---
title: "Do you need a K-rated transformer? Nonlinear loads | Entogo"
description: "A K-rated transformer is built to carry the harmonic currents that nonlinear loads draw without overheating. When switch-mode power supplies, drives, and…"
url: https://entogo.ca/insights/do-you-need-a-k-rated-transformer/
lang: en
type: article
image: https://entogo.ca/_astro/do-you-need-a-k-rated-transformer.DvNYhKke.jpg
datePublished: 2026-08-15
dateModified: 2026-08-15
site: https://entogo.ca/
llms: https://entogo.ca/llms.txt
---

Home › Insights › Do you need a K-rated transformer? Specifying for nonlinear loads

Power & Distribution

# Do you need a K-rated transformer? Specifying for nonlinear loads

Entogo August 15, 2026

![Data center server racks fed by dry-type distribution transformers handling nonlinear harmonic loads](https://entogo.ca/_astro/do-you-need-a-k-rated-transformer.DvNYhKke_RDEMi.webp)

In short

A K-rated transformer is built to carry the harmonic currents that nonlinear loads draw without overheating. When switch-mode power supplies, drives, and LED lighting dominate a panel, the transformer's K-rating and an oversized neutral decide whether a standard unit will survive the load.

## What makes a load nonlinear, and why does it matter to a transformer?

A linear load draws current in proportion to the applied voltage, so the current stays a clean sine wave. Most modern electronic loads do not. Switch-mode power supplies in servers and IT gear, variable-frequency drives, LED drivers, UPS rectifiers, and electronic ballasts draw current in short pulses near the voltage peak. Those pulses are the sum of the fundamental 60 Hz current plus a series of **harmonic** currents at multiples of 60 Hz — the 3rd at 180 Hz, the 5th at 300 Hz, the 7th at 420 Hz, and so on.

Harmonic currents matter to a transformer because losses do not rise in proportion to frequency — they rise faster. Winding **eddy-current losses** climb roughly with the square of the harmonic frequency, so a 300 Hz current heats the windings far more than the same RMS value at 60 Hz. A transformer sized only for its nameplate kVA can therefore run hot, lose insulation life, and fail early on a heavily nonlinear load even though the RMS current never exceeds the rating. **IEEE C57.110-2018** is the recommended practice for establishing how much a transformer can carry when the load current is non-sinusoidal.

## How a K-factor rating works

The K-factor is a single number that weights each harmonic by how much extra eddy-current heating it produces. Under **UL 1561** it is defined as:

K = Σ (Ih / I1)² × h²

where h is the harmonic order and Ih/I1 is that harmonic’s current as a fraction of the fundamental. Because the h² term grows quickly, high-order harmonics dominate the result. A purely linear load gives K = 1; the more harmonic content, the higher the number.

UL recognizes a standard scale — **K-1, K-4, K-9, K-13, K-20, K-30, K-40, and K-50**. A K-rated transformer is not simply a derated standard unit; it is built to dissipate the additional heating its rating implies, using measures such as smaller, transposed, or paralleled conductors, more conductor cross-section, and often an electrostatic shield between windings. A standard distribution transformer is effectively a K-1 machine.

### Matching the rating to the load

The right rating depends on how much of the connected load is nonlinear and what kind. A rough guide used in practice:

| Rating | Typical application |
| --- | --- |
| K-1 | Motors, resistance heating, incandescent — linear loads |
| K-4 | Mixed receptacle, HVAC, light electronic content |
| K-13 | Schools, health-care, multiple drives, mixed IT |
| K-20 | Data centers, dense server rooms, heavy electronics |

These are starting points, not code minimums. Where the load mix is known, the K-factor can be calculated from a harmonic survey; where it is unknown or expected to grow, designers commonly specify the next rating up rather than risk an undersized unit.

## Why the neutral is the other half of the problem

The third harmonic and its odd multiples — the 9th, 15th, 21st, called **triplen** harmonics — behave differently from the rest. On a 4-wire wye system they are in phase across all three phases, so instead of cancelling at the neutral point they add. The neutral of a panel serving heavy single-phase nonlinear load can therefore carry more current than any phase conductor — theoretically as much as **173%** of the phase current under worst-case conditions.

