[RE-wrenches] EGC ground impedance

[Andrew Truitt]

Wrenches - What do you consider to be a safe ground impedance for a PV
array equipment grounding conductor?  NEC2011 250.53(A) says that if the
impedance between the grounding *electrode* and earth is less than 25 ohms
then no supplementary grounding electrode is needed, but I'm not clear on
what that means for the ground impedance measurement within the array.  For
example, I am inspecting a ballasted landfill system (i.e. no penetrating
the earth allowed - just a UFER at the equipment pad) where the ground
impedance at the GEC is very low (<1 Ohm), but the array EGCs are showing
ground impedances of between 500 and 1000 Ohms.  The EGCs (#4) are
connected to Cope cable trays, which are in turn connected to the inverter
grounding bar via #4 bonding jumpers.  The cable tray is grounded per
manufacturer specs (large bonding jumpers at each joint), but the runs can
be quite long (300'+) with many connections, though nothing appears to be
loose.  I intend to recommend that continuous conductors be installed to
reduce array ground impedance, but any input on the issue would be very
much appreciated.



For a brighter energy future,


Andrew Truitt

Principal
Truitt Renewable Energy Consulting, LLC

(202) 486-7507

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Company Website <http://truittreconsulting.weebly.com/>

NABCEP Certified PV Installation Professional ID: 032407-66

Colorado Journeyman Electrician License No.: 600132

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