Schneider Electric MH62D9VWP Owners Manual

AC Combiner Box Application Guide
for Circuit Breaker Panelboards

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Solar engineers and installers often overlook the utility and
convenience of using an AC combiner box when designing
a photovoltaic (PV) system. This application guide details the
benets of incorporating a panelboard as an AC combiner box
into a solar system and assists engineers in selecting the most
suitable combiner box/panelboard options for each PV system.

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The role of AC combiner
boxes in solar systems

The function of a combiner box is to consolidate the output of multiple string inverters into one main feed, reducing
labor and material costs by using less wiring. A combiner box may also provide a disconnecting means to the
string inverters that feed them, and overvoltage protection to the electrical equipment they feed.

AC combiner box functions

For solar installations in the Photovoltaic (PV) industry, AC combiner boxes may perform several functions. They can
consolidate power from multiple string inverters into a central location and provide overcurrent protection and a
disconnecting means for each string inverter input, making the system easier to install, maintain, and operate. They
may also incorporate surge protective devices and a main output disconnecting means in one centralized enclosure.
For commercial applications, the box maximizes power and saves on material and labor costs by eciently
consolidating and distributing the combined connections, enabling PV power to be sent to the power grid or loads.
Choosing the right combiner box can vastly improve the ease of installation, maintenance, and operational eciency
of a solar application.

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UTILITY
SERVICE

UTILITY
TRANSFORMER

UTILITY
METER

MAIN CIRCUIT
BREAKER

AC COMBINER BOX
(SOLAR LOAD CENTER)

INVERTERS

VAC

VDC

VAC

VDC

VAC

VDC

VAC

VDC

VAC

VDC

VAC

VDC

VAC

VDC

SOLAR
PANELS

DC COMBINER BOXES

Square DTM brand panelboards from Schneider Electric may be used as AC combiner boxes. They support

voltage systems up to 600 Vac and are more readily available than traditional combiner boxes. Merchandised Square D
brand panelboards from Schneider Electric may be purchased Ready-To-Install (RTI) from the inventory of an authorized
electrical distributor and assembled at the jobsite. Square D panelboards can house electrical connections and disconnects
for multiple inverter output circuits, allowing for maintenance of a single inverter and its strings without suffering power loss of
other inverters and PV strings. A single disconnect for the combiner output circuit allows for emergency shutdowns, switching
to alternate power sources, and maintenance.

Like standard electrical power systems, PV systems require overcurrent and short-circuit protection for inverters, conductors,
and other equipment. Continuous improvements in PV technology have led to increased currents in solar systems,
underscoring the need for effective protection. By consolidating the output of multiple string inverters into one AC combiner
box, overcurrent protection and disconnects can be coordinated, localizing power disruption to a single inverter and set of PV
strings — rather than shutting down the entire system.

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Overcurrent protection in
AC combiner boxes

Solar combiner boxes commonly provide overcurrent protection and a disconnecting means for each circuit that feeds them.
Historically, fuse boxes/panels were the most popular form of overcurrent protection in home and commercial electrical
systems, though they have since been largely eclipsed by circuit breakers. While fuses can provide overcurrent protection,
circuit breakers are easier to maintain and provide an integral disconnect switch. Because each fuse protects the circuit by
melting, a blown fuse needs to be replaced immediately for the circuit to function again, whereas a circuit breaker “trips” and
interrupts the current flow from the power source. To restore power after diagnosing and addressing the reason that a circuit
breaker trips, the user merely flips the switch off and on again, instead of needing to replace a fuse.

Square D circuit breaker panelboards provide several inherent benefits over fuses when implemented in a solar system:

Square D circuit breakers are 100% factory tested. Fuses, on the other hand, cannot be tested

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because a blown fuse would need to be replaced.

Schneider Electric and Square D have been the brands trusted by electrical contractors and

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consulting engineers for over 100 years.

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NF Panelboards in vented
NEMA Type 3R enclosures

Square D brand panelboard
advantages

Schneider Electric has been the market leader in circuit breaker panelboards for nearly a

century. First developed in the 1960s, the I-Line™ Power Distribution Panelboard features

Schneider Electric’s unique circuit breaker engagement system and is used to feed NQ and NF

Panelboards. I-Line Panelboards are designed to manage currents up to 1200 A and are utilized

in nearly every market segment — including solar. NQ Panelboards use QO™ circuit breakers

developed in the 1950s, and NF Panelboards were rst introduced in 1994, then updated several

times as the replacement for previous Square D circuit breaker panelboard ranges dating back to

the 1920s. All these technologies have undergone continuous improvements since their inception.

