Understanding Transformers: What Every Contractor Needs to Know

Updated: August 21, 2025

TL;DR

Pick kVA from actual VA with correct multipliers, set taps to hold voltage under load, size OCPD and secondary conductors per NEC 2023, ground and bond correctly at XO when the secondary is a separately derived system, and give the unit room to breathe. When in doubt, confirm with the AHJ on Long Island before you land wires or order gear. For quick sourcing across dry-type, buck-boost, and mini power centers, see the Electrical transformers category.

Why it matters

Pick the wrong transformer and you invite nuisance trips, overheated gear, and inspection delays that eat schedules. Pick the right one and your feeders run cooler, voltage stays where it belongs, and the punch list is shorter. On Long Island, towns and counties can adopt amendments, so always verify requirements with the Authority Having Jurisdiction (AHJ) before you buy or land conductors. For help sourcing gear fast, see Revco Lighting & Electrical Supply.

Fundamentals

What a transformer does

A transformer transfers energy between windings by electromagnetic induction. It adjusts voltage up or down and provides isolation when you have separate primary and secondary windings. Autotransformers share a winding, which saves size and cost, but they do not provide isolation.

Nameplate terms that matter

• kVA rating: the apparent power the unit can deliver continuously at rated temperature rise.
• Primary and secondary voltage: example 480 V to 208Y/120 V. Verify tap settings if provided, typically ±2.5% steps.
• Impedance (%Z): affects short-circuit current and voltage drop. Lower %Z gives higher available fault current.
• Temperature rise: common dry-type rises are 80 C, 115 C, 150 C. Lower rise usually means higher efficiency and life.
• K-factor: applies to non-linear loads like VFDs and switch-mode power supplies. K-13 or higher handles harmonic heating better.
• Sound level: consider occupied spaces. Follow manufacturer tables and place units away from quiet rooms when possible.
• Enclosure rating: NEMA 1 for dry indoor, 3R for outdoor, 12 for dust, 4/4X for wet or corrosive locations.

Common transformer categories

• Dry-type distribution: indoor schools, offices, retail, light industrial.
• Liquid-immersed: higher kVA or outdoor pads where losses and noise must be minimized.
• Buck-boost: small corrections like 240 V to 208 V equipment or vice versa.
• Control/Class 2: panel controls and low-voltage circuits.
• Mini power centers: compact package with transformer, primary OCPD, and panelboard for temp power or tight spaces.

Code & compliance (NEC 2023)

For selection and installation, anchor on the following NEC 2023 sections. Do not quote code text; read the articles directly and confirm with the AHJ.

• Article 450: transformer construction and installation. See 450.3 for overcurrent protection, 450.9 for ventilation, 450.11 for markings, 450.13 for accessibility and mounting, 450.21 for dry-type indoor thresholds, and Part III for vaults.
• Article 240.21(C): transformer secondary conductor rules, including protection methods and length limits by condition.
• Article 250.30: grounding and bonding for separately derived systems, including the system bonding jumper and XO treatments.
• Article 110.3(B): follow listing and labeling instructions.
• Article 110.26: provide required working space where equipment with serviceable parts is installed.

Energy performance may be subject to federal efficiency rules for distribution transformers. When in doubt, select models that meet the current Department of Energy efficiency tables. Local utility programs may also require specific efficiency levels for incentives.

Selection steps

Use this field checklist before you place a PO.

1. Define load and duty. Add up VA or kVA, include motor inrush and continuous load factors. For continuous loads, size conductors and OCPD at 125 percent of the continuous portion.
2. Confirm voltages and phasing. Example: 3-phase 480 V primary to 208Y/120 V secondary. Verify system grounding method and neutral loading.
3. Account for environment. Temperature, moisture, dust, and corrosion drive enclosure choice. Provide ventilation clearance per nameplate and NEC 450.9.
4. Harmonics and K-factor. For VFDs, LED drivers, and office electronics, consider K-rated dry-type units or design with adequate capacity and temperature rise.
5. Impedance and coordination. Check available fault current and let-through. Verify primary and secondary OCPD coordination with %Z and transformer inrush.
6. Noise and placement. Keep units out of quiet rooms. Use vibration pads and flexible conduit where allowed to limit structure-borne noise.
7. Efficiency and losses. Compare core and coil losses. Lower temperature rise often correlates to better efficiency and life.
8. Accessories and taps. Specify wall brackets, weathershields, lug kits, and temperature sensors. Set taps to keep secondary within tolerance under typical load.
9. Listings and documentation. Require UL listing appropriate to type, provide submittals, and keep nameplate data in the closeout package.

