Avoiding Costly Mistakes in Commercial Solar Design

In this webinar with BayWa r.e. and Wyssling Consulting’s engineering team, they discuss what commercial site surveys need to capture, how the ASCE 7-22 snow load update affects structural reviews, common electrical and interconnection issues on C&I projects, and design choices that reduce project cost. Wyssling’s team works across commercial and residential design and engineering and is licensed in all 50 states.

Structural surveys: get to the framing

Some commercial surveys come back with nothing but ground-level photos. That isn’t enough for a structural review, and it means the engineer has to send you back out. The fix is to bring a lift or confirm there’s a roof hatch ladder, so you can get up to the framing.

For joists or purlins, capture four things: size, thickness, spacing, and the span between supports. Those numbers tell the engineer how much roof load each member carries and whether it can take more. Use calipers for purlin thickness, because a tape measure can’t tell 14 gauge from 16 gauge, and without a thickness the engineer has to assume conservatively. Depth and width are fine with a tape.

When the columns are 50 feet apart and your tape won’t reach, photograph the area and sketch the measurement right on the picture. On a standing seam roof, shoot the seam profile. Without it, picking the right clamp is close to impossible.

If the customer still has the original building plans, ask for them. Plans carry the structural information and show what the building was designed to carry, which can save you much of the measuring.

Pre-engineered metal buildings need extra attention. Light-gauge Z and C purlins are usually designed very tight, with roughly 2 lb per square foot of collateral load allowed for insulation and hanging equipment, so there’s little reserve capacity. Newer buildings sometimes include an allowance for solar, around 10 lb per square foot, and California may require one. Older buildings usually don’t. Find out what the building was designed for before you commit to a ballasted system.

If the roof’s capacity is in doubt, Wyssling can send a PE out to inspect it. Learning late that a roof can’t support the system is expensive, which is why some customers want an engineer on site first. And if you have questions before a survey, the engineers will walk through what they need up front, then hop on a call (or FaceTime, if a surveyor sees something unfamiliar) while you’re out in the field.

ASCE 7-22: higher snow loads, smaller jump than they look

Ground snow loads pulled from the ASCE 7 Hazard Tool can look alarming under the 2022 edition. One example location shows a ground snow load of 46 and an ASD snow load of 32, because the revised equations allow a 30% reduction. After that reduction, expect probably a 10 to 20% increase over ASCE 7-16.

Two details help. Some of the older data came from 1980s research, and the new values are based on newer research. And locations that used to return “case study” results, such as higher elevations in California and New York, now appear to return values directly, so you may not have to track down a local snow map from the AHJ.

Tornado loads apply only to risk category III and IV buildings (schools, churches, hospitals) and mostly in the Midwest. On a typical 300 kW roof they most likely won’t govern. Only a handful of states have adopted ASCE 7-22 so far, and manufacturers’ ballast tools have mostly caught up.

Electrical surveys: photograph what the engineer needs

Everything in electrical design starts with clearances, and NEC 110.26 requires working space around the equipment. A quick check: picture a residential refrigerator, about 30 inches wide, 3 feet deep, and 6.5 feet tall. If it can’t stand in front of the equipment without tipping, find another location.

Your photos are the engineer’s eyes, so think like an engineer and ask what you’re looking at and whether it matters. More photos are better, as long as they’re clear.

• Check your photos before you leave the site. Blurry breaker labels can’t be read.
• Capture breaker sizes, panel schedules, and door placards. When equipment name plates aren’t accessible, these let the engineer complete a load calculation.
• Note where the dedicated transformer sits and how close it is to the panel board. Physical size and proximity matter.
• Open as many panels as you can safely, with proper PPE. Some switchgear has bus bar openings in unexpected locations that allow interconnection.

On interconnection, the meter section with the current transformers is locked by the utility, so the section next to the meter isn’t automatically available to you. When a supply-side tap isn’t an option, the 120% rule isn’t your only path. With a fault current study and load calculations under engineering supervision, you can connect up to the rating of the service or switchboard, whichever is lower. In Southern California Edison territory, utility approval for a major supply-side project can be required before the AHJ will issue a permit, so that route adds time.

Two other items are showing up more often. Arc flash calculations and labeling under NFPA 70E are requested more frequently, and glare studies are required in many jurisdictions, especially near airports. Some require them on everything.

For EV chargers without load management, code requires AHJs to size the load at the charger’s full output current, which can mean an expensive panel or switchboard change. Load management changes the math. Four chargers on two two-port stanchions can run from a 100 amp circuit because the chargers throttle back to stay within it. Getting AHJ approval takes code support and manufacturer documentation.

Where commercial design saves money

Installers moving from residential into commercial often run into details they don’t know to look for. These are the ones that came up.

The neutral can often be downsized, or eliminated entirely, with commercial inverters. It also often doesn’t count as a current-carrying conductor for derating, though sometimes it does, so check. Smaller wire and conduit follow.

On long runs, aluminum generally costs less than copper even after you upsize for voltage drop and add a parallel run. Wyssling sometimes puts both options on the plans so the installer can buy to current pricing. Conduit goes the other way. A 39.9% fill is acceptable on paper, but going up one size can save hours of wire pulling on a long run.

UL 3741 can save you from buying rapid shutdown devices, which adds up on larger systems. It requires that the inverter and racking are listed together, the inverter is close enough to the racking, and the racking is unbroken. On many inverters, if any module needs rapid shutdown, all of them do, so partial arrays need care. Cost it both ways: splitting one 100k inverter into two 50k units can come out ahead, while replacing a 200k with five 40k units may cost more than buying the rapid shutdown devices.

Voltage drop gives you one more lever. The DC side tolerates more drop than the AC side, and excess AC drop can prevent exporting. Placing inverters farther from the array and taking the drop on DC can save on equipment and conductors.

Wyssling’s engineers say the savings they find can sometimes exceed what the engineering itself costs.

Energy storage: define the purpose first

Settle what the battery is for before you size it. Arbitrage, peak shaving, and frequency control each affect how the storage is set up and how big it needs to be.

On zero-export sites, power the facility can’t use is lost. Storage can capture it, which can improve the customer’s savings even after the battery cost. And on a project with a high DC-to-AC ratio, an AC-coupled battery doesn’t prevent inverter clipping. Wyssling redesigned one customer’s system as DC-coupled so the excess generation went into the batteries instead.

Working with BayWa r.e. on C&I projects

Wyssling is one of BayWa r.e.’s design and engineering referral partners, and BayWa r.e. can connect customers with their team. BayWa r.e.’s commercial services team and service desk can also review project design criteria and evaluate product alternatives, and each customer’s dedicated account manager provides updates on product availability, pricing, and market trends. 

Utilize BayWa r.e. and Wyssling’s commercial services or just talk through an upcoming C&I project with experts: