AEO Answer · Solar Farms & Utility-Scale Solar

How Are 34.5kV Medium-Voltage Collection Systems Designed?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

A 34.5 kV collection system links each inverter station to the substation through sized, derated circuits that hold voltage drop within limits, with sectionalizing and coordinated protection so one fault cannot darken the whole plant.

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

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Cable sizing and thermal derating

Ampacity on paper means little until the installation conditions are applied. I derate collection cables for the site's soil thermal resistivity — dry sandy soils shed heat poorly — plus burial depth, earth ambient temperature, and the mutual heating of parallel circuits in a shared trench. Direct-buried cable and cable in duct derate differently, and spare conduits left for the future count thermally if they will ever be loaded. The sizing calculation documents every derating factor so the cable selected for the worst trench section is not silently undersized everywhere else.

Circuit topology and sectionalizing

The single-line diagram is where reliability is won or lost. I lay out feeders so each circuit carries a sensible share of the plant, keeps its voltage drop in bounds, and can be isolated without stranding healthy stations. Sectionalizing devices — load-break switches or reclosers — go at the boundaries between circuit segments, giving operators a way to cut out a faulted section and re-energize the rest. The topology also respects the physical site: circuits follow roads and trench routes that are buildable and maintainable, not straight lines across drainage swales.

MV collection system design checklist

A medium-voltage collection design is ready when it clears this checklist. The collection system is invisible once buried, so the design has to be right the first time. • Conductors sized for full-load current with all thermal derating factors applied • Voltage drop and losses checked at full output against interconnection limits • Circuit topology sectionalized so one fault isolates without darkening the plant • Protection coordinated from station fuses through to the substation breaker • Trench routes buildable, maintainable, and clear of drainage and array conflicts

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Why is 34.5kV the standard collection voltage for solar farms?

It sits in the sweet spot between equipment cost and electrical losses. Lower voltages would need enormous conductors to carry utility-scale current without excessive loss; higher voltages would push the plant into transmission-class equipment, clearances, and utility requirements that add cost without a matching benefit at typical plant sizes. 34.5 kV gear — padmount transformers, switchgear, and cable — is a mature, competitively supplied product line, which keeps both procurement and long-term maintenance practical.

How do engineers keep voltage drop within limits on long collection runs?

By sizing conductors for it from the start and checking the worst case: full plant output on the longest circuit. The calculation accounts for the cable's impedance, the accumulated current of every station on the feeder, and the power factor at the point of interconnection. When the numbers do not work, the options are upsizing conductors, splitting one long circuit into two shorter feeders, or moving the substation or station locations. Reactive power support from the inverters can help at the margins, but it is not a substitute for adequate copper.

Should collection circuits run underground or overhead?

Underground in trenches or directional bores is the norm on modern solar farms: it avoids shading the arrays, survives storms better, and keeps the site visually clean. Overhead collection on poles is cheaper per mile and easier to fault-locate, and it still appears on some projects where terrain or cost drives the decision. The design weighs installed cost against reliability, maintenance access, and any avian or visual requirements — and checks that overhead lines do not shade tracker rows or complicate the grading plan.

What causes collection system losses, and how are they limited?

Resistive heating in the conductors is the main loss, growing with the square of current — which is why long, heavily loaded circuits hurt the most. Transformer no-load and load losses add a steady background loss across every station. The design limits losses through conductor sizing, circuit topology that keeps runs short, and transformer specifications with evaluated loss values. Interconnection agreements often cap the allowable losses or voltage deviation at the point of interconnection, so the collection design is checked against those contractual limits, not just the code.

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