Sizing and configuration examples (shown math)

Example 1: 1-phase continuous lighting load

Given: 120 V lighting at 48 A continuous. Continuous loads are sized at 125 percent.

• Required rated current = 48 A × 1.25 = 60 A
• kVA required = 120 V × 60 A ÷ 1000 = 7.2 kVA
• Selection: standard 7.5 kVA 120/240 V single-phase transformer meets the load. Consider 80 C or 115 C rise for lower losses and longer life.

Example 2: 3-phase office panel, 480 V to 208Y/120 V

Given: 45 kVA dry-type, 480 V primary, 208Y/120 V secondary.

• Secondary full-load current Isec = 45,000 VA ÷ (√3 × 208 V) = 45,000 ÷ 360.256 ≈ 124.9 A (round 125 A)
• Primary full-load current Ipri = 45,000 VA ÷ (√3 × 480 V) = 45,000 ÷ 831.36 ≈ 54.1 A
• Notes: size OCPD and conductors per NEC 2023, consider continuous portions at 125 percent, and verify secondary conductor method under 240.21(C). Keep voltage drop to design targets, commonly 3 percent feeder, 5 percent total.

Example 3: Buck-boost to raise 208 V supply to a 230 V single-phase load

Given: 230 V, 22 A nameplate load on a 208 V circuit. Needed boost is 22 V.

• Required buck-boost kVA ≈ Vboost × Iload ÷ 1000 = 22 V × 22 A ÷ 1000 = 0.484 kVA
• Selection: a 0.5 kVA buck-boost transformer wired per manufacturer diagram. Confirm listing and tap connections.

Need gear fast for any of the above scenarios? Compare dry-type and oil-filled options in our short dry-type vs oil-filled overview.

Installation and wiring notes

• Location and access: Provide clear working space where serviceable parts exist per 110.26. Follow mounting and accessibility rules in 450.13.
• Ventilation: Keep vents clear. Respect manufacturer clearance and the requirements in 450.9. Avoid closets or ceiling plenums unless specifically listed for that use.
• Noise control: Mount away from quiet rooms. Use vibration isolation pads and flexible conduit whips where allowed to reduce structure-borne noise.
• Grounding and bonding: If the secondary is a separately derived system, bond XO with a system bonding jumper sized per NEC and connect the grounding electrode conductor as required by 250.30. Do not bond neutral in more than one location.
• Overcurrent protection: Apply 450.3 for primary and secondary protection. Coordinate with available fault current and transformer inrush.
• Secondary conductors: Choose an allowed 240.21(C) method and keep within its length and protection limits. Record which method you used in the closeout package.
• Taps and voltage: Set taps so the secondary is within tolerance under typical load. Re-check after the first week of operation.
• Listing and instructions: Install per 110.3(B) and the unit’s data sheet. Torque lugs to spec and re-torque after thermal cycling if the manufacturer requires it.

Testing, commissioning, documentation

• Pre-energization: Inspect enclosure, verify tap position, check clearances, confirm bonding and grounding, and megger windings if the manufacturer permits.
• Functional checks: Energize the primary with the secondary open, verify no abnormal noise, then check no-load secondary voltage at all phases and neutral.
• Load checks: Record secondary voltage and current at 25, 50, and 100 percent load if possible. Balance single-phase panel loading across phases.
• Documentation: Save nameplate photos, tap settings, torque records, test results, and the chosen 240.21(C) method. Include the submittal and O&M in the closeout packet.

Troubleshooting

• Overheating or hot spots: Verify load vs kVA and duty cycle. Check ambient temperature and ventilation per 450.9. Confirm tap settings and tighten terminations to manufacturer torque. Harmonic loading may require a K-rated unit.
• Breaker trips on energization: Magnetizing inrush can nuisance-trip thermal-magnetic devices. Use a properly sized primary OCPD per 450.3 and consider time-delay characteristics where permitted. Verify coordination with upstream devices.
• Low secondary voltage: Measure primary voltage first. If primary is low, adjust taps if available. Check voltage drop on secondary conductors, especially under 240.21(C) conditions. Balance single-phase loads on 3-phase secondaries.
• Audible hum complaints: Relocate away from quiet areas, add vibration isolation pads, and verify mounting hardware is snug. Inspect for loose core clamps if the manufacturer allows field service.
• Grounding issues or nuisance GFCI trips: If the secondary is a separately derived system, confirm the XO bond and grounding electrode conductor per 250.30. Remove any stray neutral-to-case bonds downstream.

