Ultimate Guide to Distribution Transformers: Key Features, Applications, and Benefits

Updated: August 21, 2025

TL;DR

Pick the right kVA and voltage, follow NEC 2023 Article 450 plus the product listing, and plan ventilation, working space, and protection before you pour a pad or hang a unit. Treat SDS grounding per 250.30 and size bonding jumpers per 250.102(C). Size primary OCPD from 450.3 and apply 240.21(C) rules to secondary conductors. Verify DOE efficiency, temperature rise, and impedance to control heat, losses, and noise. Commission with tap checks, polarity and insulation tests, and load measurements, then document results for the AHJ and the owner.

Why this matters on the job

On Long Island projects, a wrong transformer choice can snowball into voltage sag, nuisance trips, and change orders you do not have time for. The right unit keeps lights steady, HVAC happy, and machinery within nameplate limits. It also helps you pass inspection without rework. If you need a quick starting point, browse the Electrical transformers category and match kVA, voltage, and enclosure to the site conditions.

Fundamentals

What is a distribution transformer? It is a step-down transformer that takes medium or higher voltage from the utility or a building service and delivers a lower, usable voltage to panels and equipment. Common secondary voltages include 120/240 V single phase, 208Y/120 V, 480Y/277 V, and 240/120 V delta systems.

Core types and cooling

• Liquid-filled units use mineral oil or approved alternatives for cooling. They offer high kVA in compact footprints and often serve outdoor pad-mount applications.
• Dry-type units use air for cooling. They avoid oil handling and are common indoors in electrical rooms, schools, and retail. See current dry-type distribution transformers.

Nameplate basics

• kVA rating is apparent power capacity. Size to continuous load and consider future growth and motor inrush.
• Voltage class identifies primary and secondary voltage. Watch for delta or wye connections and whether the system will be separately derived.
• Impedance (%) affects short-circuit current. Lower impedance means higher fault current; coordinate with upstream OCPD.
• Temperature rise (for example 150 C) affects efficiency and enclosure heat. Lower rise generally means higher efficiency and longer life.

Where you will use them

• Residential developments for 120/240 V services and site lighting.
• Commercial fit-outs feeding 208Y/120 V panels for lighting and receptacles.
• Light industrial where 480 V motors and process loads need stable voltage.
• Renewables and EV support stepping utility distribution to building systems or EVSE panels.

Long Island notes Work with the local AHJ and utility service rules before you pick pad-mounted gear locations, clearances, and grounding details. Township amendments and utility standards can be stricter than the base code. We include an AHJ checklist and call points later in this guide.

Code and compliance (NEC 2023 quick map)

Use the NEC, then the listing, then the AHJ. Start with Article 450 for transformers. Follow the product’s listing and installation instructions per 110.3(B). Keep working space clear per 110.26. If your transformer creates a separately derived system (SDS), apply 250.30 for grounding and bonding. Size bonding jumpers using 250.102(C). Ventilation and location rules appear in 450.9. Ground the enclosure per 450.10. Provide a disconnecting means as required by 450.14. Coordinate overcurrent protection using 450.3, and remember that secondary conductors must meet 240.21(C). For medium-voltage primaries, use the applicable 450.3(A) tables. Local amendments and utility standards on Long Island may add spacing, enclosure, and pad requirements. Always confirm with the AHJ before you pour a pad or pull conductors.

Selection steps

Use this field checklist to land on the right unit the first time.

1. Define the system. Note primary and secondary voltage, phase, and connection. Decide whether the secondary will be an SDS that needs a system bonding jumper and grounding electrode connection.
2. Calculate kVA. Size to the maximum continuous load, then add a margin for growth and motor inrush. For multi-tenant spaces, consider diversity but avoid undersizing.
3. Choose liquid vs dry-type. Outdoors or tight kVA in small footprints often favors liquid-filled pad-mounts. Indoors, schools, retail, and renovation work tend to favor dry-type. If the job calls for dry-type, you can shop dry-type distribution transformers.
4. Pick enclosure and location. Match NEMA rating to the environment. Check ventilation needs and keep 110.26 working space clear. In mechanical rooms, confirm 450.9 ventilation and heat rise implications.
5. Verify efficiency and temperature rise. Use the current DOE efficiency class for distribution transformers and pick a temperature rise that fits the room heat load. Lower temperature rise generally means lower losses and quieter operation.
6. Plan protection and coordination. Start primary OCPD sizing from transformer full-load amps per 450.3. Coordinate with upstream devices and available fault current. Remember 240.21(C) rules for secondary conductors. For fuse solutions on primary or secondary, see UL-Class fuses.
7. Grounding and bonding. If it is an SDS, install the system bonding jumper, grounding electrode conductor, and bonding jumpers per 250.30 and 250.102(C). If not an SDS, bond the enclosure and follow 250.4(A)(5) principles.
8. Accessories and options. Consider taps, sound attenuation, K-factor for nonlinear loads, surge protection, and provisions for monitoring or temperature switches.