That is why K-rated dry-type transformers are built with a neutral bus rated for at least **200%** of full-load current under UL 1561, and why designers often run a double-size neutral from the transformer to the first distribution point. It is also why **NEC 220.61(C)** does not permit the usual neutral-load reduction to be applied to the nonlinear portion of a wye-connected load — the neutral has to be sized for current that will actually be there.

## Where a K-rated transformer makes sense

The strongest case is any facility where electronic loads dominate a step-down transformer’s secondary — [data centers](https://entogo.ca/solutions/data-centers/), trading floors, broadcast and imaging suites, and similar. It is also common on the low-voltage transformers inside [substation and power-distribution](https://entogo.ca/solutions/substations-power-distribution/) line-ups feeding modern commercial and [industrial](https://entogo.ca/solutions/industrial-epc/) buildings, where drives and switch-mode supplies now make up a large share of the load.

Where load is predominantly linear — motors, resistance heating, most HVAC — a standard [three-phase dry-type distribution transformer](https://entogo.ca/products/three-phase-dry-type-distribution-transformer/) is the correct and more economical choice; a K-rated unit adds cost and mass with no benefit. Low-loss designs such as an [amorphous-core dry-type transformer](https://entogo.ca/products/amorphous-core-dry-type-transformer/) address a different problem — no-load core loss — and are not a substitute for K-rating on a harmonic-heavy load. K-rating is also distinct from power-factor correction: a [power-factor-correction capacitor bank](https://entogo.ca/products/power-factor-correction-capacitor-bank/) trims reactive demand and any utility penalty but does nothing about harmonic heating, and unswitched capacitors can resonate with harmonic currents. The two are specified separately, against different problems.

## What a buyer should specify

- The **K-rating**, from a harmonic survey where possible, or from the load type where not.
- A **200% neutral bus**, and where warranted 200% neutral feeders to the downstream [distribution switchboard or panelboard](https://entogo.ca/products/distribution-switchboard-panelboard/).
- The temperature-rise and insulation class, and whether the transformer is evaluated to UL 1561 / CSA C22.2 No. 47 with its capability established per IEEE C57.110.
- Whether an electrostatic shield is required for transient and common-mode-noise attenuation.

Getting the rating right protects insulation life; over-specifying wastes capital. On projects where nonlinear load is the design driver, transformers designed and built to UL 1561 and IEEE C57.110 — with the neutral, shielding, and thermal margin matched to the actual harmonic profile — keep the equipment within its ratings for the life of the load; UL (cULus)/CSA certifiable on request.

- Transformers
- Harmonics
- Power Quality
- Nonlinear Loads
- Data Centers

Glossary: [60 Hz vs. 50 Hz](https://entogo.ca/glossary/#60-hz-vs-50-hz) [K-factor](https://entogo.ca/glossary/#k-factor) [Dry-type transformer](https://entogo.ca/glossary/#dry-type-transformer) [Power-factor-correction capacitor bank](https://entogo.ca/glossary/#power-factor-correction) [Switchboard](https://entogo.ca/glossary/#switchboard-ul-891) [Panelboard](https://entogo.ca/glossary/#panelboard-ul-67)

FAQ

## Common questions

- **What is a K-rated transformer**: A transformer built to carry harmonic currents from nonlinear loads without overheating. Its K-rating shows how much extra winding eddy-current heating it can handle compared with a standard K-1 unit.
- **Do I need a K-rated transformer for a data center**: Usually yes. Server power supplies are strongly nonlinear, so dense IT loads commonly call for K-13 to K-20 transformers with 200% neutrals rather than a standard unit.
- **What does the K-factor number mean**: It weights each harmonic by how much it heats the windings, using the square of the harmonic order. A linear load gives K-1, and the number rises as harmonic content grows.
- **Why do nonlinear loads need an oversized neutral**: Triplen harmonics add in the neutral of a 4-wire wye system instead of cancelling, so neutral current can reach up to 173% of phase current. UL 1561 K-rated units use a 200% neutral bus.

Keep reading

## Related insights

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