Common mistakes

• Ignoring continuous load multipliers: Undersizing leads to heat and early failure. Apply the 125 percent rule to the continuous portion of the load when sizing feeders and OCPD.
• Wrong secondary conductor method: Picking a 240.21(C) option but exceeding its length, protection, or routing limits. Document the chosen method on drawings and in closeout.
• Missing working space: Squeezing units into mechanical rooms without 110.26 clearances creates service hazards and inspection delays.
• No ventilation clearance: Blocking side or top vents violates 450.9 guidance and cooks the windings.
• Neutral bonded twice: Bonding neutral in the transformer and again in the first panel. Only bond once at the derived system per 250.30.
• Skipping tap checks: Leaving factory taps at nominal when the primary is high or low. Result is 5 to 10 percent off on the secondary.

For a quick refresher on pitfalls and how to avoid them, bookmark our practical roundup on common transformer installation mistakes.

Parts to stock + Shop at Revco

Keep the following on the truck or in the shop so you can finish in one trip:

• Dry-type distribution transformers in core sizes like 15, 30, 45, 75, and 112.5 kVA
• K-rated units for non-linear office and retail loads
• Buck-boost transformers for small voltage corrections
• Mini power centers for temp power and tight electrical rooms
• Wall brackets, weathershields, vibration isolation pads
• Lug kits, grounding bushings, bonding jumpers, labeling kits

Compare current lead times and prices in our dry-type distribution transformers and Electrical transformers category. For low-voltage lighting jobs, see low-voltage lighting transformers.

When to call the AHJ or an engineer

• Ambiguous secondary conductor conditions: If your layout does not cleanly match a 240.21(C) method, consult the AHJ before rough-in.
• Unusual grounding and bonding: Corner-grounded delta, high-leg delta, or separately derived systems tied to site electrodes that are difficult to access merit an engineer’s review per 250.30 intent.
• High available fault current: If utility short-circuit levels are elevated, request coordination and arc flash study support from an engineer.
• Special occupancies: Healthcare, assembly, educational, and high-rise projects often carry local amendments on Long Island. Verify adoption and interpretations with the AHJ.
• Transformer vaults or rooftop pads: If you need a vault or outdoor pad near openings, coordinate Article 450 Part III details and structural loads with an engineer and the AHJ.
• Oil-filled near buildings: Clearances, fire barriers, and spill control vary by jurisdiction. Get AHJ guidance before setting gear.
• Utility coordination: Short-circuit levels, meter positions, and service-routing assumptions affect OCPD and arc flash results. Involve an engineer.
• Energy compliance: If a spec cites federal distribution-transformer efficiency tables, confirm model compliance and documentation.

Safety disclaimer

This guide is general information for licensed professionals. It is not a substitute for the NEC, local amendments, manufacturer instructions, or a stamped design. On Long Island, adoption and interpretations vary by AHJ. Installations must comply with NEC 2023 Articles 450, 240.21(C), 250.30, 110.3(B), and 110.26 as applicable. Follow all listings and labels. When in doubt, stop and verify with the AHJ before energizing.

FAQ

• What is the difference between isolation and autotransformers? Isolation units have separate windings and can create a separately derived system. Autotransformers share a winding, do not isolate, and usually are not separately derived.
• Do I need a K-rated transformer for LED lighting and office loads? If harmonic content is significant, a K-13 or similar rating helps manage heating. Also consider a lower temperature rise and adequate kVA capacity.
• Can I parallel transformers? Only when the nameplates match closely in kVA, voltage ratio, impedance, and polarity, and the manufacturer permits it. Mismatches cause circulating current and overheating.
• Can a 480 V to 208Y/120 V transformer serve 240 V loads? Not directly. You would need a separate 240 V source or a different transformer configuration rated for that output.
• Are transformers allowed in ceiling plenums? Only if specifically listed and installed per the listing and NEC requirements for that space. Many dry-type units are not permitted in plenums.
• Where do I bond the neutral? For a separately derived system, bond at the transformer or first disconnecting means as allowed by 250.30. Do not bond again downstream.
• Does lower temperature rise matter? Yes. Lower rise often means lower losses and longer insulation life. It can also help in noise-sensitive or warm rooms.
• How do I extend transformer life? Keep it cool, within kVA, and clean. Verify torque, ventilation, and tap settings. See our practical tips in how to extend the life of your transformers.

Credits

Author: Revco Editorial Team — Electrical Content Editor

Technical review: Pending — add approved name/credential

Contact: Revco Lighting & Electrical Supply, (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. Accessed August 2025.
2. U.S. Department of Energy. 10 CFR 431 Subpart K, Energy Conservation Program for Distribution Transformers. Accessed August 2025.
3. UL Standards. UL 1561, Dry-Type General Purpose and Power Transformers. Accessed August 2025.
4. UL Standards. UL 5085-3, Low Voltage Transformers, Part 3: Class 2 and Class 3 Transformers. Accessed August 2025.