What is your largest inverter size?

Example:

Busbar = 200A

Main = 200A

What is your largest inverter size?

                                 200A X 1.2 = 240A

                                 240A – 200A = 40A

                                  40A / 1.25 = 32A

Largest inverter is 32A

You can multiply by the ac voltage to get the power!

705.12(D)(2)(3)(C): SOLAR + LOAD BREAKER METHOD

Load and supply breakers do not exceed the main breaker (this also works for subpanels).

New in the 2014 NEC, 705.12(D)(2)(3)(c) allows us to add up the load and supply breakers and as long as they don’t exceed the rating of the main supply breaker (or the busbar rating) then we are good.

                                  Solar + Load ≤ Main

Note that for the solar + load breaker accounting method, we do not use

125% of the inverter current, we use the size of the breaker. This is easy and convenient when combining inverters with an ac combiner subpanel.

Ten-foot inverter tap rule

240.21(B)(1), the ten-foot tap rule (for taps of ten feet or less):

• Tap ampacity > tap load

• Ampacity of tap conductor has to be greater than the load.

Pa

Example:

• ten-foot tap rule

• 200A source breaker

• 16A inverter.

The tap conductor must be at least 10% of (source breaker + 125% inverter

current):

                                             16A inverter 3 1.25 = 20A

                                       20A + 200A source breaker = 220A

                                                   0.1 x 220A = 22A

Tap conductor must be at least 22A

Additionally, the inverter breaker must be less than 22A and for a 16A inverter;

we would use a 20A breaker (16A 3 1.25 = 20A breaker).

One more thing to check is the 230A feeder, which needs to be enough to carry

currents from the source and 125% of the inverter.

DC DISCONNECT LABEL CALCULATION

1. Operating current:

               A. Imp X number of source circuits.

2. Operating voltage:

               A. Vmp X number of modules in series per source circuit.

3. Max system voltage:

               A.Voc X Max number of modules in series per 

                source circuit X Correction Factor of Ambient temp.

4. Short circuit current:

               A. Isc X 1.25 X number of source circuits 

MARKING OF MODULES

•   Open-circuit voltage                                       Voc
•   Operating voltage                                           Vmp
•   Maximum permissible system voltage           Maximum system voltage
•   Operating current                                            Imp
•   Short-circuit current                                        Isc
•   Maximum power                                             Power of module (Vmp 3Imp)
•   Polarity                                                            + or –
•   Max series fuse rating                                     Max fuse size

How to find out Voltage drop ?

Voltage drop example question

You have a PV array in a field and an inverter at a house 250 feet away. You are

using 10AWG stranded copper wire. There is just one string and the voltage of

the string operates at Vmp of 200V and an Imp of 5A. What is the percentage

power loss due to dc voltage drop?

Answer

Voltage drop is calculated with Ohm’s Law, which states:

                                  Vdrop = I X R

We know that current is I = 5A

We will solve for resistance by multiplying:

ohm / kFT X kFT = ohm

The distance from the PV to the inverter is 250 feet, so there will be two wires

to complete the circuit, therefore the wire length is:

                        250 feet X 2 directions = 500 feet

                      500 feet / 1000 feet per kFT = 0.5kFT

                                  Distance = 0.5kFT

From looking up the properties of 10AWG stranded copper wire, we get

1.24 ohms/kFT, so:

                              ohm / kFT X kFT = ohm

                      1.24 ohms / kFT X 0.5 kFT = 0.62 ohm

                               Resistance = 0.62 ohm

Now back to Ohm’s Law:

                                     Vdrop = I X R

                             Vdrop = 5A X 0.62 ohm

                                 Vdrop = 3.1 volts

We now know that during peak sun conditions, we are losing 3.1 volts on the

wire and if we measured the voltage at the array, it would be 3.1 volts higher

than at the inverter.

To figure out our losses as a percentage we will divide voltage drop by system

voltage and then turn that into a percentage.

