PV System Design (PV 202)
Module 7
Electrical Design & Code Requirements
1
Module 7:
Electrical Design & Code Requirements
Photovoltaic Systems: Chapter 11
Solar Electric Handbook: Chapters 14, 15 & 16
Although the NABCEP PV Associate Exam does not require the use or
interpretation of electrical code rules, a general understanding of conductor sizing, wiring methods, and grounding is essential to PV system design.
Content outline
• Introduction to the National Electrical Code
• Conductor sizing and derating
The National Electrical Code (NEC)
• Published every three years
• Adopted by state or local jurisdiction
• Many articles in the NEC are applicable to the design and installation of a PV system, particularly Articles 690 & 705.
• The first four chapters of the NEC apply to all electrical installations, so many of those rules also apply:
• Conductors & conductor sizing (Article 310)
• Overcurrent protection (Article 240)
• Grounding (Article 250)
3
The National Electrical Code
Published by the National Fire Protection Association (NFPA).
Available in soft cover, as a Handbook with notes
(hard cover) and can be viewed for free on the NFPA website.
Electrical
Integration
The NEC defines the various
circuits and
components in PV systems and specifies their requirements.
5Source: Dunlop, James. Photovoltaic Systems, 3rd ed.
Conductor Sizes
Conductor Properties
Larger conductors have lower resistance for a given length.
7Source: National Electrical Code (NEC)
Conductor Insulation: Article 310
AWG size marked every 24”
Other info marked every 40”
• Maximum voltage
• Insulation type letters
• Manufacturer
Conductor Insulation Letters
T = Thermoplastic N = Nylon coating W = Wet locations U = Underground
S = Sunlight resistant
H = 75˚C temperature rating HH = 90˚C temperature rating H-2 = 90˚C temperature rating
& wet locations
9Source: Dunlop, James. Photovoltaic Systems, 3rd ed.
Conductor
Ampacity:
Article 310
Ampacity is the current- carrying capacity of a conductor.
Depends on conductor material and size, type of insulation, and ambient temperature.
Ampacity
Correction
Factors for High
Temperatures
Conductor ampacity must be corrected (de-rated) for ambient temperatures
above 86˚F.
11Source: National Electrical Code (NEC)
Ampacity Adjustment Factors for
Multiple Conductors
Conductor ampacity must also be adjusted (de-rated) when
there are more than three current-carrying conductors bundled together in conduit or cable.
Temperature Correction Factors for Rooftop
Installations Exposed to Sunlight
• This rule has changed in every Code cycle since introduced.
• Formerly tables with “temperature adders” based on
distance between bottom of raceway/cable to roof surface.
2017 & 2020 NEC:
Add 60˚F (33˚C) if bottom of raceway is less than 7/8” to roof surface. If 7/8” or greater, no temperature adder is required.
Follow your local building and electrical codes, and rules in whichever NEC is enforced.
13
Voltage Correction Factors for Low
Temperatures – Table 690.7
Open-circuit voltage is corrected for low
temperatures to find the maximum
possible array voltage.
Source Circuit Wiring: Article 690
PV source circuits are
usually wired with exposed conductors that must be suitable for the conditions, like high temperatures and sunlight (UV).
• PV Wire
• USE-2 conductors
15Sources: https://www.iewc.com/products/wire-and-cable/building-and-fixture-wire/photovoltaic- wire, graybar.com & vwcable.com
Module Connectors
Modules are manufactured and should be connected with external, locking, male-female connectors that require a tool to disconnect.
Combiner Boxes
Multiple PV source circuits can be brought to a combiner box and create the PV output circuit to the inverter.
17Source: MidNite Solar Inc.
Combiner Box: Terminal Blocks &
Series Fuses
Overcurrent Protection Devices
(OCPD): Article 240
Fuses & Circuit Breakers Table 240.6(A)
Standard Sizes:
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100…
ALSO:
1, 3, 6 & 10
Source: Dunlop, James. Photovoltaic Systems, 3rd ed. 19
PV Array (DC) Disconnecting Means
The array disconnect switch opens all ungrounded, current- carrying conductors in the PV output circuit before going to the inverter, charge controller, or battery bank.
• PV Disconnect
• DC Disconnect
Equipment Disconnecting Means
All major components must include switches or circuit breakers as a means to
disconnect them from the rest of the system.
Isolate all sources of energy for service & repair:
• PV array
• Battery bank
• Grid power
21Source: Dunlop, James. Photovoltaic Systems, 3rd ed.
