Wiring & Protection
Transcript of Wiring & Protection
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7/30/2019 Wiring & Protection
1/204 home power 147 february & march 2012
Chapter 2 of the National Electrical CodeWiring and
Protectionis lengthy and important, covering a number of
topics. PV installers must appropriately apply those sections
that pertain to PV systems.
Conductor IdentificationChapter 2 starts with Article 200, Use and Identification ofGrounded Conductors. Many in the PV industry refer to
the DC conductors from the PV modules as positive and
negative, but that nomenclature is of less importance in
modern systems.
In the majority of PV systems installed today, the inverters
use an isolation transformer. For PV systems using these
inverters, the NEC requires bonding one of the current-
carrying conductors on the PV side of the inverter to
ground. Typically, this is accomplished across the ground-
fault protection device located in the inverterbonding the
negative conductor to ground creates a negatively groundedsystem. Some PV systems require bonding the positive
conductor to ground. (Note that nearly all PV modules can
be positively grounded.) To top off that variability, there are
now ungrounded inverters, in which neither PV conductor
is bonded to ground. These systems are covered in Section
690.35, Ungrounded Photovoltaic Power Systems.
Given the different wiring configurations, its most
accurate to refer to the conductors in terms of grounded
current-carrying; ungrounded current-carrying; and
grounding conductors rather than negative, positive,
and ground conductors. This terminology also helps clarify
how each of these conductors should be identified, which
brings us back to Article 200, which sets the requirements for
identifying the grounded current-carrying conductors for all
electrical systems, including PV systems.
Section 200.6 details those identification methods.
Subsection 200.6(A) deals with identifying conductors
that are 6 AWG and smaller. In residential grid-tied PV
systems, it is likely that all the conductors used will fall into
this size category. Section 200.6(A) lists eight appropriate
identification methods, one of which is specific to PV systems
(discussed later). The first three methods and the PV-specific
requirement are the most commonly used. The grounded
current-carrying conductors can either be white, gray, or
marked with three continuous white stripes on top of any
color insulation other than green, which is reserved solely for
grounding conductors.
Given those requirements, for most PV systems the
negative is the grounded conductor and should follow one
of those identification methods. If the positive conductor is
the grounded conductor, it will follow those rules. And forungrounded systems, neither conductor is marked that way.
Now that you have identified the grounded current-
carrying conductor(s), you can identify and mark the
ungrounded current-carrying conductor(s). These can be any
color other than those listed in 200.6not white, gray, or
with three continuous white stripes; nor green or green with
yellow stripes. In the case of negative-grounded PV systems, I
recommend that the ungrounded color be red. This will meet
Code and helps identify the positive polarity of the conductor.
Regardless of the chosen color-coding, I also recommend
using marking tape at the termination to identify the polarity
of all of the conductors.It is important to cover the marking allowance specific
to PV systems200.6(A)(6) allows the installer to mark
small PV source circuit with distinctive white markings at all
terminations. This is an exception to the general rule for small
conductors, so you may need to bring this to an inspectors
attention. The exposed USE-2 or PV cable can be black and
marked at the terminations made inside junction or combiner
boxes. Just remember that once you transition to an interior
wire such as THWN-2, that conductor type is outside the
scope of this allowance and must meet one of the other
requirements listed.
For conductors 4 AWG and larger, 200.6(B) outlines the
rules you are required to follow. The first three methods are
identical as the requirement for small conductors. The fourth
method allows for distinctive white or gray markings that
encircle the conductor at the termination point.
Overcurrent ProtectionArticle 240, Overcurrent Protection, sets the rules on how to
properly select, locate, and enclose overcurrent protection
devices (OCPDs) to protect conductors from overload, short-
circuit, and ground-fault conditions. Section 240.4 contains
some critical information on the required methods to
properly protect conductors. Conductors must be protected
against overcurrent in accordance with the conductors
codecornerrenewable energy and the national electrical code
Wiring & Protectionby Ryan Mayfield
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7/30/2019 Wiring & Protection
2/210www.homepower.com
ampacity values (as listed in Section 310.15), unless otherwise
required or allowed within 240.4(A) through (G). In general,
a conductor needs overcurrent protection at a value less than
or equal to the conductors ability to carry current.
