Keyboard shortcuts

Press ← or → to navigate between chapters

Press S or / to search in the book

Press ? to show this help

Press Esc to hide this help

20. Electrical Theory for PV

Learning objectives

  • Apply series/parallel reasoning to real array wiring and its consequences.
  • Trace power flow through a complete system.
  • Distinguish the AC service types you’ll interconnect to.

20.1 Series and parallel, applied

From Chapter 1.7 and Chapter 4.2: series adds voltage, parallel adds current. The design consequences you now understand (Part IV):

  • More modules in series → higher string voltage → fewer, smaller conductors, but the cold-Voc ceiling (Ch 15) limits how far you can go.
  • More strings in parallel → higher current → larger conductors and the 156% conductor rule (Ch 16) bites harder. Good design balances these against the inverter’s window and the BOS cost.
BOS (Balance of System): all components in a PV installation other than the modules themselves, including racking, wiring, disconnects, inverter, and overcurrent protection. BOS cost is a major driver of installed system price.

20.2 Power flow through the system

Trace the energy: DC array (variable V and I with sun/temperature) → inverter (MPPT holds the array at its power knee; converts to AC) → main panel (feeds loads; back-feeds excess) → grid or battery. Every transition is a place the NEC requires protection, disconnection, and marking, which is why the code’s structure mirrors this flow.

MPPT (Maximum Power Point Tracking): a control algorithm the inverter runs continuously to find and hold the operating point where the array delivers its maximum instantaneous power. The array's I-V curve shifts with irradiance and temperature, so MPPT adjusts the inverter's input voltage in real time.

20.3 AC service types

You interconnect to one of a few service configurations:

  • Single-phase 120/240 V split-phase: the standard North American residential service; two 120 V “legs” plus neutral.
  • Three-phase 120/208 V (wye): common in light commercial/multifamily.
  • Three-phase 277/480 V: larger commercial/industrial; higher voltage means lower current for the same power (smaller conductors), which is why big systems favor it. Matching the inverter’s output configuration to the service (voltage, phases) is a basic but essential interconnection check (Chapter 16.4).

20.4 Power flow and where the code lives

  [PV ARRAY]──DC──►[DC disconnect]──►[INVERTER]──AC──►[AC disconnect]──►[MAIN PANEL]──►[GRID]
   690.7/8/9         690.13            690 / UL 1741      705              705.12         utility
   string V/I        de-energize       MPPT + DC→AC      interconnect     120% busbar    PTO
        │                                                                      │
        └── "DC side": always live in light, arcs don't self-extinguish        └── "AC side": LOTO-able

Every box is a place the NEC requires protection, a disconnect, and marking, which is why the code’s structure mirrors the energy path. The vertical split between “DC side” (always live, Ch 28) and “AC side” (lockout-able) organizes the safety logic too.

Chapter 20 summary

Series/parallel choices set the voltage/current tradeoffs that Part IV sizes. Power flows DC array → inverter → panel → grid/battery, and the NEC protects each transition. Know your service type: 120/240 split-phase residential, 208 V or 480 V three-phase commercial, and match the inverter to it.

  • Series connection: modules wired positive-to-negative; voltages add, current stays constant.
  • Parallel connection: strings wired positive-to-positive; currents add, voltage stays constant.
  • BOS (Balance of System): all system components other than modules: racking, wiring, disconnects, inverter, overcurrent protection.
  • MPPT (Maximum Power Point Tracking): inverter algorithm that continuously adjusts input voltage to harvest maximum array power.
  • Split-phase (120/240 V): standard North American residential service with two 120 V hot legs and a neutral; 240 V appears between the legs.
  • Three-phase wye (120/208 V): common light-commercial service; three 120 V phase conductors produce 208 V line-to-line.
  • 480 V three-phase: large commercial/industrial service; higher voltage means lower current for the same power, enabling smaller conductors.

Full definitions: Appendix A (glossary).

Practice Problems: Chapter 20

  1. A commercial building offers both 208 V and 480 V three-phase service. For the same power, which gives lower current and therefore smaller conductors?
  2. Why does the NEC place protection and disconnect requirements at each transition in the power-flow chain?
  3. A residential service is “120/240 split-phase.” How many hot legs does it have, and what’s the voltage between them?
  4. Trace, in order, the major equipment a watt of solar energy passes through from array to grid.

Solutions: Chapter 20

  1. 480 V: higher voltage means lower current for the same power (P = V × I), so conductors can be smaller; this is why large systems favor 480 V.
  2. Each transition (DC, conversion, AC, interconnection) carries distinct hazards and must be independently protected, de-energizable, and labeled for safety and serviceability.
  3. Two hot legs (each 120 V to neutral); 240 V between the two legs.
  4. PV array → DC disconnect → inverter → AC disconnect → main panel → grid.