Code issues (was adding MX-60's in PP) [RE-wrenches]

[William Miller]

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At 09:37 AM 3/15/2004, you wrote:
>Wrenches:
>
>When thinking about the "which side to connect to what" question you
>have to carefully consider the ampacity of the cables going to the
>battery AND the inverter. It is easy to end up with a Code violation if
>you do it the way Trace used to  ? recommend.  For those of you who
>remember the old APT power centers they had a 3 pole fused disconnect -
>1 or 2 poles for loads and a separate one for sources.  This was the
>right way to do it but was not cost effective against the DC 250 due to
>the extra pole of OC protection.  The basic problems are shown below:
>
>If you add load breakers by tapping in on the battery side, then you
>risk exceeding the ampacity of the battery cables.  This is especially
>true with the combination of 4/0 and 250 Amp breakers which is already
>using the round up exception at 75C (breaker terminal temperature
>rating).  Essentially, this means you can not add any additional load to
>the battery side.  You should always connect additional load to the
>inverter side of the breaker.  In this scenario the 250 Amp breaker is
>then acting as a main and doing it's job to protect the battery cable.
>With this method there is no impact on the inverter cable since it is
>already covered by the 250 Amp limit.
>

John:

Remember, if the MX60 is charging in tandem with the inverter, the 
inverters constant-voltage charging circuitry should start tapering 
immediately as it senses the increase in battery voltage caused by the 
MX60.  It seems highly unlikely that an inverter in charge mode, in tandem 
with an MX60, could generate more current than the system, designed around 
inverter DC loads, could safely handle.

I have always detested the mechanical ineptitude of connecting to the BACK 
of the lugs on the DC breakers in the Xantrex disconnect.  First of all, 
the breaker hold downs are razor sharp and I have lacerated the heel of my 
hand more than once.  It is now SOP in our shop to file the bracket corners 
on any new or existing installation.  Secondly, the factory lead is too 
small for any normal charge applications.  Thirdly, how lame is it to have 
to work behind the breaker????  A positive DC buss makes way more sense 
here!  (I shoehorn in a Square D LBA163104, if possible--tough if you have 
a DCBB and a shunt.)

It seems to me like it is a no-brainer that one would connect charging 
sources to the batteries with dedicated over-current protection.  What if 
one wanted to charge batteries from PV while the inverter breaker was 
off?  True also, the need to protect inverters, monitors, tracker controls, 
DC loads, etc. from PV OC voltages.  I can't see any argument to the 
contrary that would overcome this logic.

William

PS:  I did like the DC side of the APT units.  The AC side made much less 
sense.

William

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