PowerPacTTM electronic trip circuit breakers now oer advanced functions such as adjustable

long time, short time, instantaneous trip, and ground fault protection that safeguards equipment.

Square D brand circuit breaker panelboards can be purchased ready-to-install from various

electrical distributor inventories and assembled at the jobsite. The lead time to deliver factory-

assembled panelboards to the jobsite is typically less than two months, supporting a timely setup

and installation.

Integration of surge

These circuit breaker panelboards
are fully customizable to suit a
variety of solar projects and
systems. Circuit breakers can be
back-fed for combiner box inputs (in
this case, solar inverters) and
support common North American
(NAM) AC voltage systems — 3Ø
4W up to 600 Vac and 1Ø 3W up to
600Y/347 Vac. Panelboards are
available in NEMA Type
3R/4/4X/5/12 enclosures with
lockable handles that provide a
degree of protection to personnel
against access to hazardous parts
and against the ingress of water or
solid foreign objects.

protection and metering

Surge protection devices designed specically for PV systems can also be installed in the

AC combiner box, safeguarding equipment from lightning or other voltage surges. Square D

TM

Panelboards may be congured with integral SurgeLogic

current ratings up to 240 kA.

Power metering may also be installed in Square D brand panelboards to monitor each circuit

into and/or out of the panelboard. Branch circuit power monitoring is available for NF and

NQ Panelboards, and embedded power metering for feeder circuit breaker trip units and the

outgoing circuit are available options in I-Line Panelboards.

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Surge Protection Devices with surge

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Inverter output current vs.
circuit breaker trip current

Rated current has a dierent meaning for solar inverters than for circuit breakers. Inverters may deliver surge power well in excess of their

continuous current rating for sustained periods. Circuit breakers may be calibrated to open when continuously operated at as little as 80% of

their nominal trip current (or handle rating) at a UL specied ambient temperature of 40°C (104°F).

ED/EG/EJ FAMILY MOLDED CASE CIRCUIT BREAKERS

CHARACTERISTIC TRIP CURVE NO. 515-5

CIRCUIT BREAKER INFORMATION

Circuit Breaker Continuous Maximum Number

Prefix Ampere Rating AC Voltage of Poles

ED, EG, EJ 80–100 600Y/347 2, 3

This curve is to be used for application and coordination purposes only. The
EZ-AMP overlay feature at the bottom of the page should be used during
coordination studies.

All time/current characteristic curve data is based on 40°C ambient cold start.
Terminations are made with conductors of appropriate length and ratings.

Curve No. 0515TC1305

January 2013

Drawing No. B48095-515-05

© 2013 Schneider Electric all rights reserved

(Replaces 0515TC0005)

For this reason, a circuit breaker with a nominal current rating of 100A may open when back-fed continuously with an inverter output current as

low as 80 A. Therefore, to avoid nuisance trips, the nominal trip current of a typical circuit breaker should typically be 125% of the continuous

output current of the string inverter that feeds it. The trip curve above shows that that the E-frame branch circuit breakers installed in NF

Panelboards will operate continuously at >90% of their nominal trip current (at 40 C), making them highly suitable to support the sustained high

currents produced by solar string inverters.

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Current carrying capacity in
extreme conditions

Circuit breakers are sensitive to both current and ambient temperature. When applied
at ambient temperatures other than 40°C, at frequencies other than 60 Hz, or in other
extreme conditions, circuit breaker performance may be affected. In these cases, it
becomes necessary to determine a circuit breaker’s actual current carrying capacity
under such conditions. By correctly applying an appropriate rerating curve, solar
system designers can accurately predict the circuit breaker’s real current carrying
capacity.

Schneider Electric recognizes that equipment in solar system applications may endure
harsher environmental conditions than governing standards assume and offers several
solutions for reliable operation. Additionally, electronic trip units in Square D PowerPacT
circuit breakers are ambient insensitive from -10°C (14°F) to 60°C (140°F)
Electric recommends that outdoor solar array panelboards be configured with remote­mounted main circuit breakers with electronic trip units or remote-mounted switches.

