground fault, on the wire that was used to
series the modules, for this to be a problem. Does anyone remember this or
which issue it was in?
If we really can fry modules, we will certainly want to cover our
liabilities (asses) by using the fuses. At least, now we can get by with
half as many whips as we were using previously. Possibly we could use in
line fuses, installed in J-boxes, as the supplementary overcurrent
protection required for protection for the modules. .
Thanks for "asking the right questions."
Drake
----- Original Message -----
From: Pat Kiernan
To: RE-wrenches at topica.com
Sent: Friday, April 28, 2000 3:50 PM
Subject: RE: Module Wiring [RE-wrenches]
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Drake,
Good point about the wires being cooler when the modules are cooler and
output current is higher.
I believe that the series overcurrent device requirement for each parallel
string is a UL 1703 requirement (not NEC). For a Siemens SP75 the maximum
fuse is 8 amps (4.8 x 1.25 x 1.25 = 7.5A). This value should be indicated on
the module label.
It sure would be nice to find some way to eliminate the series fuses.
Thanks for your effort in pursuing this.
Pat
Re: RE Lab: Module Wiring [RE-wrenches]Hi Travis,
We do need to multiply the module currents by 1.25 twice to get the
code required current rating. I inserted the note from Siemens as it was
written. The important thrust of the message to me was that we only need to
follow code, that we don't need extra fuses in the system.
Different sets of modules will, of course, require their own calculations.
Here is an example that I believe is in compliance with both the NEC and
Siemens requirements.
Eight Siemens SP 75s wired in series-parallel for 24 volts, have a short
circuit rating of 19.2 amps (4.8 X 4) . Multiply that times 1.25 twice (to
comply with code) and you get 30 amps. A pair of number 10 THWN-2 wires
should satisfy any inspector. (* Note: The rating of THWN-2 comes on many
THHN conductors).
An interesting point here is that if the modules are going to be so cold
that they will produce enough extra current that the second 1.25
multiplication factor needs to be used, the wire will also be cold, and thus
its current carrying capacity will be higher.
What I understand from the message, from Arthur Rudin of Siemens, is that
each series set does not require its own fuse. The old code seemed to
require a fuse for each series set.
The total series - parallel module configuration must be wired so that the
code required calculations don't show more amperage than the wiring and
fuses would be able to handle.
Thanks for helping sort out this tangled issue.
Drake
RE Lab
----- Original Message -----
From: Travis Creswell
To: RE-wre- at topica.com
Sent: Thursday, April 27, 2000 7:07 PM
Subject: Re: RE Lab: Module Wiring [RE-wrenches]
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Drake,
What is the Siemens module rating for max series fuse and short circuit
current? They must be rated quite differently than the modules that I am
used to.
Why do you only use 1.25 de-rate instead of1.56?
See Table 690-7 Page 849 of the 1999 NEC Handbook. In areas where the temp
can get to -5 F then you need to use 1.25 in addition to the 1.25 de-rate
required in 690.8(a)(1). 1.25x.125=1.56
Some of the modules here at my shop are rated as follows
Max Series Fuse Isc x 1.56 de-rate
AP1206 10 Amps 7.7 12
US64 8 amps 5.1 7.96
BP585 8 Amps 5.0 7.8 (I
think, they are on the roof and I don't feel like getting up there right
now)
>From the above numbers it looks like the Astro Power 120 has a problem
but the ones I've installed over the last year have upped the max series
fuse rating to 15 amps. What this means to me is that I can only series
the above modules. We installed some Solarex MSX 60's recently and I think
they had a max series fuse rating of 20 amps which I thought I was very
high. Can someone check out a MSX60 and reply with that info?
If you are allowed to use the 1.25 de-rate it still means that you can
only parallel two modules. Are you implying that you can parallel as many
modules as you want? In parallel, as I see it, you are forcing the
downstream modules to pass the current from the upstream modules through
them.
Keep up the hard work on the research you are doing out there.
Travis Creswell
Ozark Solar
----------
From: Drake Chamberlin - Electrical Energy <sol- at eagle-access.net>
To: RE-wre- at topica.com
Subject: RE Lab: Module Wiring [RE-wrenches]
Date: Thu, Apr 27, 2000, 6:07 PM
Hi Wrenches,
I went through the Siemens Web site to get answers to the question of
daisy chaining series strings of modules. It appears that there is no fuse
required between the Series sets anymore.
Below is the response from Arthur Rudin of Siemens concerning my
question.
****************************************************************************
*****************
Hi Drake,
Sorry for the delay in this response. I have been travelling and did
not
have access to my e-mail. The NEC does require a fuse for each source
circuit. Modules or strings can be wired in parallel as long as the
total
short circuit current x 1.25 does not exceed the series fuse rating of
the
module. This is explained in article 690 of the NEC and the NEC
handbook.
You can check your library for this information.
Best regards,
Arthur
****************************************************************************
*************
Siemens apparently has no other requirement for their module wiring
than the NEC specifications.
So if Siemens is going to let us meet the Code requirements for module
wiring, without using extra fuses between the series pairs, we are safe
daisy chaining strings of Siemens modules, and then going to one overcurrent
device.
We can omit those pesky fuses between series strings of modules. There
is no need for multiple whips to combiner boxes, with all the terminal
strips, etc. This is the new Code, and it is a lot easier than the last.
Drake
RE
Lab
****************************************************************************
*******************
Pat Kiernan
Eco Electric, Inc.
PO Box 2389
Basalt, CO 81621-2389
tel/fax (970) 927-9270
e-mail: ecoelec at rof.net
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