Sizing service tap conductors and disconnect. [RE-wrenches]

[Allan Sindelar]

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Dan, did you get answers to this post? f so, could you forward them, or
direct me to where they are in the Wrenches archives?
Thanks,
Allan

----- Original Message ----- 
From: "Dan Rice" <danrice at scinternet.net>
To: <RE-wrenches at topica.com>
Sent: Thursday, September 01, 2005 7:37 PM
Subject: Sizing service tap conductors and disconnect. [RE-wrenches]



Wrenches,

Sorry for a repeat of a topic I remember being addressed awhile back. I'm
installing a grid-tie system on a 400A meter-main on a commercial building.
For ease of meeting (avoiding) the requirements for not exceeding the rating
of the panelboard busbar, I plan to tap the service conductors on the supply
side of one of the two 200A main disconnect breakers, which are conveniently
fed by accessible 4/0 AL conductors. My tap conductors will leave the main
service panelboard through a short rigid conduit nipple and terminate in the
requisite "lockable, visible-break" fused disconnect.

Question: While I'm somewhat familiar with the tap rules in NEC 240.21(B)
for feeders, but 240.21(D) "Service Conductors" refers to 230.91, which does
not address conductor size. How have you been sizing your service tap
conductors, and what is the code justification? It seems as though the tap
conductors may be sized based on the overcurrent protection device rating of
the disconnect that they supply, as 230.90(A) suggests. Is that how you size
the tap?

Next question: NEC 230.79 says that the rating of the service disconnecting
means for a single branch circuit installation may be as small as 15 amps,
or 30 amps for two branch circuits. Except for dwellings (min 100 amps), all
others must be 60 amps minimum. Since the circuit is a PV power source and
not a "branch circuit" (if using the Article 100 definition), does this mean
that the smallest service tap supplied disconnect for a PV system must be at
least 60 amps? This would be overkill for the single GT3.0 to be
installed...

Thank you very kindly,

Dan Rice
Abundant Sun, LLC.

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