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

[Dan Rice]

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Wrenches,

On the question of sizing service tap conductors and additional service
equipment for a utility-side connection to an existing service for a
grid-tied PV system, I spoke to John Wiles. After combing through the NEC,
he wrote up the following, which covers this topic in detail. I hope this is
useful to others, too.

Dan Rice
Abundant Sun, LLC.
Service Entrance Conductor Taps for Utility-Interactive Inverter Systems

Section 690.64 of the National Electrical Code (NEC) establishes how and
where a utility-interactive PV system may be connected to the utility
system. The point of connection may be either on the load side of the
service disconnect or the utility (supply) side of the service disconnect.
In many cases, the complex requirements for load side connections
established by 690.64(B)(2) make such a connection not practical and dictate
that the utility-interactive inverter be connected on the supply side of the
service disconnect. Here are some, but not all, of the major code sections
that address the requirements for such a connection.

Section 690.64(A) allows a supply (utility) side connection as permitted in
230.82(6).

Section 230.82(6) lists solar photovoltaic equipment as permitted to be
connected to the supply side of the service disconnect.

It is evident that the connection of a utility-interactive inverter to the
supply side of a service disconnect is essentially connecting a second
service entrance disconnect to the existing service and many, if not all, of
the rules for service entrance equipment must be followed.

Section 240.21(D) allows the service conductors to be tapped and refers to
230.91.

Section 230.91 requires that the service overcurrent device be co-located
with the service disconnect. A circuit breaker or a fused disconnect would
meet these requirements. A utility-accessible, visible break, lockable
(open) fused disconnect (aka safety switch) may also meet utility
requirements for an external PV ac disconnect.

Section 230.71 specifies that the service disconnecting means for each set
of service entrance conductors shall be a combination of no more than six
switches and sets of circuit breakers mounted in a single enclosure or in a
group of enclosures. The addition of the photovoltaic equipment disconnect
would be one of the six.

Section 230.70(A) establishes the location requirements for the service
disconnect. Section 705.10 requires that a directory be placed showing the
location of all power sources for a building. Locating the PV service
disconnect adjacent or near the existing service disconnect may facilitate
the installation, inspection, and operation of the system.

Section 230.79(D) requires that the disconnect have a minimum rating of 60
amps. This would apply to a service-entrance rated circuit breaker or fused
disconnect.

Section 230.42 requires that the service entrance conductors be sized at
125% of the continuous loads (all currents in a PV system are worst-case
continuous). The actual rating should be based on 125% of the rated output
current for the utility-interactive PV inverter as required by 690.8. The
disconnect must have a 60-amp minimum rating. Larger conductors may be
required after temperature and conduit fill factors have been applied.

For a small PV system, say a 2500 watt 240 Volt inverter requiring a 15-amp
circuit and overcurrent protection, these requirements would appear to
require a minimum 60-amp rated disconnect, but 15-amp fuses could be used;
fuse adapters would be required. While 15-amp conductors could be used
between the inverter and the 15-amp fuses in the disconnect, Section
230.42(B) requires that the conductors between the service tap and the
disconnect be rated not less than the rating of the disconnect; in this case
60 amps.

How we would deal with the 60-amp disconnect, 15-amp over current
requirements using circuit breakers is not as straightforward. A circuit
breaker rated at 60-amps would serve as a disconnect and it could be
connected in series with a 15-amp circuit breaker to meet the inverter
overcurrent device requirements. In this case the requirements of
690.64(B)(2) should be applied for the series connection.

Section 110.9 requires that the interrupt capability of the equipment be
equal to the available fault current. The interrupt rating of the new
disconnect/overcurrent device should at least equal the interrupt rating of
the existing service equipment. The utility service should be investigated
to ensure that the available fault currents have not been increased above
the rating of the existing equipment. Fused disconnects with RK-5 fuses are
available with interrupt ratings up to 200,000 amps.

Section 230.43 allows a number of different service entrance wiring systems.
However, considering that the tap conductors are unprotected from faults, it
is suggested that the conductors be as short as possible with the new PV
service/disconnect be mounted adjacent to the tap point. Conductors
installed in rigid metal conduit would provide the highest level of fault
protection. All equipment must be properly grounded per Article 250
requirements.

Additional service entrance disconnect requirements in Article 230 and other
articles of the NEC will apply to this connection.

John Wiles jwiles at nmsu.edu 505-646-6105   9/16/2005



----- Original Message ----- 

From: "Dan Rice" <danrice at scinternet.net>
To: <RE-wrenches at topica.com>
Sent: Thursday, September 01, 2005 6:37 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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