Dry-Type vs. Oil-Filled Transformers: A Contractor’s Guide for Long Island Jobsites

Updated: August 21, 2025

TL;DR

• Use dry-type for most indoor commercial rooms, schools, and healthcare where low maintenance, no liquid, and simpler code path matter.
• Use oil-filled outdoors or at higher kVA where heat rejection and footprint win. Indoor oil-filled can trigger vaults, fire protection, and liquid containment per Article 450.
• Plan early for ventilation, secondary conductor routing, grounding of separately derived systems, and working space. Coordinate with the AHJ on Long Island before you buy.
• Math check: at 75 kVA, 480 to 208/120 V three-phase, primary ≈ 90 A, secondary ≈ 208 A. Lay out OCPD and tap lengths to comply with 450.3 and 240.21(C).
• Browse Revco’s guide to distribution transformers and shop dry-type units to standardize specs.

Why this matters

Choosing between a dry-type and an oil-filled transformer affects safety, footprint, maintenance, and cost for the life of the installation. On Long Island, salt air, tight mechanical rooms, mixed-use occupancies, and coastal flood concerns add more variables. Picking the right style at design time avoids rework, noisy equipment rooms, and costly containment or ventilation retrofits. For product options and mounting hardware, you can shop transformers and accessories from Revco.

Local adoption note: New York State and local jurisdictions on Long Island may apply amendments or plan-review conditions. Always confirm requirements with the Authority Having Jurisdiction (AHJ) before submittals or purchase.

Fundamentals

How dry-type transformers work

Dry-type units use air for cooling and solid insulation systems around the windings. Common constructions include ventilated (core-and-coil with air passages) and cast-coil or resin-encapsulated designs that resist moisture and contaminants. Typical temperature-rise ratings are 80°C, 115°C, or 150°C. Lower rise usually means lower losses and quieter operation, but it can increase upfront cost. Dry-type units are popular indoors because there is no liquid to manage, so there is no risk of oil leaks or related containment requirements.

How oil-filled transformers work

Oil-filled units use a dielectric liquid for both insulation and heat removal. Mineral oil is common, but many projects now specify listed less-flammable fluids with higher fire points. The tank and radiators move heat to ambient air by natural convection or with fans on larger units. These transformers handle high kVA in compact footprints and generally run cooler for a given rating, which supports long service life in heavy-duty or outdoor applications.

Cooling, insulation, temperature rise, and sound

Cooling method and insulation class drive efficiency and sound. A dry-type 150°C rise model will typically have higher no-load and load losses than an 80°C rise model of the same kVA. Oil-filled designs often achieve equal or lower temperature rise at a smaller size. Sound levels are governed by design and NEMA limits; lower flux density, vibration control, and better enclosure design reduce noise. If a space is sensitive, such as classrooms or offices, consider a lower temperature-rise dry-type or remote-mount an oil-filled unit outdoors.

Indoor vs. outdoor environments

Dry-type units excel indoors where ventilation is adequate and combustible clearances are manageable. When outdoor, specify a weatherproof enclosure and corrosion-resistant features. Oil-filled units shine outdoors, especially at higher kVA, but indoor use can trigger vaults, rated rooms, fire protection, and liquid containment. In flood-prone coastal zones, plan tank heights, anchorage, and secondary containment accordingly.

For indoor distribution in commercial spaces, many contractors standardize on ventilated dry-type units. For site power, larger services, or pad-mount distribution, oil-filled often pencils out on efficiency and first cost per kVA. If you are comparing SKUs, Revco’s distribution dry-type transformers category is a good starting point.

Code and compliance (NEC 2023 quick hits)

Article 450 applies to both dry-type and oil-filled units. Provide guarding where subject to damage and install so ventilation is not blocked. Follow the nameplate and any marked clearance. Use the overcurrent tables in 450.3 based on system voltage. Where the installation uses liquid insulation indoors, check 450.23 and 450.27 for extra conditions like fire suppression and liquid confinement. See common field pitfalls.

• Guarding and physical protection. Provide guarding by the methods in 450.8 and protect from vehicle damage where applicable.
• Ventilation. 450.9 requires clear air paths and honoring marked spacings so the unit does not exceed its temperature rise.
• Marking and source marking. 450.11 requires specific nameplate data; 450.11(B) permits reverse feeding only where the installation follows manufacturer instructions.
• Overcurrent protection. Use 450.3(A) for over 1,000 V, 450.3(B) for 1,000 V and below. Remember that 450.3 protects the transformer, not the conductors. Conductor protection is elsewhere.
• Secondary conductors. Tap rules live in 240.21(C). Ten-foot and 25-foot conditions are common in commercial rooms; design the layout so your first OCPD location meets one of the allowed methods.
• Separately derived systems. Grounding and bonding per 250.30, including grounding electrode conductor sizing and bonding jumpers. Plan terminations early to avoid rework.
• Working space. Provide working space per 110.26 where required for safe operation and maintenance of electrical equipment. Coordinate door swings, egress, and equipment placement during layout.

