[RE-wrenches] Two systems on the same steel rack; different GEC's

Jim MacDonald jmac at solaresystems.com
Wed May 20 14:53:28 PDT 2009


Hello wrenchers

 

We've got 2 systems [23kW and 44kW] going up on Solarmount onto a single
large custom steel dunnage structure on top of a warehouse.  

>From the ground it will look like a single massive array, but actually 

1/3 of the modules (8 strings) will be fed into (2) Fronius [which will
feed one bldg service, with its own water main ground], and 

2/3 of the modules (16 strings) will be fed into (4) Fronius [which will
feed a different bldg service, with its own water main ground.]

 

All of the mods will be weebed onto rails, which will be mechanically
connected to the I-beams, which is welded to the building steel, which
goes into the ground.

 

We plan to land all of the DC equipment grounds onto a common bus bar
near the inverter bank; from that we plan to run a bare solid #6 copper
to each inverter.

 

We'll run #6 GEC's out of each inverter, to be irreversibly crimped onto
a dedicated single #4 GEC for each building, which will land on their
respective water mains.

 

The question is: does this mean we're also bonding two neutrals
together, which means current could flow into the service at Building A,
through a lightbulb, back to their neutral buss, up our EGC/GEC, down to
Building B's neutral buss, and out through their service neutral?  That
may pose two issues:

1) If Building A has a 400A service and Building B has a 100A service,
building B's neutral is now in major danger of an overcurrent situation.
(There are probably other potential problems as well, if the two
services are out of phase for example)
2) The bonded GEC now becomes a grounded current-carrying conductor

This example seems pretty unique; we haven't found anything in the code
yet that specifically addresses the situation, but 250.32 prohibits
something similar on the grounds that it causes point #2 as above. A
possible counter-argument to point #1 : does potential exist between
L1/2/3 of transformer A and the neutral of transformer B?

  

Fun stuff..cheers for reading this

Jim Mac

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