Small & Distributed Wind: Siting, Towers & NEC 694

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

You already know how sunlight becomes electricity. Wind does the same job with a spinning rotor instead of a silicon cell — and it fails in a very different way. A solar array almost always produces close to what the software predicted. A small wind turbine on a short tower in the wrong spot can produce a fraction of what the brochure promised. This lesson is about why, and what the pros do differently.

Educational overview only — not electrical, mechanical, structural, or drilling design, a safety program, or a substitute for work under licensed electricians, mechanical contractors, well drillers, and engineers. Follow the NEC as adopted by your authority having jurisdiction (AHJ), the building, mechanical, plumbing, and fire codes, OSHA, state water and well regulations, the manufacturers' instructions, and your utility's rules. Code editions change and jurisdictions adopt different ones — always check the edition your AHJ enforces.

How a Wind Turbine Makes Power

Moving air pushes on the blades, which are shaped like airfoils so lift turns the rotor. The rotor spins a generator — on most small turbines a permanent-magnet alternator connected directly to the rotor, with no gearbox. The output is "wild" AC whose voltage and frequency change with wind speed, so it is rectified and then fed to a grid-tied inverter or a battery charge controller.

Two kinds of turbines show up in the field:

• Horizontal-axis (HAWT) — the familiar propeller shape, with a tail vane (small units) or a yaw drive pointing it into the wind. This is the design that delivers most of the energy produced worldwide.
• Vertical-axis (VAWT) — eggbeater or helical shapes. They accept wind from any direction and look interesting on buildings, but in general they produce less energy for their size.

Every turbine needs a way to protect itself in high winds: furling (the rotor tilts or swings out of the wind), blade pitch control, or electrical/mechanical braking. A turbine that cannot shed load in a storm can destroy itself.

Why Wind Speed Is Everything

The power in the wind rises with roughly the cube of wind speed. Double the wind speed and the available power goes up about eight times; increase it 10 percent and you get roughly a third more. That is why a few miles per hour of difference in average wind speed decides whether a project makes sense.

Wind near the ground is slow and turbulent because trees, buildings, and terrain drag on it. A widely used rule of thumb: the bottom of the rotor should be at least about 30 feet above anything within roughly 300 to 500 feet of the tower. In practice that often means towers of 80 to 120 feet or more for residential and farm turbines — much taller than most homeowners expect.

Towers and Foundations

• Tilt-up towers (guyed tubes) — lowered with a winch or gin pole for maintenance; need room for guy anchors in several directions.
• Guyed lattice towers — economical and tall, with a large guy footprint.
• Freestanding lattice or monopole towers — small footprint, but large engineered foundations and higher cost.

The tower and foundation are structural systems. They need engineering for the turbine's thrust loads and the local wind and soil conditions, and most jurisdictions want stamped drawings. Height also triggers zoning reviews (setbacks, fall zones, noise) and, for tall structures or sites near airports, FAA notification under 14 CFR Part 77.

NEC Article 694

NEC Article 694 covers wind electric systems — the turbine output circuits, the disconnecting means, overcurrent protection, grounding, and the connection to an inverter, battery, or the grid. Interconnection with the utility is also governed by Article 705 and the utility's rules, and grid-tied inverters are typically listed to UL 1741 and meet IEEE 1547. Many turbines have no true "off" while the wind blows — the Code and the manufacturer require a way to stop or brake the rotor and to disconnect the electrical output safely.

Why Small Wind Often Underperforms

• Towers too short — the single biggest cause; slow, turbulent air near trees and rooftops.
• Rooftop and building-mounted turbines — turbulence, vibration, and noise; usually poor production.
• Optimistic wind estimates — regional maps and the brochure's "rated power" at a high wind speed rarely match a real backyard.
• Uncertified equipment — look for turbines certified by an independent program to recognized small-wind standards (based on the IEC 61400 family), which report realistic power curves and annual energy estimates.
• Maintenance neglect — turbines are moving machines; bearings, blades, and brakes need inspection.

Going Deeper (Intermediate)

A proper small-wind assessment starts with data: wind-resource maps, nearby airport records, and — for larger projects — an anemometer on a met tower for a year. The designer models annual energy using the turbine's certified power curve and the site's wind-speed distribution, then applies losses for turbulence, availability, and wire runs. Compare the result against a same-budget solar array; on many residential sites, solar wins. Small wind shines on open, consistently windy rural land, at farms, and in off-grid hybrid systems where wind fills in during winter and at night when solar is weak.

Advanced / Pro-Level

Pros size the wire run from tower to inverter carefully — towers sit far from buildings, so voltage drop is real. They specify a diversion load or dump load for battery systems so the turbine always has somewhere to send power, coordinate grounding and lightning protection for a tall conductive structure, and plan tower climbing and rescue for maintenance (or choose a tilt-up so nobody climbs). They also read the local ordinance before the sale: height limits, setbacks equal to tower height, and noise limits kill more projects than engineering does.

Practice Challenge

A homeowner wants a 10 kW turbine on a 40-foot tower in a backyard with 60-foot trees on two sides. What do you tell them? (Answer: the rotor would sit in slow, turbulent air below the treeline and would likely produce a small fraction of its rating. Either the tower must clear the trees by about 30 feet — roughly 90 feet or more, subject to zoning — or the client is better served by solar. Show them the certified annual energy estimate, not the rated kW.)

In Practice

Experienced small-wind installers spend more time on siting conversations than on installation. They walk away from bad sites, which protects their reputation — a dead turbine on a hill advertises against you for years.

Common Mistakes to Avoid

• Selling a turbine by its rated kW instead of its certified annual energy
• Putting turbines on roofs or short towers among trees
• Skipping the zoning, setback, and FAA questions until after the deposit
• Forgetting a reliable stop/brake method and safe disconnect for maintenance

Takeaway: Small wind lives or dies on tower height and real wind data — get the turbine well above the turbulence, follow NEC 694, and be honest when solar is the better answer.