Calculating string amperages [RE-wrenches]

[John Berdner]

Ray/Wrenches:
 
Yes, you could use a breaker rated for continuous duty (typically a
magnetic hydraulic type breaker). 
This would eliminate the second 1.25 safety factor for continuous duty
of the breaker.
(Don't forget however that the 1.56 factor would still apply to wire
ampacity calculations)
 
Assuming a continuous duty breaker for 2 strings you would need a
minimum breaker of 12.75 Amps which rounds up to the next even breaker
size of 15 Amps as you noted in your post.
The breaker limits the possible back feed from the battery via the
charge controller to 15 Amps.  I agree with Bill Brooks that you have to
be careful about generalizing the no back feed issue.  Not all Inverters
and charge controllers prevent back feed.  Most of charge controllers I
am aware of do prevent reverse current back into the array at night but
this does not necessarily mean they prevent reverse current flow during
fault conditions.  Also, if there are other sets of modules paralleled
on the charge controller side of the breaker (multiple sets of modules
each protected by a breaker) then the other set of modules can
contribute fault current which would be limited to 15 Amps. 
 
So, to meet Code you can use 2 parallel sets of SolarWorld 165 on a
single continuous duty 15 amp breaker if:
1) The Charge controller will not allow reverse current to back feed
the fault from the battery; and
2) There are no other sets of modules connected in parallel between the
breaker and the charge controller.
e.g. only one set of 4 modules connected to the charge controller
If both conditions are not met then the installation would be in
violation of the Code.
 
For Example: Consider a system with a charge controller that does NOT
allow back feed current and an additional set(s) of module/breakers
connected in parallel, e.g. 8 (or more) modules wired as sets of 2
series x 2 parallel with each set protected by a 15 Amp breaker as shown
in "ASCII Draw" below:
 
2 series x 2 parallel > 15 Amp breaker >|
2 series x 2 parallel > 15 Amp breaker >|> charge controller

 

In the case of two parallel sets (as shown above) the possible back
feed current from the other modules back through the breaker would be
2*1.25*Isc so the possible fault current to a fault in one of the
strings is: Ifault = (1*1.25*5.1 Amps) + (2*1.25*5.1) or Ifault = 19.125
Amps. This is correct from an engineering/calculation point of view but
may not meet the requirements of 690.8.  For three or more parallel sets
the breaker would clearly be the limiting factor (this is the situation
considered by the language in 690.8) so Ifault= (1*1.25*5.1 Amps)+15 =
21.375.  In either case, the fault current is still higher than the UL
series fuse rating of 15 Amps so these installations would not be Code
compliant.  
 

I hope this helps clarify the effect of using a continuous duty breaker
with multiple modules in parallel.  Unfortunately this still leaves you
with a Code issue for more than 4 modules.  In my experience there are
very few, if any, crystalline modules that can be paralleled to a single
over current device. I have seen some thin film modules that have a high
UL series fuse rating and a small Isc and in those cases you can do it. 
Even then you may face an issue with the AHJ because the role / purpose
of the UL series fuse rating is not well understood.  
 
Best Regards,
 
John Berdner
 

>>> walters at taosnet.com 2/8/2008 11:11:11 AM >>>


Well we do have some discrepancies here. William was saying we could 
go to 3 parallel Solar World strings, John Berdner here says only 
one, and I'm still saying 2 is good.
John I have 2 issues with your excellent analysis.
1) 690.8 B) (1) exception: "Circuits containing an assembly, together 
with its overcurrent device(s), that is listed for continuous 
operation at 100% of its rating shall be permitted to be utilized at 
100% of its rating."
This means that with the right breaker, we don't have to use the 1.56 
multiplier for array current, only 1.25 which is for edge of cloud 
effect, etc. I have seen Solar world modules exceed their nameplate 
rating, so this multiplier is appropriate.
2 x 5.1 A x 1.25 = 12.75 Amps on a 15A breaker that is rated for 100% 
duty is acceptable. (most Outback and Midnite solar offerings are 
100% rated BTW)

2) All this argument is based on the possibility of back feed from 
the grid or batteries, but my understanding is that both grid tied 
inverters, and charge controllers prevent back feed. So I don't think 
this is applicable to most work that we are doing. (I am aware of 
relay type controllers APT... that could allow backfeed possibly?) I 
suppose with large enough arrays, the backfeed would be from adjacent 
subarrays?

So unless I'm missing something (please help me out here) I think I'm 
going to keep paralleling the SW165s in lower voltage off grid 
systems. I like to set up 4 modules per breaker, with the modules 
wired 2 in series, 2 parallels. If I'm running series strings over 48 
v nom., I'll take another look at not paralleling.

Ray Walters



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