Sizing and configuration examples (shown math)

Example 1: 75 kVA, 480 V to 208Y/120 V, three phase

Given: 75 kVA transformer. Primary 480 V, secondary 208Y/120 V. Three phase.

• Primary full-load current (3φ): I = kVA × 1000 ÷ (√3 × V). Using √3 = 1.732, denominator = 1.732 × 480 = 831.4. Current = 75,000 ÷ 831.4 = 90.2 A.
• Secondary full-load current at 208 V (3φ): denominator = 1.732 × 208 = 360.3. Current = 75,000 ÷ 360.3 = 208.2 A.
• Primary OCPD starting point: begin at 125% of primary FLA where permitted by 450.3. 1.25 × 90.2 A = 112.8 A. Round only to the next standard size after verifying conductor ampacity and coordination.
• Secondary conductors: apply 240.21(C) for length, routing, and first OCPD location. Coordinate with the secondary panel main rating.
• Grounding: a wye secondary that is not solidly connected to another source is typically an SDS. Apply 250.30 for the system bonding jumper and grounding electrode conductor, and size bonding jumpers per 250.102(C).

Example 2: 25 kVA, 240 x 480 V to 120/240 V, single phase

Given: 25 kVA transformer. Secondary 120/240 V, single phase.

• Secondary full-load current at 240 V: I = 25,000 W ÷ 240 V = 104.2 A.
• Secondary full-load current if the load is entirely at 120 V: I = 25,000 W ÷ 120 V = 208.3 A. Split-phase panels balance 120 V loads across legs to limit neutral current.
• Primary full-load current at 480 V: I = 25,000 W ÷ 480 V = 52.1 A. At 240 V primary: I = 25,000 W ÷ 240 V = 104.2 A.
• OCPD and conductors: start from 450.3 and applicable ampacity tables. Round to standard sizes only after confirming termination temperature ratings and coordination with available fault current.
• Neutral and grounding: if the 120/240 V secondary is an SDS, place the system bonding jumper at the transformer or the first disconnect as allowed by 250.30, and bond enclosures per 450.10.

Tip: Transformer impedance influences available fault current on the secondary. Lower impedance increases prospective short-circuit current. Verify equipment short-circuit ratings and coordinate protective devices accordingly.

Installation and wiring notes

• Location and working space. Keep clearances per 110.26 and the nameplate. Do not block ventilation openings. For outdoor pad mounts, plan bollards and drainage, and use a listed pad sized for the footprint. If you need pads, see typical transformer pad options.
• Mounting and handling. Use lifting lugs and rigging points provided by the manufacturer. Level the base so oil or air can circulate as designed. Shim only where the instructions permit.
• Primary and secondary terminations. Land conductors on the correct lugs, torque to the listing, and record values. Use listed mechanical lugs and bonding hardware. Stock common grounding and bonding cable lugs to save trips.
• Taps and voltage adjustment. Many units include ±2.5% or ±5% taps. Set taps with power off. Match the tap to the measured primary so the secondary lands near nameplate voltage under load.
• Grounding and bonding. If the secondary is a separately derived system, install the system bonding jumper and grounding electrode conductor per 250.30 and size bonding jumpers per 250.102(C). Otherwise, bond the enclosure and follow 250.4(A)(5).
• Overcurrent protection and secondary conductors. Choose primary OCPD using 450.3. Coordinate with upstream devices and available fault current. Apply 240.21(C) rules to secondary conductors if the first OCPD is not part of the transformer assembly.
• Harmonics and K-factor. Nonlinear loads like VFDs and servers can overheat a standard transformer. Use a K-rated unit when THD is high. Place sensitive electronics downstream of surge protection.
• Surge protection. Add a listed SPD at the service and at sensitive distribution points to improve uptime and protect insulation. Shop typical AC surge protective devices.
• Noise control. Isolate mounts from lightweight walls. Keep transformers out of quiet offices and classrooms when possible. Lower temperature rise usually tracks with quieter operation.

Testing, commissioning, and documentation

• Pre-energization checks. Inspect enclosure, verify tap position, tighten terminations to recorded torque, confirm grounding and bonding, and check that ventilation paths are clear.
• Insulation and ratio tests. Perform an insulation resistance test on windings to ground per the manufacturer. Verify ratio or phase rotation where applicable. Record ambient temperature for context.
• Energize and measure. With no load, verify primary and secondary voltage at the lugs. Under load, confirm voltage drop stays within equipment tolerance. Measure neutral-to-ground at the SDS to confirm the correct bonding point.
• Thermal scan. After an hour at typical load, perform an IR scan of terminations and bus. Investigate any hot spots that sit well above similar phases.
• Labeling. Mark the disconnecting means per 450.14 and identify the system as SDS where applicable. Label arc-flash boundaries per the facility program.
• Turnover package. Include nameplate photo, wiring diagram, tap setting, torque log, test results, OCPD size, conductor sizes and lengths, and as-built drawings. This speeds AHJ review and future service calls.

