[RE-wrenches] Fuse sizing in battery circuits

[Drake]

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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