AHJ note: Suffolk and Nassau County agencies and local towns sometimes add plan-review conditions for indoor oil-filled units, vaults, and spill control. Confirm at design.

Selection steps

Use this field-tested sequence to choose the right style and model. It works for classroom panels, retail TI, or a pad-mount at a light-industrial site.

1. Define voltages and kVA. List primary and secondary voltages and total connected kVA, then apply realistic demand and diversity. For 3-phase, use I = kVA × 1,000 ÷ (√3 × V) to estimate currents for conductor and OCPD planning.
2. Map the environment. Indoor or outdoor? Corrosive, sandy air, or coastal moisture? Dry-type is common indoors. Oil-filled is typical outdoors or at higher kVA where heat rejection matters. If indoors with liquid insulation, check the additional conditions in 450.23 and 450.27 before you lock the spec.
3. Pick enclosure and corrosion protection. For outdoors, select NEMA 3R or better and consider stainless hardware or coatings. For mechanical rooms, verify ventilation paths so marked openings are not blocked.
4. Select temperature rise and sound profile. Lower temperature-rise dry-type models run cooler and usually quieter, which helps in schools and offices. For noise-sensitive spaces, consider remote placement or isolation pads.
5. Check code triggers. Verify guarding per 450.8, ventilation per 450.9, marking per 450.11, and overcurrent per 450.3. Lay out secondary conductors to comply with 240.21(C). For separately derived systems, plan grounding and bonding per 250.30 with clear terminations.
6. Plan losses and efficiency. Compare temperature rise options and core designs for lifecycle cost. For non-linear loads, specify K-factor rated dry-type units to keep heating under control.
7. Decide serviceability. Consider access for terminations, future IR scans, and fan kits. Verify working space early so electrical room doors and egress aren’t compromised.
8. Confirm accessories and protection. Pick primary OCPD type and rating using 450.3 and the manufacturer’s inrush data, then coordinate downstream OCPDs. Choose monitoring, temperature sensors, and drip pans or containment if required by the AHJ.
9. Finalize procurement. Standardize where possible to simplify spares and training. See Revco’s dry-type distribution options for common footprints and kVA ranges.

Sizing & configuration examples

Example A: 75 kVA, 480 V to 208/120 V, 3-phase

Secondary current: I_sec = (75 kVA × 1,000) ÷ (√3 × 208 V). √3 ≈ 1.732. Denominator = 1.732 × 208 = 360.256 V. I_sec ≈ 75,000 ÷ 360.256 = 208 A (amperes).
Primary current: I_pri = 75,000 ÷ (1.732 × 480) = 75,000 ÷ 831.36 = 90.2 A (amperes).

• Code path: Select overcurrent per Table 450.3(B) for 1,000 V and below. Protect conductors separately and apply 240.21(C) for secondary conductor rules. Treat as a separately derived system and bond per 250.30. Provide working space per 110.26. Follow all nameplate and instruction markings per 110.3(B).
• Practical layout: Keep the first secondary OCPD within an allowed tap method and route conductors to minimize length and bends. Where noise matters, pick a lower temperature-rise dry-type or mount an oil-filled unit outdoors.

Example B: 15 kVA, 480 V to 240/120 V, single-phase

Secondary current: I_sec = 15,000 ÷ 240 = 62.5 A.
Primary current: I_pri = 15,000 ÷ 480 = 31.25 A.

• Use case: Small panels, controls, and lighting. Indoors, a ventilated dry-type is typical. For outdoor or harsher sites, specify a NEMA 3R or better enclosure.
• Accessory pick: Add vibration pads and wall brackets. For small voltage adjustments on equipment, consider a buck-boost instead of a full isolation transformer; see the Hubbell buck-boost transformer example.

Example C: Buck-boost for a 230 V load on a 208 V system

When a piece of gear needs 230 V but the site only has 208 V, a pair of small buck-boost units in auto-transformer connection can raise the voltage. Size per the connected load current and the manufacturer’s tables. Confirm labeling and overcurrent per 110.3(B) and 450. If the load is lighting, compare with purpose-built low-voltage lighting transformers before you commit.

Installation & wiring notes

• Ventilation and clearances: Do not block factory vents. Honor any clearance marked on the nameplate or in instructions. See 450.9 and 110.3(B).
• Guarding and protection: Where subject to physical damage, provide guarding per 450.8. In loading docks or parking structures, add bollards or curbs.
• Indoor oil-filled: Review 450.23 and related sections for less-flammable fluids, liquid confinement, and fire protection. Indoor mineral-oil units can trigger vaults; see 450.41 to 450.48.
• Outdoor placement: Use rated enclosures, level pads, and correct working clearances. Protect radiators and bushings from vehicles and snow removal.
• Noise control: Isolate the base, avoid mounting on light partitions, and rotate the core to minimize sound toward occupied spaces. Lower temperature-rise models often help.
• Labeling: Mark the source and system bonding point. Identify the first OCPD location and any tap method used. Provide directory updates for downstream panels.
• Corrosion and coastal details: On Long Island, specify stainless hardware, gasketed doors, and periodic rinsing for salt exposure. Seal conduit entries to limit salt fog.