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3

. Schneider

Electronic trip units in
Square D PowerPacT
circuit breakers are
ambient insensitive
from -10°C (14°F) to
60°C (140°F).

Schneider Electric oers
Square D brand panelboards
for use in AC combiner boxes

Square D brand I-Line Panelboards are the most capable Schneider Electric circuit

breaker panelboards for North American AC Voltage systems — 3Ø 4 W up to 600
Vac and 1Ø 3 W up to 600Y/347 Vac. Molded case circuit breakers (MCCBs) can
handle input currents of up to 960 A and back-fed circuit breakers up to 1200 A. For
overcurrent protection, thermal magnetic or electronic trip units are fully rated with
Short Circuit Current Ratings (SCCR) up to 200 kA at 480Y/277 Vac. The main bus
output supports currents up to 1200 A, with a main circuit breaker, main switch, or
main lugs, and supports output lugs up to (4) AWG 3/0 - 500 kcmil. For environmental
protection, NEMA Type 3R/4/4X/5/12 enclosures are available.

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Data Bulletin 0100DB0101

(Determining Current Carrying Capacity in Special Applications)

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Data Bulletin 0100DB1901

(Proper Application of Panelboards in Solar Arrays)

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NQ panelboards are suited to solar applications and may be used as low cost combiner boxes for all AC voltage
systems up to 240 Vac (Wye or Delta). These panelboards support continuous input currents up to 120 A with back­fed circuit breakers up to 150 A. All circuit breakers feature thermal magnetic overcurrent protection with a fully
rated SCCR up to 22 kA, and lugs that accept copper or aluminum wires up to 2/0 AWG. The main bus supports
output up to 600 A with main lugs that accept (two) AWG 1/0-750 kcmil cables per phase. An 80% rated main
circuit breaker or main switch up to 600A may also be chosen that will accept (two) AWG 2/0-500 kcmil aluminum
or copper cables per phase. Neutral lugs are available that will accept up to (four) AWG 1/0-750 kcmil aluminum or
copper conductors. Integral surge protective devices, vented NEMA Type 3R enclosures, and electronic trip units
for output circuit breakers are available for added protection. NQ panelboards may also be configured for use in 48
Vdc 2 wire voltage systems with a maximum available fault current of 5 kA.

NF panelboards provide another low-cost option for AC combiner box applications for voltage systems up to
600Y/347 Vac. These panelboards support continuous input currents up to 100 A with back-fed circuit breakers
rated to 125 A at 480Y/277 Vac, lugs for wires up to 2/0 AWG, thermal magnetic overcurrent protection, and fully
rated SCCRs up to 65 kA at 480Y/277. The main bus supports output up to 600 A, with main lugs up to (2) 1/0-750
kcmil main lugs per phase, or an 80% rated main circuit breaker or main switch accepting (2) 2/0-500 kcmil wires
per phase. Neutral lugs are available that accept up to (4) AWG 1/0 – 750 kcmil aluminum or copper conductors.
Electronic trip units on output circuit breakers, vented NEMA Type 3R enclosures, and surge protective devices are
available for added protection.

Back-fed circuit breakers in panelboards used as AC combiner boxes support many North American string inverters.
Because thermal magnetic circuit breakers are sensitive to ambient temperature, it may be important to minimize the
temperature rise inside their enclosure due to the absorption of incident solar radiation (ISR). ISR absorption in outdoor
panelboard applications may be minimized by installing a solar shield, or by painting their enclosure white. To reduce the
ambient temperature around each circuit breaker, Schneider Electric recommends leaving blank spaces between circuit
breakers in a panelboard.

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Solar system designers are encouraged to remotely mount the main disconnect switch or circuit
breaker for each panelboard to lower the ambient temperature inside its enclosure and enable installation of an internal surge
protective device.

Opening the inner
door enables access
to all circuit breaker
handles (and Visi­tripTM) indicators).

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The flow of current combined with ambient temperature increases the

NF Main Circuit
Breaker Panelboards
in vented NEMA
Type 3R enclosures

temperature of circuit breakers. Please consult the temperature derating
table for PowerPacT circuit breakers to avoid reaching their thermal
withstand value.

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Qualied personnel
gain access to all
electrical connections
by opening the outer
door (when the
panelboard is not
energized).

To learn more about energy management solutions,
visit our website.

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Schneider Electric

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Suite 2700, Boston
MA 02108, USA

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