Isolation transformers · kVA and voltages, electrostatic shield, insulation class and temperature rise, K-factor for non-linear loads, minimum efficiency at 35% load, enclosure
An isolation transformer is a transformer with separate primary and secondary windings on a laminated steel core, with or without an earthed electrostatic shield between them, that transfers power while galvanically isolating the load from the supply, to protect equipment and people, break earth loops and reduce common-mode noise. Ask three things first: the kVA, voltages and connections needed, which insulation class, temperature rise and shielding suit the loads and environment, and what efficiency and enclosure rating it must meet.
Single- and three-phase dry-type isolation transformers for power, control and equipment supplies. Signal and LAN isolation magnetics, airfield lighting series transformers, autotransformers and liquid-filled distribution transformers are outside this page.
01
Hammond Power Solutions' Sentinel K K-factor transformer listing (45 kVA, three-phase, 600 V delta to 208Y/120 V, copper wound) describes an electrostatic shield, a 150 °C temperature rise and a K13 rating, a Type 3R enclosure for indoor or outdoor use, cUL/UL and CSA listing, and states that all units use a uniform 220 °C insulation system with an 80, 115 or 150 °C temperature rise. K-factor units are designed and tested for non-linear loads. [2]
Editorial calculation: with a 220 °C insulation system and a 150 °C average winding rise, about 70 °C remains for the ambient temperature and the difference between average and hottest-spot winding temperature; an 80 °C or 115 °C rise leaves more temperature margin. Ask the supplier how its rating allocates ambient and hot-spot allowance. [2]
Editorial note: if the loads include drives, rectifiers, UPS or LED lighting, ask for a K-factor or harmonic-rated design and a neutral sized for triplen currents rather than a standard unit.
The product safety standard for this equipment is IEC 61558-2-4:2021, which covers isolating transformers for general applications and power supply units that incorporate them, including transformers with electronic circuits; it is used together with IEC 61558-1:2017 and has group safety publication status under IEC Guide 104. This third edition replaced the 2009 edition, moving construction descriptions into IEC 61558-1:2017 and adding a symbol for power supply units with linearly regulated output voltage. [3] The 2009 edition had raised the covered supply voltages to 1,100 V in line with the TC 14 standards. [4]
Give the supplier the kVA, primary and secondary voltages and connections (delta, wye, single-phase), frequency, taps, shield requirement, insulation class and temperature rise, K-factor or harmonic load profile, impedance, sound level, efficiency standard, enclosure type and IP/NEMA rating, ambient and altitude, cable entry, approvals (for example IEC 61558-2-4 or cUL/UL and CSA listing) and quantities.
02
The US energy conservation standard 10 CFR 431.196 sets minimum efficiencies for low-voltage dry-type distribution transformers at 35% of nameplate load. For units made from 1 January 2016 to 22 April 2029 the three-phase minimums include 97.89% at 15 kVA, 98.40% at 45 kVA, 98.60% at 75 kVA, 98.83% at 150 kVA and 99.02% at 300 kVA; for units made from 23 April 2029 they rise to 98.31%, 98.72%, 98.88%, 99.06% and 99.22%. Ratings not listed are interpolated. [1]
Editorial calculation: at 35% load a 45 kVA three-phase unit delivers about 15.75 kW (unity power factor assumed); at 98.40% efficiency its losses are up to about 15.75 × (1 ÷ 0.984 − 1) ≈ 256 W, and at the 2029 level of 98.72% about 204 W. Whether a given isolation transformer falls under the DOE definition depends on its type and rating; ask the supplier which efficiency standard it is certified to. [1]
03
Suppliers linked to this page list isolation transformers; each card links to the page reviewed.
Listings include single- and three-phase isolation transformers, low-voltage isolation transformers and dry-type transformers.
04
This is a functional reference built from the sources cited; it is not one maker's design.
| Part | Role | Fix in the order |
|---|---|---|
| Primary and secondary windings | Transfer power with isolation. [2] | Copper or aluminium; connections. |
| Electrostatic shield | Reduces noise coupling. [2] | Shield and earthing. |
| Core | Magnetic path; losses. [1] | Steel grade; losses. |
| Insulation system | Sets temperature class. [2] | Class and rise. |
| Electronic circuits (where fitted) | Within scope of IEC 61558-2-4. [3] | Circuit function; standard. |
| Neutral and K-rating | Harmonic loads. [2] | K-factor; neutral size. |
| Enclosure | Protection and cooling. [2] | Type, IP or NEMA. |
05
An isolation transformer quotation needs values for your supply. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Rated Power (kVA) | 15–1,000 kVA covered by DOE tables. [1] | kVA and overload. |
| Primary Voltage (V AC) | 600 V delta in one example; IEC 61558 series covers supply voltages up to 1,100 V. [2] [4] | Voltage and taps. |
| Secondary Voltage (V AC) | 208Y/120 V in one example. [2] | Voltage and connection. |
| Frequency (Hz) | 50 or 60 Hz. | Frequency. |
| Efficiency (%) | 98.40% min at 35% load for 45 kVA 3-ph (2016). [1] | Efficiency and standard. |
| Insulation Class (class) | 220 °C system in one range. [2] | Class and rise. |
| Dielectric Strength (kV) | Insulation test; requirements and tests set in IEC 61558-2-4 with IEC 61558-1. [3] | Test voltages and the standard tested to. |
| Operating Temperature (°C) | Rise 80/115/150 °C options. [2] | Ambient and rise. |
| Humidity Range (% RH) | Environment. | Range. |
| Protection Rating (IP) | Type 3R in one example. [2] | IP or NEMA type. |
| Core Material (grade) | Electrical steel. | Grade and losses. |
| Winding Material (grade) | Copper in one example. [2] | Copper or aluminium. |
| Weight (kg) | Handling. | Weight. |
| Dimensions (L×W×H) (mm) | Space. | Drawing. |
06
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Rating: [kVA; voltages; connections; taps] Loads: [linear or non-linear; K-factor] Insulation: [class; temperature rise] Shielding: [electrostatic shield; earthing] Efficiency: [standard; load point] Supply: [enclosure; ambient; approvals; quantity]
These companies list isolation transformers.
Lists three-phase and single-phase isolation transformers alongside autotransformers and voltage stabilisers.
Product scope checked 2026-10-11
Lists isolation transformers among dry-type, power and substation transformers.
Product scope checked 2026-10-11
Lists low-voltage isolation transformers among power and high-frequency transformers.
Product scope checked 2026-10-11
Lists AC isolation transformers among line power transformers.
Product scope checked 2026-10-11
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
Hammond's Sentinel K range uses a 220 °C insulation system with 80, 115 or 150 °C temperature rise options. [2]
98.40% at 35% load for units made from 2016, rising to 98.72% for units made from April 2029, per 10 CFR 431.196. [1]
Hammond describes K-factor units as designed and tested for applications with non-linear loads. [2]
IEC 61558-2-4:2021, used together with IEC 61558-1:2017; it also covers power supply units incorporating isolating transformers and transformers with electronic circuits. [3]
Sources checked 2026-10-12. Hammond describes its own transformer range; the US rule sets minimum efficiencies where it applies; the IEC abstracts define the scope of the isolating-transformer safety standard. Values for your transformer must come from the supplier's test report.
Ask for use case, specification boundaries, supplier type, and RFQ preparation information for this product.
Compare manufacturer profiles with relevant product and process capability.