Interpretation of NEC code / can of worms [RE-wrenches]

Jason Fisher jfisher at nahbrc.org
Wed Sep 3 07:13:58 PDT 2003


Hi Vince. You Wrote:

"The inspector wants me to replace the thhn with a 90 deg. rated conductor
all the way to the inverter, even though the conductors are not in a high
temperature location. "

Two thoughts on this, if the THHN conduit and junction box is not in a wet
location (indoors) than the THHN is rated to 90 degrees C (dry) so is fine
under any interpretation. If not, first make sure the conductors are THWN
rated as well. Though I would personally take issue with this request, it is
a hard nut to crack simply quoting code articles and they seem to have made
their position clear. So you can either find a source for THWN-2 (rated 90C
Wet and Dry) and replace it, track down the wire manufacturer and see if
this insulation is also rated -2 (many wire suppliers are now rating their
off-the-shelf THWN as -2), or get Sharp to approve this configuration and
clarify their 90C requirement. This is an important point since the 90C
requirement by the inspectors relates to the fact that the insulation must
meet the manufacturer's requirements and they typically state 90C on their
panels. The "whole circuit" issue is an odd one since most connectors you
would be using to make this splice would only be rated to 75C, the terminals
on the DC disconnect will probably be 75C, and most likely the terminals on
the SB are 75C terminals. Even the MC connectors on the factory installed
USE-2 tails are only rated for 70C max according to MC's literature so how
everything in the circuit could be 90C is questionable. This really is an
issue that needs to be dealt with by the panel manufacturers.

"The other issue that I have is most of the inspectors I'm working with want
me to install an additional grounding electrode with a #6 grounding
electrode conductor coming straight down from the array."

As you note, the important point is not to create new grounding paths at
different potentials than the main building's grounding electrode. This
would happen if a new rod were installed with an additional grounding wire
from the array to this rod without bonding this new rod to the old one. The
difference in potential during a fault or surge could create potential
problems to both equipment and personal safety. If the inspectors want this
wire perhaps they have a good reason why, and they could explain this to
you. If they can't explain it to you perhaps you could explain it to them.
I personally think it's a good idea to run a separate ground (#6) from a
rooftop-mounted array in a short path directly to either the existing
grounding electrode or a new one that I'll add. If a new one is added, it
must be bonded to the existing electrode(s) preferably by a wire run below
grade. I am not aware of any code requirement for this however, as this
ground wire is technically just an equipment ground that only needs to be
sized to meet the requirements of 250.122 and could (though maybe not
should) be run in the same conduit with the PV circuit wiring.

My experience with these issues is that somebody needs to bend a little to
resolve these disputes. Typically, it is in our best interest to be the one
that bends. After all, they have the authority. That said, I would make sure
you fill yourself up on all the facts around these issues and have your
points ready, perhaps with some copied reference material to present to them
personally during a re-inspection. You goal for this exchange should be to
demonstrate to them that you are a knowledgeable and experienced
professional who is aware of the safety issues involving these systems and
is not willing to cut corners just to get something sold. They also should
understand that you are trying to make a living at this and should not be
expected to perform duties or provide equipment that does not add to the
safety of the PV installation. Remember though, that the Code is the
"minimum" standard and the AHJ has the final word. Issues of "reliability"
or "performance" however, do not fall within the mandate of the inspector's
duties though can influence the way they approach inspecting your work.
These situations are complicated when your customer is sitting there
wondering why you are so incompetent that you failed your inspection (just
kidding!). If you are lucky, the inspectors will briefly meet the homeowner
when they return and confirm that you are very competent and that they are
pleased with your work. Either that or maybe the customer will march down to
City Hall and throw the bums out!

Hope this helps and good luck with it.

Jason Fisher

Aurora Energy LLC
P.O.Box 3315
Annapolis, MD 21403

Voice: 410-268-3088 / fax: 410-268-3684
email: jason at aurora-energy.com

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