Two load types that look nothing like the industrial plant most magnetics specifications were written for.
Most magnetics specifications carry assumptions inherited from conventional industrial plant: motor-dominated load, predictable duty cycles, a stiff utility service, generous electrical rooms. Data centers and battery energy storage systems violate most of those assumptions, and components selected on plant intuition frequently underperform.
In an industrial plant, rectifier load is a fraction of the total and motors dominate. In a data center the proportion inverts. Server power supplies, UPS rectifier front ends and PDU-level electronics mean the connected load is overwhelmingly non-linear, and the design consequences follow directly.
Distribution transformers feeding IT load carry harmonic current continuously, not intermittently, and at a high proportion of total load. This is genuine K-13 to K-20 territory — one of the few applications where a high K-rating is routinely justified rather than defensive over-specification.
Two further points are frequently missed:
Modern UPS systems with IGBT rectifier front ends draw relatively clean current — often below 5% THD without external mitigation — which is a meaningful change from older six-pulse or twelve-pulse designs. Before specifying filtering, establish which generation of UPS is actually being installed. Mitigation sized for a legacy UPS front end can be entirely unnecessary on a modern one.
Conversely, when the facility transfers to generator, the generator sees the UPS as its load. Input filters and the generator's own impedance interact, and UPS manufacturers generally publish a required generator sizing ratio for exactly this reason.
A BESS presents a different problem again. The power conversion system is bidirectional, so current flows both ways and the harmonic spectrum differs between charge and discharge. Grid-forming and grid-following inverters behave differently still.
Grid-interconnected storage is governed by IEEE 1547 for distributed energy resource interconnection, alongside the harmonic limits of IEEE 519. IEEE 1547 imposes requirements on voltage and frequency ride-through, anti-islanding, and power quality at the point of interconnection that do not apply to a purely consuming industrial load. The utility interconnection agreement usually sets the binding numbers, and it should be in hand before magnetics are specified.
Generally K-13 to K-20. Data center load is overwhelmingly non-linear and runs continuously, which is one of the few applications where a high K rating is routinely justified rather than defensive over-specification. Calculate from the measured or specified spectrum where possible.
Often not. UPS systems with IGBT rectifier front ends frequently draw below 5% THD without external mitigation, unlike older six-pulse and twelve-pulse designs. Establish which UPS generation is actually being installed before sizing filtering for it.
Single-phase IT loads on a four-wire wye system produce substantial third harmonic current, which is zero-sequence and adds rather than cancels in the neutral. Neutral current can exceed phase current. K-rated transformers include a 200% neutral, but downstream feeder and panelboard neutrals are frequently still sized at 100%.
The power conversion system is bidirectional, so harmonic content differs between charge and discharge, and thermal sizing must reflect the actual cycling duty rather than a continuous nameplate rating. Interconnection transformers see inverter output rather than utility sinusoid, and most installations are outdoor and containerised with wide ambient swings.
IEEE 1547 governs distributed energy resource interconnection, covering ride-through, anti-islanding and power quality at the point of interconnection, alongside the harmonic limits of IEEE 519. The utility interconnection agreement usually sets the binding numbers and should be in hand before equipment is specified.
The passive-versus-active decision is rarely about performance at full load, where both work. It is about part load, generator operation, future expansion, and whether your load profile is actually as steady as the calculation assumed.
Most harmonic specifications are written against the wrong measurement point. Understanding where IEEE 519 limits apply, and how the Isc/IL ratio sets them, is what separates a $4,000 line reactor from a $40,000 filter.
NEMA 1, 3R, 4X and 12 are not a simple ladder of increasing protection. Each addresses different hazards, and the wrong choice either wastes money or fails in service — often through heat rather than ingress.