For a refresher on selection tradeoffs, skim Revco’s guide to distribution transformers.

Testing, commissioning, and documentation

• Pre-energization checks: Inspect nameplate, verify taps, torque terminations, and confirm ventilation paths. Verify bonding per 250.30.
• Electrical tests: Take insulation resistance, turns ratio, and winding resistance where specified. After energization, record no-load secondary voltage and load voltage under a typical condition.
• Thermal checks: Use an IR camera after load stabilizes. Compare hotspots to vendor guidance. Investigate loose lugs, blocked airflow, or harmonics if temperatures trend high.
• Records: Capture serial numbers, tap positions, IR images, torque logs, and the grounding and bonding diagram. File the as-built. This supports future maintenance and warranty.
• Maintenance cadence: Schedule dust removal for dry-type units and periodic oil sampling for oil-filled units per the manufacturer. See Revco’s tips to extend transformer lifespan.

Troubleshooting

• Runs hot at light load: Check ventilation obstructions and tap settings. Verify harmonic content on non-linear loads; a K-rated dry-type may be needed.
• Buzz or hum complaints: Confirm mounting. Add vibration pads, rotate the core orientation, and confirm bolt torque to spec. Consider lower temperature-rise models for quieter operation.
• Nuisance trips at energization: Inrush can be high. Coordinate primary OCPD per 450.3 and manufacturer data. Use time-delay fuses where appropriate.
• Oil level or color off (oil-filled): Inspect for leaks. Send a sample for dissolved gas analysis per manufacturer recommendations.
• Low secondary voltage: Measure primary voltage first, then verify tap position and conductor voltage drop. Correct any undersized secondary conductors.

Common mistakes

• Blocking factory vents or ignoring minimum clearances on a dry-type.
• Routing secondary conductors beyond allowed lengths without an OCPD meeting 240.21(C).
• Specifying indoor oil-filled units without planning for vaults, fire protection, and liquid confinement where required by Article 450.
• Missing the separately derived bonding point or grounding electrode conductor per 250.30.
• Placing noisy units against light partitions next to offices or classrooms.
• Skipping torque checks and IR scans during commissioning.

Parts to stock + Shop at Revco

• Distribution dry-type transformers for common 15–225 kVA indoor applications.
• Hubbell buck-boost transformer for small voltage corrections on equipment.
• Low-voltage lighting transformers for dedicated lighting loads.
• Wall brackets and seismic-rated stands, vibration pads, drip pans or containment, primary and secondary OCPDs, lugs, and labeling kits.

When to call the AHJ or an engineer

• Any indoor oil-filled installation, especially in assembly, health care, or high-rise occupancies.
• When vault requirements, fire suppression, or spill containment are unclear for Article 450.
• Large pad-mounts near property lines, flood zones, or traffic paths that need bollards or barriers.
• Nonstandard grounding or bonding for separately derived systems, or multiple building electrodes.
• Harmonic-heavy loads where K-factor sizing or derating is likely.

Safety disclaimer

This guide is for licensed professionals. Follow NEC 2023, the listing and labeling on the equipment, and the manufacturer’s instructions. Local amendments on Long Island may change requirements. When in doubt, coordinate with the AHJ before purchasing or installing equipment.

FAQ

• Can a dry-type be used outdoors? Yes, with a properly rated enclosure and attention to ventilation and corrosion. Indoors remains the typical choice.
• Do oil-filled units always need a vault indoors? Not always. Requirements depend on the fluid type, rating, and occupancy. Check Article 450 and local rules.
• Are cast-coil dry-types better for harsh rooms? They resist moisture and contaminants better than ventilated units, at higher cost.
• What about efficiency credits? Lower temperature-rise and premium-core designs reduce losses. Compare lifecycle cost, not just first cost.
• How loud are transformers? Sound varies by design and temperature rise. For quiet spaces, specify low-rise dry-type or move oil-filled units outdoors.
• What maintenance is typical? Dry-type: cleaning and torque checks. Oil-filled: visual inspections plus periodic oil sampling per the manufacturer.

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.”

Credits

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

Sources

1. EC&M Magazine, “NEC Requirements for Transformers (Article 450 Highlights),” retrieved 08-2025, ecmweb.com.
2. EC&M Magazine, “NEC Transformers, Part 2: Oil and Vault Provisions,” retrieved 08-2025, ecmweb.com.
3. NEMA, “Transformers, Regulators, and Reactors (TP 80050), sound and temperature guidance,” retrieved 08-2025, nema.org.
4. Electrical Contractor Magazine, “Rules for Separately Derived Systems (250.30),” retrieved 08-2025, ecmag.com.