This is a fairly complicated issue. I agree that an ungrounded
battery system is safer than a grounded one, since the hot wire will
not arc to the casing of the enclosure.
If a system has a charge control and an array, the negative wire of
the array usually needs to be grounded (or have a fuse in each
leg). If there is an MPPT charge control, the voltage will almost
certainly be over 50 volts. However, supposedly even a 24 volt system
with no MPPT can go over 50 V.
So, in a standard battery system, with a charge control, ground fault
protection and an array, the DC system will likely need
grounding. If the DC system is grounded, then the battery cables are
grounded also. In 250.166 (B) it says the grounding electrode
conductor (GEC) shall not be smaller than the largest conductor
supplied by the system.
If the battery cables are "supplied by the system" doesn't this imply
that the GEC should be the size of the battery cable? This is the
logic that has led some inspectors to require a 4/0 copper wire to be
bonded to a 1/2" water pipe. This is, of course, technically absurd.
Are a lot of battery based systems now going in with no DC grounding
and fuses in both legs? How does a GFP work in this scenario?
The Sunny Island is a different animal, as the DC system in it does
not connect to the array. It is AC coupled instead of DC coupled.
Therefore, it is connected with AC. The connection to the batteries
is isolated. So maybe the issue of DC grounding doesn't apply. The
DC system in the Sunny Island will, however, normally operate at over
50 volts, unless the batteries are low.
What is the bottom line here?
At 04:47 PM 4/15/2013, you wrote:
Hi August;
250.162(A) says "operating at greater than 50 v", which a 48 v
nominal system operates most of the time at over 50 v, at least when
solar charging and/or selling back.
We used to get dinged many years ago for using breakers rated at 50v
dc on 48 v systems. John Wiles used to insist we use the highest
possible voltage (Voc of modules on coldest day)
Personally I prefer ungrounded systems, so I'm not saying there's
anything wrong from a safety stand point, just that inspectors here
don't see it the same way.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760On 4/15/2013 2:20 PM, August Goers wrote:
Hi Drake,
The Sunny Island System runs at 48 V nominal so I don't believe
250.162(A) applies. We have some really savvy inspectors in the Bay
Area and they were happy for us to be ungrounded since we're
operating at 48 V. Yes, the actual voltage might go above 50 V when
charging but I believe it is the nominal voltage that the code cares about.
Best,
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