Thanks for a great response Matt, and an interesting lead to a product
solution...
Let me preface this with saying I am installing another company's design, so
all you are saying about proper
gear at the service is totally heard, and this tap would not be something I
would design...
The situation is a Wadsworth tandem 100amp main. Really rare and weird panel
from the early 60s. It has
2 service entrance masts, and a 8 jaw meter socket feeding 2 separate fused
100 amp load centers.
The design was to tap into one of these with 40amp fused disconnect from a sma
7000.
Unfortunately the wires in the Wadsworth gear is type TW which is not included
in the Feeder exception of
310.16. When this was discussed. I threw out there the idea of changing the
wires. These guys wanted to
void the listing for piercing taps and any other taps
The REAL problem is the wording of NEC 690.64(B) because all these guys that
are not willing to wrap
their head around what the signal path/ back feeding/ over-current/ 230
tapping scenarios are... read 120
and wants to apply it to everything in the system. These inspectors were
explaining to me that the feeder IS
the buss and 120%... and its really hard to explain the intent with the
language of the paragraph. It needs to
be 2 separately described scenarios in my opinion, to avoid the constant
misunderstanding.
Also, the NEC says in 690.64 and the 230 reference there, that taps are
permitted for solar... not much
detail, not much help.
It seems like these inspectors consider the "voiding the UL listing" is a dead
wall conversation ender. I'm
almost willing to admit you are right, it is totally valid, which makes the
point of .64 come into question.
I hope you are right about newer gear having listed tapping means and large
buses for solar. It seems like
that will take a while to filter down because so many people work with
electricians different than the solar
installers they are working with (and because so many installers focus on
solar and don't wire houses). I
think the glimmer of hope that a tap will pass on a retrofit is gonna keep
them around for some time. And I
really think that the code needs to address the options better.
looks like someone involved here is buying a service upgrade. surprise!
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