A general rule is often followed up with specific cases
and/or exceptions. In 240.4(B), an allowance is made forovercurrent devices rated at 800 A or less. When specific
conditions are met, a conductor can be protected by an
overcurrent device with an ampere rating greater than the
conductor. The next higher standard-rated device can be used
to protect the conductor if the conductors ampacity doesnt
correspond to a standard OCPD ampere rating, the next
standard OCPD doesnt exceed 800 A, and the conductors
being protected are not part of a branch circuit supplying
multiple receptacles. For PV systems, once you adjust a
conductors ampacity for conditions of use and continuous
duty, that conductor can be placed on an overcurrent device
with a rating greater than the conductor. For example, letssay you are using a 10 AWG conductor to connect a string of
modules to an inverter. After applying correction factors if
that 10 AWG conductor has an ampacity value of 21 A, you
can use a 25 A OCPD.
Section 240.4(D) lists special requirements for small
conductors, namely the limitations of OCPD ampere ratings
based on the conductor size. So, unless specifically allowed in
240.4(E) or (G), the OCPD protecting small copper conductors
cannot exceed:
15 A for 14 AWG Copper
20 A for 12 AWG Copper 30 A for 10 AWG Copper
When sizing conductors, you may have a scenario where
one of the above listed conductors must be protected by an
OCPD with a smaller rating than the conductors ampacity.
The 240.4(D) section simply defines the maximum OCPD
rating, even if the conductors ampacity exceeds the OCPD
rating. For example, after correction factors are applied, a 10
AWG conductor may have an ampacity value of 35 A. The
Code requires that this conductor is protected by an OCPD
with a maximum ampere rating of 30 A.
Article 240.6(A) lists standard OCPD ratings, the values
referenced when determining the proper OCPD for protecting
conductors. The standard values start at 15 A and go up to 6,000
A. Per 690.9(C), for OCPDs protecting PV source circuits, the
standard ratings are 1 A to 15 A in 1 A increments. After that,
the standard values listed in 240.6 are applicable. Practical
examples of this selection process, as well as conductor sizing
will be covered in an upcoming Code Corner.
Section 240.24 covers the location and accessibility
requirements for OCPDs. Generally, all OCPDs must be
readily accessible and the center of the grip on the OCPDs
operating handle may not exceed 6 feet 7 inches above
the working platform when in its highest position. Two
allowances to this rule may apply to a PV installation. One
is that OCPDs may be accessed by portable means, such as
ladders, if the OCPDs are mounted adjacent to the equipment
they are protecting. The other is not an exception listed in 240
but rather an allowance in 690. In 690.9(C), the Code allows
the OCPDs protecting PV source circuits to be accessible,
but doesnt require that they be readily accessible (per NECdefinition of readily accessibleagain we see that it is
acceptable to access particular OCPDs via a ladder and so
forth).
Section 240.24 specifies that OCPDs be located where
they are not exposed to potential physical damage. This may
require protecting the OCPDs with barriers (for example,
bollards may be used to prevent potential damage to OCPDs
by vehicular traffic). Section 240.24(D) prohibits locating
OCPDs in the vicinity of easily ignitable material, such as
in clothes closets. And finally, OCPDs are not allowed in
bathrooms [240.24(E)] or over steps [240.24(F)].
Part III of Article 240, which covers enclosures forOCPDs, affects many PV installations, and Section 240.32 is
of particular importance. If the OCPDs are located outside,
youll need to verify that the enclosure used to house the
OCPD complies with 312.2, as referenced in 240.32. This is to
prevent moisture from entering and accumulating within the
enclosure. Article 312.2 has requirements for raceways and
cables entering the enclosure, stating that if an enclosure is
being entered above uninsulated live parts, the fittings used
to enter the box must be listed for wet locations. Thankfully,
PV installers have multiple options for enclosures listed for
various applications. For enclosures that will be mounted
on an outdoor vertical surface, you can use NEMA 3R-ratedenclosures. Some manufacturers of NEMA 3R enclosures
have listed their products for installation on a tilted surface;
this should be verified on a per-model basis. Typically, for
enclosures mounted in a non-vertical orientation, a NEMA
4 enclosure should be used (see NEC table 110.28 for more
information on enclosure types).
Setting up Your InstallationThese two Code articles, 200 and 240, set the foundation for PV
installations. Be sure to read through to the end of Article 240,
as it covers the various types of OCPDs. As with nearly all of
the Code, I suggest you read and re-read these articles. Youd
be surprised how often something new jumps out at you and
changes your thinking.
AccessRyan Mayfield ([email protected]) is the principal at a
design, consulting, and educational firm with a focus on PV systems in
Corvallis, Oregon. He is an ISPQ Affiliated Master Trainer.
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