Troubleshooting

• Low secondary voltage. Check tap setting, conductor length and size, and upstream voltage. Measure under load. If only one phase sags, look for a loose lug or an unbalanced multiwire branch circuit.
• Excessive heating. Verify ventilation per 450.9, load level vs kVA, and harmonic content. Look for blocked airflow or dust. Consider a lower temperature-rise unit for hot rooms.
• Nuisance tripping. Recheck OCPD sizing per 450.3 and coordination with motor inrush. Confirm impedance and available fault current assumptions.
• Hum or vibration complaints. Confirm mounting method, add isolation pads, and relocate if the space is noise sensitive. Verify core bolts are tight per the listing.
• Ground fault indications. Inspect for incorrect SDS bonding point or parallel neutral–ground paths. Verify 250.30 implementation and remove improper bonds downstream.
• Equipment damage after storms. Check for surge events and failed MOVs. Install or upgrade SPDs on the service and feeders. See AC surge protective devices for replacements.

Common mistakes

• Skipping the efficiency check. Ignoring DOE efficiency classes raises operating cost and heat. Verify the nameplate before you buy.
• Wrong tap setting. Leaving taps at default when the primary is high or low produces chronic under- or overvoltage on the secondary.
• Undersized secondary conductors. The 240.21(C) rules for length and protection are easy to miss in a rush. Confirm the first OCPD location.
• Missing SDS details. Forgetting the system bonding jumper or mixing neutral and equipment grounds after the bonding point creates parallel paths and nuisance trips.
• Ventilation blind spots. Installing in a tight room without 450.9 ventilation consideration shortens life and invites thermal alarms.
• Noise surprises. Mounting next to offices or classrooms without isolation pads leads to complaints and rework.

Parts to stock

• Dry-type distribution transformers for indoor rooms and retrofits. Shop dry-type units.
• Grounding and bonding cable lugs for copper or aluminum terminations. See grounding lugs.
• AC surge protective devices for service and feeder protection. Browse SPDs.
• Transformer pads and hardware for outdoor pad-mounts. Check pad options.
• Labeling and identification for disconnects and SDS markings.

Shop at Revco for stocked transformers and accessories that match Long Island job requirements. If you do not see a spec in stock, ask for a quote and lead time.

When to call the AHJ or an engineer

• Before you pour pads or set gear outdoors. Confirm clearances, bollards, and setbacks. Utilities on Long Island often apply stricter placement and access rules.
• Medium voltage primaries. Get engineering review for 5 kV, 15 kV, or higher, especially when fault current and protection studies are needed.
• Transformer vaults or special rooms. Where fire-resistance, ventilation, or drainage is required by Article 450, involve the design team early.
• Separately derived systems. When in doubt about the bonding point or electrodes, request an AHJ interpretation referencing 250.30 and 250.102(C).
• Harmonic-heavy loads. For data rooms, VFD clusters, or EVSE with high THD, have an engineer size K-factor and ventilation.

Safety disclaimer

Follow the 2023 NEC, the product listing, and the manufacturer’s instructions. Local amendments and utility standards on Long Island may be more restrictive. The AHJ has final authority. Do not work on energized equipment. Use proper PPE and lockout procedures.

FAQ

• Do I have to bond the neutral on a wye secondary? Only if it is the designated bonding point of an SDS. Otherwise, keep neutral isolated from equipment grounds.
• How many taps should I use? Use the minimum tap change that lands the secondary near nameplate voltage under typical load. Recheck under load before handoff.
• What temperature rise should I pick? 150 C is common. Lower rise units run cooler and usually have lower losses, which can help with room heat and noise.
• What about K-rated transformers? Use them when nonlinear loads produce significant harmonics. They handle extra heating without premature insulation damage.
• Can I put the first OCPD on the secondary panel? Sometimes. If the transformer does not include the secondary OCPD, apply 240.21(C) limits on conductor length and routing.
• Are pad-mounts allowed near parking? Often yes, with bollards and clear working space. Verify utility and AHJ spacing before setting the pad.

Credits

Author: Revco Editorial Team - Electrical Content Editor
Technical review: Pending - add approved name/credential
Contact: (631) 283-3600

About Revco Lighting & Electrical Supply

Since 1978, Revco Lighting & Electrical Supply has been helping professionals bring their projects to light—literally. As a go-to source for lighting and electrical products across Long Island, NY and nearby areas, we specialize in supporting contractors, builders, and industry experts with practical solutions and dependable service. Whether it’s a complex commercial build or a simple residential upgrade, we’re here to make sure you have what you need, when you need it.

Sources

1. NFPA. NFPA 70, National Electrical Code, 2023 Edition. Articles 110, 240, 250, 450. NFPA LiNK page for 2023 NEC. Retrieved August 2025.
2. U.S. Department of Energy. 10 CFR Part 431, Subpart K — Distribution Transformers. eCFR Subpart K. Retrieved August 2025.
3. IAEI Magazine. Article 450 Transformers. IAEI Article 450. Retrieved August 2025.
4. Eaton. Dry-type distribution transformers design guide (DG009001EN). Eaton design guide DG009001EN. Retrieved August 2025.