SW inverter to battery cable calculation [RE-wrenches]

[John Berdner]

Wrenches:
 
I spoke to John Wiles today regarding the example in his book.
The following is his quick response:
 
J. WIles: See the PV/NEC briefing sheets on the SW2024 that are part of
my standard presentation.
CALCULATED DC input currents are about 214 amps at 4 kW, 22 volts and
.85.
We measured a SW4024 under these conditions (4 kW and 22 volts) and got
an average current of 254 amps (so much for 85% efficiency) and an RMS
value of 311 amps.
I suspect that the Heimenann 250 amp breaker would eventually trip on
these currents depending on how it responds, as a magnetic hydraulic
unit, to the ripple currents.  For code purposes, it would have to be
part of a listed assembly to use the 250 amp rating without an 80%
derating.
I have heard that the 250 amp breraker would trip (after a time) at
Trace when the inverters were being burned in with 4 kW loads.
Fuses as resistive devices would respond to the ripple.  Conductor
ampacity based on heating would also be based on the RMS value.
 
>>>
 
J. Berdner:  After receiving the above email, I called John Wiles and
we discussed inverter current measurement methods.  This is presently a
topic of some debate in UL 1741 Standards development circles.  J. Wiles
said he used a high speed Data Acquisition System (DAS) and then post
processed the raw current measurement data to mathematically calculate
the Average and RMS values of the dc current.  Obviously this is an
accurate, albeit costly and complex, way to get the real values of the
inverter's dc input current.  In the real world this is not a viable
option so how the heck do you measure it becomes a valid question.
As it turns out, you can't use any old RMS DVM.  Most have >> either <<
dc coupling or ac coupling but not both.  The Fluke 180 Series (and
possibly others) have a special position for RMS measurement of signals
with combined ac and dc components.  With one of these babies all you
need is a current shunt, a big battery at it lowest voltage, and enough
resistive load to drive the inverter to full power.  Then you can
measure the current and find numbers that are probably consistent with
those reported by B. Brooks and J. Wiles.  Some of you may be asking why
all of this matters at all and why inverter manufacturers don't just
give you the RMS value for the combined dc plus ac ripple current.  I
suspect it is not made more available because, if it were, the Code says
you can't round the ampacity up to the next breaker size of 250 Amps. 
Instead you would need a 275 Amp breaker and 250 MCM cable as opposed to
a 250 Amp breaker and 4/0 cable used today.  Then 2" conduit becomes a
problem and the existing wire bending space is an issue, and...
 
Fear not!, You are not alone in your quest for truth.  Many of the
folks in the Standards development community are asking too.
I would bet, with long odds, that this information will be a
requirement for the manuals in the next version of UL 1741.
 
Best Regards,
 
John Berdner

>>> allan at positiveenergysolar.com 11/28/2003 12:47:33 PM >>>
John,
OK, that helps a little. Wiles addresses it to a degree in Appendix F.
Using
the 4024 example, the calculated 267A is less than the 311A RMS value
of the
current waveform. Simply extrapolating: 311/267 X 184A = 213.4A RMS,
which
is 29.4A additional because of AC ripple, close to the 28A of the
original
5548 calculation.

But it leaves us with a circular (il)logic: Wiles' last paragraph reads
"The
systems designer should contact the inverter manufacturer in cases
where it
is expected that the inverter may operate at loads approaching the
full
power rating of the inverter. The inverter manufacturer should provide
an
appropriate value for the dc input current under the expected load
conditions". Yet this whole discussion began because Joel "asked
someone at
Xantrex technical service for the fuse, breaker and wire loss
calculations
for the cable between the SW inverter
and the battery, but the person I talked to did not know the
calculations."

It amazes me how one company's reputation can change so drastically and
so
quickly.

Allan at Positive Energy, a Certified Dealer






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