Selling tactic for MPPT [RE-wrenches]

[William Miller]

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Friends:

I'm not sure where this thread is going, but please allow me to elaborate 
on a few points:

1. I tried to convince myself and the collective group that QO breakers are 
OK for 48 V nominal systems.  I even have Square D factory certification 
indicating approval for use at 125VDC.  As succinctly put by some member 
here, however, UL trumps the factory every time.  If the job is not going 
to be inspected, I might use these again.  There are, however, several 
excellent combiners available now, so the need is no longer as great to use 
standard electrical components.

Also, the QO breakers are listed for DC use in certain load centers, those 
with copper buss bars, IIR.   I hope some year Square D will UL test QO 
breakers at 125VDC for UL approval.

2. I'm working on an off-grid install I didn't get the first time 
around.  The original builders apparently used their electrician, and 
he/she trashed the job pretty good.  Among other problems, the voltage drop 
from the array calculates out at 8%.   The system was using a C-40 charge 
controller. The system is a nominal 24 volts.  With the 8% drop, it seems 
unlikely there was ever enough voltage headroom at current to provide any 
charging.

I installed a Blue Sky 3048 at the battery end of the feed and changed the 
array voltage to a nominal 48.  Right away, this changed calculated drop to 
2% at 48 VDC.  Figuring the array runs at more like 66 VDC, the numbers are 
even better.  This, in my opinion, is an excellent use of MPPT, especially 
with a controller that allows 48 in and 24 out.

(This also underscores the point that being an electrician does not 
automatically qualify you to install RE.)

William Miller

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