          (Vdrop / system voltage) X 100% = voltage drop percentage

                       (3.1V / 200V) X 100% = 1.55% voltage drop

Since voltage multiplied by current is power, then voltage drop percentage is the

same as power loss, so we are losing 1.55% of our power in this case.

           Answer to voltage drop question: 1.55% power loss at STC

                                          table use for ohm/KFT on wire.

Article Raceway or cable covered

320 Armored Cable: AC

330 Metal-Clad Cable: MC

334 Non-metallic-Sheathed Cable: NM, NMC and NMS (romex)

338 Service Entrance Cable: SE and USE (USE-2)

340 Underground Feeder and Branch-Circuit Cable: UF

342 Intermediate Metal Conduit: IMC

344 Rigid Metal Conduit: RMC

348 Flexible Metal Conduit: FMC

350 Liquidtight Flexible Metal Conduit: LFMC

352 Rigid Polyvinyl Chloride Conduit: PVC

356 Liquidtight Flexible Nonmetallic Conduit: LFNC

358 Electrical Metallic Tubing: EMT

NEC PV ARTICLES & SECTION

690 PV systems705 Interconnections

110.14(C) Terminal temperatures

110.21(B) Hazard marking

110.26(A)(1) Working spaces

110.28 Enclosure selection

200.6 6AWG and smaller can be marked white for PV

230 Services

240 OCPD

240.4(D) Small conductor rule

250 Grounding and bonding

250.52 Electrodes

250.66 AC GEC

250.166 DC GEC

250.122 ECG (ac and dc)

300.5 Underground installations (how deep to bury conduit)

300.7 Raceways exposed to different temperatures

310 Wire sizing

310.15(B)(2)(a) Ambient temperature correction factors

310.15(B)(3)(a) Adjustment for >3 current carrying conductors in conduit

310.15(B)(3)(c) Temperature adjustment in conduit in sun on roof

310.16 Conductor ampacity in raceway cable or buried

310.17 Conductor ampacity in free air

314 Junction boxes, enclosures, outlets

320–362 Raceways (conduit) and cables

330 Metal-Clad: MC cable

334 Non-metallic sheathed cable: NM, NMC and NMS

338 Service entrance cable: SE, USE (USE-2)

342 Intermediate metal conduit: IMC

344 Rigid metal conduit: RMC

352 Rigid PVC

356 Liquid tight flexible non-metallic conduit: LFNC

358 Electrical metallic tubing: EMT

480 Batteries (also in 690.71–690.74)

690.2 Definitions

690.5 Ground-fault protection

690.7 Maximum voltage

690.8 Currents and circuit sizing

690.9 Overcurrent protection

690.10 Stand-alone systems

690.11 Arc-fault protection (dc)

690.12 Rapid shutdown

690.13–.18 Part III Disconnecting means

690.31–.35 Part IV Wiring methods

690.41–.50 Part V Grounding

690.51–.56 Part VI Marking

690.71–.74 Part VIII Batteries

705.10 Directory

705.12 Point of connection

705.12(A) Supply-sideMemorize and familiarize

705.12(D) Load side

705.12(D)(2)(1) Feeders

705.12(D)(2)(2) Feeder taps

705.12(D)(2)(3) Busbars

705.12(D)(2)(3)(a) 100% rule

705.12(D)(2)(3)(b) 120% rule

705.12(D)(2)(3)(c) Sum rule (loads + breakers ≤ busbar)

705.31 Location of supply-side connection disconnect < 10 ft

705.32 Connect to supply-side of ground-fault protection

CONDUICT TYPES

Accessible (equipment): Not behind locked doors, elevation or other means.

Readily accessible: Not requiring tools, ladders or climbing to gain access. May be behind locked doors.

Bonding: Connected for electrical continuity.

Multiwire branch circuit: Branch circuit with two or more ungrounded conductors and one neutral grounded conductor.

Control circuit: Circuit that controls performance of another circuit. (Can be used with 690.12 rapid shutdown for turning off power at combiner.)

