Transformer Sizing for Outbuilding Feeder

Why this matters

A detached garage, workshop, barn, or guest house fed from the main service often runs hundreds of feet of underground conductor. Run that on the existing house voltage and the voltage drop pushes the outbuilding into nuisance-low conditions: starting motors stall, inverter welders trip, EV chargers cut back current. The conventional fix is a step-up transformer at the main service and a step-down at the outbuilding, which lets you push the same kW at a fraction of the current and a tenth of the voltage drop. This is the most cost-effective long-feeder solution above about 300 ft on residential, and skipping the analysis usually leaves the customer with a chronically underperforming subpanel they live with for years.

When step-up/step-down makes sense

Comparison points: bigger conductor vs. transformer pair vs. relocate service.

The breakpoint moves with load. A 100 A subpanel at 500 ft on #4 copper at 240 V drops about 9 percent; the same load at 480 V on the same conductor drops about 2.3 percent. The transformers pay for themselves on conductor savings alone above roughly 400 ft for 60 A and above.

Sizing the transformer

Step 1: load calculation per NEC 220 for the outbuilding. Treat the outbuilding as a separate dwelling unit (220.84) or as a feeder calculation using actual connected load. Common outbuilding loads:

• Lighting and receptacles per 220.42 (3 VA per sq ft general)
• Specific appliances (well pump, EV charger, welder, dust collector, mini-split, irrigation pump)
• Apply demand factors per Article 220 where applicable

Step 2: size the transformer kVA at or above the calculated demand load, with a service factor.

Most residential outbuildings land at 15 to 37.5 kVA. Step-up and step-down transformer must be sized to the same kVA. Both should be the same brand and impedance class for predictable behavior under fault.

Conductor sizing at the step-up side

Compute primary current from the transformer's primary voltage and kVA:

Primary current (single-phase) = kVA x 1000 / primary voltage

For a 25 kVA transformer at 480 V primary: 25,000 / 480 = 52 A.

Size the primary feeder per NEC 240.21(B) feeder taps and 215.2 / 215.3 (feeder ampacity at 125 percent of continuous load). For 52 A continuous: 65 A minimum, choose 70 A or 80 A breaker depending on conductor selection and termination ratings.

Voltage drop at this stage is computed at the higher voltage. Same conductor that drops 9 percent at 240 V drops about 2.3 percent at 480 V, because the current is halved and the percentage is relative to a doubled voltage.

NEC 240.21(C) governs transformer secondary conductors when applying tap rules between the step-down transformer and the outbuilding panel.

Conductor sizing at the step-down side

Short conductor run from the step-down transformer to the outbuilding panel. Sized for the secondary current at the lower voltage:

25 kVA at 240 V = 104 A. Conductor sized at 125 percent of continuous = 130 A minimum. #2 copper or 1/0 aluminum typical.

The voltage drop on this short run (typically 10 to 30 ft) is negligible. The whole point of the step-up/step-down was to put the long run on the high-voltage side.

Protection coordination

Each transformer needs primary and secondary overcurrent protection per NEC 450.3.

• Primary OCPD (at the source): per Table 450.3(B). For a single-phase, less than 600 V transformer, primary OCPD not over 125 percent of primary current rating. Practical: 25 kVA at 240 V primary is 104 A; OCPD not over 130 A; use 125 A or smaller.
• Secondary OCPD (load-side, on the secondary conductors): when primary OCPD is set above 125 percent of secondary rating, secondary OCPD is required not over 125 percent of secondary current.

Both transformers (step-up at house, step-down at outbuilding) need both protections. Coordinate with the existing service main breaker upstream.

Grounding requirements

NEC 250.30 governs system grounding for separately derived systems. Each transformer creates a new separately-derived system; each new system needs:

• System bonding jumper (XO to enclosure)
• Grounding electrode conductor to a grounding electrode at the new transformer location
• Equipment grounding bonded throughout the secondary conductor system

At an outbuilding, NEC 250.32 also applies: a grounding electrode (driven rod, plate, concrete-encased) is required at any structure other than the dwelling unit served by a feeder from the main service. Many AHJs require two rods 6 ft apart unless the single-rod resistance is documented at less than 25 ohms (rarely the case without testing).

Practical install notes

• Outdoor pad-mounted transformers (dry-type or oil-filled) need to be utility-grade in some AHJs; small dry-type transformers (Square D NEMA 3R, Acme Electric T-1-53517-3S) are commonly approved for outdoor residential install.
• Indoor mounting: vault, dedicated electrical closet, or wall-mounted dry-type. Heat dissipation matters; a 25 kVA dry-type at full load dissipates 250 to 500 W as heat.
• Conductor in conduit underground: PVC schedule 80 in most jurisdictions, with proper depth per NEC 300.5 (24 in cover for direct-burial; 18 in for PVC conduit; 6 in under 4 in concrete slab in some cases).
• A long underground run in PVC is far easier to upsize later than direct-burial. Spend the extra on conduit.
• Coordinate the outbuilding load calc with the main service capacity. A 200 A service may not have headroom for a 100 A outbuilding feeder; service upgrade may be required first.

References

• NFPA 70 (NEC) 2023, Article 215 (Feeders), Article 225 (Outside Branch Circuits and Feeders), Article 240.21 (OCPD location), Article 250.32 (Two or more buildings), Article 450 (Transformers)
• NEC 220 (Branch-Circuit, Feeder, and Service Load Calculations)
• IEEE C57.12.00: General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
• IEEE C57.96: Guide for Loading Dry-Type Distribution and Power Transformers
• Square D EZB Series Dry-Type Transformer Catalog
• Acme Electric Distribution Transformer Selection Guide