When medium voltage service entrance makes sense (and when it doesn’t)

The basic tradeoff

Low-voltage (typically 480V) service entrance is simpler — one transformer pad, one switchgear lineup, no MV cable distribution. But low voltage requires substantially larger conductor sizes to carry the same kW at lower voltage, and distribution losses accumulate over distance.

Medium-voltage (15kV, 25kV, 35kV class) service entrance adds an MV distribution layer with its own switchgear, MV cable, and unit substations at the load. The capital cost is higher per kW of capacity. But distribution efficiency is better, conductor sizes downstream of the MV/LV transformer are smaller, and adding capacity later is easier.

Where the crossover happens

Rule of thumb based on a typical Texas facility:

• Below 1,000A at 480V (about 700kW connected) — Low-voltage service is almost always more economical. Distribution distances are short, transformer sizing is straightforward.
• 1,000A-2,500A at 480V (700-1,700kW) — Crossover zone. Depends on building geometry, distribution distances, and utility coordination factors. Both are plausible.
• 2,500A-4,000A at 480V (1,700-2,800kW) — MV service starts to win on conductor cost savings and operational flexibility for many facility types.
• Above 4,000A at 480V (above 2,800kW) — MV is almost always the right answer. Trying to scale low-voltage distribution at this size hits practical limits on cable sizing, panel ampacity, and conductor space.

These are not hard thresholds. Specific facility geometry, utility primary availability, and operational factors shift the crossover.

Where MV service makes obvious sense

• Cold storage facilities > 100,000 SF. Refrigeration plant load alone justifies MV service. Plus the geometry of large refrigerated buildings makes LV distribution impractical.
• Data centers > 5MW. Almost always MV at this size. Often MV at smaller sizes too.
• Manufacturing plants with significant motor load. Process motors above 500HP often run more efficiently at MV directly, avoiding LV step-up complications.
• Petrochemical facilities. MV is standard. The complexities of LV at industrial scale don’t work for plant distribution.
• Long site geometry. If load is distributed across a long building or multiple buildings, MV reduces conductor cost substantially.
• Future expansion planned. MV service is easier to upsize than LV. If the facility is likely to grow, MV future-proofs the design.

Where LV makes more sense

• Smaller commercial buildings (under 50,000 SF without major process load). The MV infrastructure cost premium isn’t justified.
• Office buildings without high-density tenant load. Even at substantial size, an office building’s actual load is often modest.
• Single-story commercial with compact footprint. Short distribution distances reduce the LV penalty.
• Buildings where MV utility primary isn’t readily available. If the utility would require substantial primary line extension to serve MV, the economics shift toward LV.

Hidden costs in the MV decision

MV service entrance adds capital cost categories that LV doesn’t have:

• MV-qualified maintenance personnel — Either in-house staff with MV qualifications or service contracts for MV equipment. NETA testing every 3-5 years on MV gear adds operational cost.
• MV protective relay coordination — Coordination studies, relay setting management, periodic verification. More complex than LV breaker coordination.
• Utility coordination differences — MV service review process is longer at most Texas utilities. Add 3-6 months to project schedules for utility-side engineering.
• NETA acceptance and maintenance testing — Required at acceptance and recommended every 3-5 years. Specialized testing crews.

The decision framework

For a facility owner or developer evaluating MV vs LV during early design:

1. Calculate connected load and projected load at year 5.
2. Identify utility primary voltage available at the site (some sites only have LV available).
3. Map the distribution geometry — where does the load actually live within the building or across the site?
4. Estimate the capital cost premium of MV vs LV for this specific project.
5. Estimate the operational cost differential over the building’s life (maintenance, energy losses, expansion capacity).
6. Discuss with the electrical contractor and utility account representative before making the decision.

This decision belongs in early design development, not during construction documents. Once CDs are issued, changing the service entrance type involves substantial rework.