Chimney Shadow, Ridge Setback, and a DC Interconnection Cap
A designer lays out 26 modules of 400 W (39 in. × 66 in.) on a south-facing roof plane 40 ft wide by 14 ft slope length with a 5/12 pitch. The plan requires a 36 in. setback at the ridge and an 18 in. egress path along one rake. A 4 in. plumbing vent sits near the center of the plane. A masonry chimney at the west end shades part of the plane from roughly 10 a.m. to 1 p.m. on December 21. The first layout places three modules inside that winter shadow. The utility's expedited interconnection cap is 10 kWdc, and the customer wants the largest array that stays under it. The specified inverter is 8 kWac with two MPPTs, and the designer has proposed placing all modules on MPPT1 to save a home run.
1. Check the layout against the setbacks and calculate the maximum module count that actually fits.
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Width. 40 ft = 480 in. Less the 18 in. egress path leaves 462 in. 462 ÷ 39 in. = 11.8, so 11 modules per row.
Slope length. 14 ft = 168 in. Less the 36 in. ridge setback leaves 132 in. Portrait modules are 66 in. tall, so 2 rows consume exactly 132 in. - the layout has zero tolerance and should be verified with field measurements, since decking and shingle irregularities consume an inch or two.
Count. 11 × 2 = 22 positions. The plumbing vent overlaps one position, leaving 21 modules = 8.4 kWdc. That is under the 10 kWdc cap, so the interconnection limit does not bind; the 8 kWac inverter is also appropriate. The 26-module plan was never buildable - it exceeds the plane's area before any setback is applied.
Conclusion: redo the layout at 21 modules, confirm the vent keep-out with a tape measure on the roof (not from a satellite image), and confirm that the 18 in. egress path is on the required side per the local fire code, since some jurisdictions require it on both rakes or at the ridge.
2. Recommend how to handle the three modules that fall in the chimney shadow, and justify the choice.
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Three shaded modules must not be wired in series with unshaded modules on the same string. Ranked options:
- Relocate. Shift the west-end modules east so the shadowed area is empty. This is the cheapest fix and keeps the array simple, though it may cost one or two modules in the final count.
- Omit them. Drop to 19-20 modules and leave the shadow zone clear. Losing 400-800 Wdc is often cheaper over 25 years than the electronics or the service calls.
- Isolate them electrically. If those positions must be used, put them on the second MPPT with module-level power electronics (optimizers or microinverters) so the shaded units do not set the current for the whole string. Keep the main array on MPPT1.
Do not leave them in series with the main array and count on bypass diodes to "take care of it." Verify with a December shade study - a June walk-through will show no problem at all and is the classic way this error ships. Also confirm the chimney's actual height and cap, since the cap often casts a wider shadow than the flue.
3. Explain why partial shading of a few modules in a series string causes a loss larger than their own output, and how that shapes the design decision.
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Modules in series carry the same current, set by the weakest cell in the string. A shaded cell generates less current and is forced into reverse bias by the unshaded cells, dissipating power as heat - the hot-spot mechanism. Bypass diodes limit the damage by routing current around a substring once reverse voltage rises enough, but activating a diode removes that substring's voltage (typically one-third or one-half of a module) and the string current is still constrained. The loss is therefore not "3 out of 21 modules = 14%." With an unmitigated shadow it can be far larger, and repeated hot-spotting degrades or destroys the shaded module and its neighbors' junction boxes.
Design consequence: the cost comparison is not optimizer price versus 1.2 kW of modules. It is optimizer price versus (a) the annual energy loss from the shadow, (b) accelerated module degradation, and (c) a callback when the bypass diode fails. For a three-module winter shadow, relocation or omission usually wins; MLPE wins when the shadow is unavoidable and the positions are needed to hit the customer's target.