Inverter Output Overcurrent
Protection & Disconnecting
Means
Overcurrent protection for the AC
inverter output circuit depends on the inverter and utility/system connection.
Overcurrent protection and
disconnecting means for the AC
inverter output may be combined by using circuit breakers or fused
AC Disconnecting Means
Since grid-tied PV inverters are UL-1741 listed with anti- islanding protection, some electrical utilities don’t
require an external AC disconnecting means.
Others do.
Check your local electrical and building codes, and your
utility interconnection policy.
23Source: Dunlop, James. Photovoltaic Systems, 3rd ed.
AC Disconnecting Means
In some cases, the AC disconnect switch
must be located in close proximity to the kWh meter.
• Lockable
• Knife switch with visual break
Grounding: Article 250
• The grid is an intentionally GROUNDED system, meaning it references voltages with respect to the EARTH (ground)
• GROUND or EARTH is assumed to be 0 volts, 0 potential.
• Everything connected to the earth should therefore have no electrical potential and be safe to touch.
• Metal water pipes
• Metal buildings, structures, signs, light posts, fences, etc.
• Concrete reinforcement rods & bars
25
Why ground electrical systems?
• So there is a reference point of zero volts
• So high voltage surges (lightning or utility faults) have a place to go
• So fuses and circuit breakers operate the way they’re supposed to
• So there is no difference in potential (voltage) between two pieces of metal (water pipe & gas pipe, e.g.)
• So people aren’t exposed to an electrical shock when they
Lightning Protection
Bond it to the premises
grounding
electrode system.
27Source: Dunlop, James. Photovoltaic Systems, 3rd ed.
Lightning Protection
Required & desired
Grounding AC Systems
• The electrical utility brings two UNGROUNDED (hot) conductors and a GROUNDED conductor to a 240-volt, residential service.
• Inside a residential service panel, the GROUNDED
conductor is WHITE and is often called the “neutral” wire.
• Caution: It is NOT “neutral!” It is a current-carrying conductor and should be treated as such, but it will measure 0 volts to GROUND.
Source: https://www.dummies.com/programming/electronics/components/alternating-current-in-
electronics-hot-neutral-and-ground-wires/ 29
Grounded
Conductors
• Black insulation = ungrounded (hot) conductors
• 240 VAC from LI to L2
• Bare wires & white insulation =
grounded “neutral”
conductors
Residential Services
• The electrical utility provides 240 VAC (L1 & L2) and a grounded conductor, which measures 120 VAC to L1 and 120 VAC to L2.
• 240 VAC is used for high-consumption, high-current loads:
• Clothes dryers, ranges, water heating, HVAC equipment, EV charging
• 30A, 40A, 50A… or more
• 120 VAC is “split” (L1 & L2) and serves lower-current loads:
• Kitchens, bathrooms & laundry circuits (20A)
• Living rooms, bedrooms, lighting, etc. (15A)
31
DC Grounding (PV Arrays)
• PV modules produce DC
• All metal parts of the array need to be connected to the premises grounding system.
• SOLIDLY GROUNDED:
• On systems with transformer-based inverters, one
conductor from the PV modules will be directly connected to ground.
• These systems generally require a separate DC Grounding
DC Grounding (PV Arrays)
• FUNCTIONALLY GROUNDED:
• On systems with transformerless inverters, the DC
conductors are not bonded to ground. There is an electrical reference to ground, performed through electronic
detection.
• There will be an Equipment Grounding Conductor (EGC) from the array to the inverter, where it is bonded to the grounding conductor going to the AC grounding system.
• Until 2017 NEC, these systems were called “ungrounded.”
33
Grounding
PV Systems
AC and DC systems must be BONDED together to form a single GROUNDING ELECTRODE SYSTEM.
Sizing the
Grounding
Electrode
Conductor:
250.66
35Source: National Electrical Code.
Module Frame
Grounding
To facilitate required equipment grounding, PV module frames should be connected to each
other, and to the rails and racking systems to ensure a continuous grounding connection.
Approved hardware and torque requirements ensure a good
Sizing the
Equipment
Grounding
Conductor:
250.122
The equipment
grounding conductor size is based on the size of the OCPD
protecting the circuit.
37Source: National Electrical Code (NEC)
Array Ground-Fault Protection
Most inverters include fuses for array ground-fault protection of the DC input circuits.
Module 7 Summary
READ
Photovoltaic Systems: Chapter 11
Solar Electric Handbook: Chapters 14, 15 & 16
• Conductor insulation, ampacity & derating
• Source circuit wiring, combiner boxes & overcurrent protection
• Disconnecting means
• Grounding
39