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NABCEP PV: System Design

NABCEP PV · Work through your exam, one topic at a time.

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System Design

Core design chain

Design starts with the customer's goal (bill offset, backup, capacity) and ends with drawings an AHJ will approve. The chain is: annual load and production target -> array size and orientation -> inverter selection and DC/AC ratio -> string sizing -> conductor and overcurrent sizing -> voltage drop -> interconnection method -> rapid shutdown and labeling.

String sizing is the highest-value calculation on the exam. Use the coldest expected ambient temperature (often the 2% ASHRAE extreme minimum, not the record low) to check maximum Voc against the inverter's absolute maximum input voltage, and the hottest expected cell temperature (ambient plus a 25-30 C rise for roof-mounted arrays) to check that Vmp stays above the inverter's MPPT window. Use manufacturer temperature coefficients when published; NEC Table 690.7 is a fallback.

DC/AC ratio of about 1.1-1.3 is normal; modest clipping is economically acceptable in most climates, heavy clipping is not. Keep DC voltage drop under about 2% and total under 3% where practical.

Traps

  • Sizing strings with Voc at STC only, then exceeding the inverter max voltage on a cold morning.
  • Forgetting that module Isc and conductor ampacity use 1.25 factors per 690.8.
  • Mixing modules of different current in series, or strings of different length on one MPPT.
  • Ignoring that bipolar or multiple orientation sub-arrays need separate string calculations.
  • Assuming all-black or bifacial modules behave identically to the datasheet used.

Worked example

Module Voc = 41.2 V, coefficient -0.29%/C, design low temperature -12 C. Delta T = 37 C, so Voc(cold) = 41.2 x (1 + 0.0029 x 37) = 45.6 V. With a 600 V inverter limit: 600/45.6 = 13.1, so 13 modules maximum. Check the warm end: Vmp 34.0 V at -0.35%/C and 65 C cell gives 34.0 x (1 - 0.0035 x 40) = 29.2 V; 13 x 29.2 = 380 V, comfortably inside the MPPT window.

Rule of thumb

Min string length = inverter minimum MPPT voltage divided by hot-cell Vmp; max string length = max input voltage divided by cold Voc, always rounding down.