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

[John Berdner]

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.
 
If you add sources by tapping in on the inverter side then you can have
a problem exceeding the ampacity of the cable from the breaker to the
inverter.  In this example, connecting source on the inverter side would
let the inverter can run all day at 310 Amps (250 Amps from the DC 250
plus 60 from the MX-60) which is well above the ampacity of 4/0.  If you
connected source on the battery side then the 250 amp breaker is once
again doing it's job to limit the current in the inverter cable.  This,
of course, leaves the question of the ampacity of the battery cable but,
in my experience, most inverters have lower current capability when
charging than when inverting.  You still need to check the battery cable
ampacity which must be less than the maximum continuous charging current
from the inverter plus the sum of rating of any source breakers
connected on the battery side of the 250 Amp breaker.
 
In addition to all of this, you still need to provide a means of
disconnect and over current protection on the PV side.  This could be
either a second breaker or a combiner box (sum of the ratings of the OC
devices can not exceed wire ampacity).
 
Best Regards,
 
John Berdner
 

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