dedicated AC disconnects [RE-wrenches]

[Geoff Greenfield]

Hi Andrew -

This is a relic of an issue and hopefully more and more utilities will be
going the way of your DC utility (and some west coast and one of the
utilities in our service area, Duke/Cinergy.)  Most of the utilities we work
with require a "visible, lockable, exterior disconnect".  We have been
educating them that this is redundant, never used, and a (most effectively)
that many utilities are skipping this requirement.  The utility linesman
still has the ability to pull the meter, but again, it is a non-issue.

This can be a very big expense in a school or large commercial building
where you could have a short, easy AC home run to the closest sub panel VS a
long difficult Home Run to an appropriate exterior wall, then back to a
point of interconnection (and factoring in AC voltage drop and the price of
copper doubling every six minutes).

Battery based systems with inverters listed for interconnection meet the
same standard.  You still need a way to isolate the inverter as per NEC -
often most easily done with the back fed AC breaker if you are next to the
panel, otherwise a non-fused Air Conditioning Disco is the low cost option.
While Fronius does have built in AC breaker - this (and their built in DC
breaker) does not meet a strict interpretation of the code.  The Xantrex GT
set up does, with the "mothership/docking" top and bottom set up.

Question:  Has anyone successfully argued that an exterior DC disco (pole
mounted for example) or wind tower mounted AC disco would meet this utility
requirement?

For a brighter energy future,

Geoff Greenfield
NABCEP Certified Energy Practitioner

THIRD SUN SOLAR & WIND POWER Ltd.
340 West State Street
Athens, OH 45701

www.third-sun.com
(740) 597-3111


Date: Wed, 23 Aug 2006 22:32:47 -0400
From: "Andrew Truitt" <atruitt at gmail.com>
Subject: dedicated AC disconnects




When I installed in California UL listed AC disconnects were mandatory
within 9' of the main meter.  Now I'm installing in DC and no such
regulation exists. I am wondering if dedicated AC disconnects are really
necessary for inverters with built in AC discos and automatic shut-off at AC
signal loss considering a) there are always 2 means of disconnection - the
breaker and at the inverter and b) the grid will not be backfed as long as
no signal is present, eliminating the need for the linesman to be able to
disconnect the system.  2 caveats:

1) Maybe, in the process of testing the lines during an outage, AC signals
are sent down the lines that could potentially restart the inverter.  Though
as soon as those signals ceased so would the inverter so I don't see it as a
problem.

2) I have very limited experience with battery/hybrid systems so if they
raise special circumstances then please enlighten me.

Andrew Truitt
Standard Solar


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