Xantrex MPPT60-150 Calcs [RE-wrenches]

[William Miller]

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Friends:

I am hoping our Xantrex Rep or knowledgeable colleague can verify my 
calculations on a Xantrex charge controller system.  The modules used will 
be Solar World SW175 mono:

Regarding voltage:  The MPPT60-150 has an open circuit voltage maximum of 
150 VDC.  I must limit my strings to 2 modules because:  With 44.4 VOC per 
module, times 3, times the NEC Correction factor of 1.13 in our climate, 
exceeds 150 VDC by 0.516 VDC.  So string quantity is 2, maximum.  Correct?

Regarding amps:  Rated amps (MPP) per module is 4.95.  If I use 10 strings, 
rated amps is 49.5.  690.8(A)(1) requires I multiply by 125%.  The answer 
is 61.875, or, in excess of the charge controller's rating.  Therefore my 
limit is 18 modules or 9 strings.

What comes to mind is that certain calculations require a 125% for 
continuous duty on top of another 125% calculation for solar applications 
(edge of cloud and all that).  I recall a conversation with Outback tech 
support some time ago and they claim the MX-60 is rated for continuous duty 
and therefore at least one of the 125% de-ratings is not necessary.  I 
assume this is true also for the Xantrex charge controller?????

However, I must still meet the requirements of 690.8(A)(1) and use one of 
the 1.25 multipliers.  Therefore I am limited to 9 strings, or 18 SW175 
modules.

How does SC current work into charge control design?  I see no reference in 
690(A)(1) to Short Circuit current.

The claim that a charge controller is assumed to be rated to operate as a 
continuous duty device is supported by the exception in this 
section:  Exception:  Circuits containing an assembly, together with its 
overcurrent device(s), that is listed for continuous operation at 100 
percent of its rating shall be permitted to be utilized at 100 percent of 
its rating.  How about the circuit breakers in the Xantrex "Chassis 
distribution Channel?"  Are these considered to be part of the assembly, or 
are standard 60 amp circuit breakers not adequate?

Thanks to Xantrex and any others who can check my math and my application 
of the NEC.  This calculation applies also to Outback MX-60 charge 
controllers so I appreciate any input form Outback.

William Miller



-- _________________________________
William Miller
Miller Power and Communications
Voice :805-438-5600     Fax: 805-438-4607
email: wrmiller at mpandc.com
http://mpandc.com
License No. C-10-773985


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