Feeder: Conductors between primary and secondary distribution.

Raceway: Channel for holding wires, cables or busbars, such as conduit.

Separately derived system: Electrical source having no direct connection to circuit conductors other than grounding and bonding. Examples are transformers.

Service: Conductors and equipment delivering (serving) electricity from utility.

FLASH CARD MATERIAL

• Area of circle is 3.14 3 radius

• Area of cylinder is depth 3 3.14 3 radius• Length 3 width 3 height = volume
• 2.54 cm/inch
• 3.28 feet per meter

• Wind category B is urban/suburban or wooded with close obstructions (typical)

• Wind category C is scattered obstructions

• Wind category D is no obstructions and wide open
• Wind category A is no longer in use
• Roof zone 3 = corners

• Roof zone 2 = edges

• Roof zone 1 = middle (best for solar)
• PV source circuit is between PV and combiner
• PV output circuit is after parallel connections in combiner
• PV maximum current definition = Isc 3 1.25Memorize and familiarize
• PV continuous current = Isc 3 1.56
• Continuous current for other than PV source and output circuits is 125%
• 156% ampacity correction only used for PV source and output circuits
• 156% not used with conditions of use; use 125% and compare
• OCPD = overcurrent protection device = fuse or circuit breaker
• OCPD size is maximum current × 1.25 and round up
• If disconnects are in different locations, then directory or plaque required at each disconnect• PV wire used for ungrounded arrays in free air
• Round-down string sizing for cold temperature high Voc calculations
• Round-up string sizing for hot temperature low Vmp calculations
• Rapid shutdown of conductors within 10 feet outside and 5 feet inside building• Pilot hole 67–80% of lag bolt size
• 3 ft space at roof ridge for fire department
• For fine-stranded cable use fine-stranded lugs
• No different orientations within source circuits
• Time of use rates are usually more expensive on summer afternoons
• Tiered utility rates are more expensive as you use more
• Sine wave is less harmonic distortion
• Conductors in conduit outside have to be wet-rated.
• Array gets 80% STC irradiance (current) at 800 watts per square meter
• Multimodal inverter, utility shuts down, power critical loads
• A grounded conductor on an insulated lug is good for measuring voltage
• A grounded conductor is always white or gray
• PV connectors are polarized
• Bypass diode failure usually decreases voltage by one-third of a module voltage• 120/240 is called split phase or single phase
• 120V inverter has single pole ac disconnect/breaker
• No multiwire branch circuit sign is required with 120V inverter
• No more than six switches to turn off PV systems on a building in a single enclosure or group of enclosures• No disconnect on grounded conductor with exceptions for GFDI, AFCI and maintenance.• PV source and output circuits must be separated from other circuits
• PV circuits must be polarized, marked, latching and identified• Load break rated manual disconnect within six feet of combiner required
• Shading short edge of module typically kicks in all bypass diodes and bypasses module• Interactive inverters do not need clamped breaker
• Bond rails to each other
• Max dc disconnect height under normal circumstances is 6.5 feet
• For lightning use lightning protection system and surge protection
• Add acid to water so acid doesn’t splash
• No disconnect or fuse on grounded conductor
• Designated safety person cannot have other duties (distractions)
• No dc disconnect in bathroom (wet feet)
• Do not bond neutral in two places (dc disconnect and inverter GFCI)
• Person who puts on the lockout tagout removes it
• Insulation tester is megohmmeter or Megger
• If disconnects not near each other use plaque or directory
• Battery bank sign should indicate grounded conductor and max voltage
• Equalization and cold temperature battery corrections increase max voltage

x

Calculator clicks for the 120% rule to determine max inverter ac amps:

Enter:

Bus 3 1.2 = –Main = /1.25 = inverter ac current

120% rule to solve for inverter power:

(((Busbar 3 1.2)–Main) 3 0.8) 3 grid voltage = max. inverter power