ANEP-100 shipboard electrical power system characteristics

What it is

ANEP-100 is the NATO Allied Naval Engineering Publication that sets the electrical characteristics a warship's power system, and the user equipment connected to it, has to meet. It covers three families of shipboard supply: 440V/230V/115V at 60Hz, 440V/115V at 400Hz, and 24/28V DC, and it is explicit that "not all these supplies may be present in a particular ship" (clause 1.1). The document describes a healthy power system, including the transients that come from ordinary events such as motor starts, and states plainly that it "does not describe system behaviour under abnormal conditions, e.g. short circuit faults" (clause 1.1). The current edition is Edition A, Version 2, promulgated 5 March 2025, superseding Edition A, Version 1.

An Allied Publication like this one is not itself the mechanism that binds anyone. ANEP-100 "is considered to be implemented when nations ensure that warships are equipped with electrical power supplies and electrical/electronic systems having characteristics according to this ANEP" (clause 1.2), and nations' agreement to use it is recorded in a separate NATO Standardization Agreement, STANAG 1008. It reaches a shipbuilder, a system designer or an equipment supplier only when a national naval procurement or a specific ship contract requires equipment to meet it. Annex E lists a domestic specification for most member navies covered - Canada, France, Germany, Italy, Norway, the UK and the US each hold one - and that national document, rather than ANEP-100 directly, is usually what a supplier actually receives.

Who the document addresses

ANEP-100 names three roles. The "Electrical Power System Designer" is "the organization charged with the responsibility of designing the ship power generation and distribution system" (Annex A.1.b). The "Naval Administration", borrowing its definition from ANEP-77, is "The Department of Government of the State responsible for providing safety regulation for naval ships" (Annex A.1.a). The third role, the party building or supplying equipment that plugs into the ship's power system, appears throughout as "the equipment supplier" or "the equipment manufacturer": where a maintained supply is needed, "the preferred approach is for the equipment supplier to provide the alternative source of power" (Annex B.3), and where a design risks breaching the harmonic distortion limits, "an early co-ordination between the Electrical Power System Designer, Naval Administration and the equipment manufacturer should be undertaken" (Annex B.9.c). No certification body, notified body or accredited auditor appears anywhere in the text.

Preferred supply, and what falls outside scope

Ships should be provided with a 440V 60Hz three-phase distribution system "in order to prevent the need for conversion equipment" (Annex B.1.a), with limited exceptions such as hovercraft using aircraft practice. Loads of 5kVA and above should be three-phase at 440V 60Hz; below that, the document sets an order of preference running from 115V 60Hz three-phase down to 440V 60Hz single-phase (Annex B.1.f). User equipment connected directly to aircraft power supplies, electric propulsion buses or submarine batteries falls outside the ANEP entirely (Annex B.1.e).

Unearthed systems and design constraints

NATO warship supply systems covered by this ANEP are unearthed, and user equipment "must not introduce direct or indirect connections between supply lines and earth" except for specific purposes such as EMI suppression, insulation monitoring or testing (Annex B.2.a). The document warns designers that phase-to-earth voltages are not necessarily equal, and that for unbalanced systems, earth-fault conditions "can be 115%" of the line-to-line voltage (Annex B.2.c).

Annex B.9 sets out a series of design constraints equipment has to meet without special negotiation. Inrush or starting current is normally to be less than 10 times full load current; 60Hz transformers rated 10kVA or greater may draw up to 25 times full load current, and smaller transformers may draw even more (Annex B.9.a). The same clause limits motor starting current on 440V circuits to 50%, 45% or 40% of generator rated current depending on generator size. Filter capacitance to earth is capped, and where the sum of harmonic-distorting loads exceeds 1% of the supply's short-circuit power (2% where each individual load is small), the equipment designer, the Electrical Power System Designer and the Naval Administration have to coordinate and "analyse... whether the ANEP-100 requirements are still met with respect to voltage harmonics" (Annex B.9.c). Paralleling the AC system at user equipment is "expressly forbidden"; paralleling after DC conversion is only permitted with transformer isolation (Annex B.9.e).

A practical guideline limits load unbalance to "5% of the arithmetic sum of the three full-load line currents" for that equipment (Annex B.5). Pulse loads - equipment such as sonar or radar that draws power in a repetitive or cyclic pattern - have to be restricted so they do not exceed the frequency and voltage modulation tolerances, achieved by limiting the load's rating, providing energy storage, early consultation with the Electrical Power System Designer and Naval Administration, or other suitable means (Annex B.7). Non-standard supplies are discouraged: requests "will be strongly resisted" and have to be notified to the Electrical Power System Designer and Naval Administration "at an early state of the design," with approval obtained "before entering into any commitment" (Annex B.4).

The tolerance tables

Annex C.2 sets the standard 60Hz supply (440V, 230V or 115V): a steady-state tolerance of ±5% on the average of three line-to-line voltages (±7% on any single one), 2% line voltage unbalance, 2% voltage modulation, a ±16% voltage transient tolerance with a 2-second recovery time, a voltage spike limit of 2.5kV, 1.4kV or 1kV across the three voltage levels, 5% total harmonic distortion, and a frequency tolerance of ±3% around 60Hz. Annex C.3 sets the 400Hz supply (440V or 115V) to the same steady-state and transient voltage percentages, with a wider frequency tolerance of ±5% around 400Hz. Annex C.4 sets the 24/28V DC supply's normal voltage range at 22-30V, an emergency range of 18-30V, an abnormal ceiling of 32V, a voltage transient band of 18V to 35V, 2% voltage modulation, 2.5V peak-to-peak ripple and a 600V voltage spike limit. Annex C.5 covers the abnormal condition on the 60Hz supply caused by a malfunctioning voltage regulator or governor: up to +50% voltage and -10% to +17% frequency, each for up to 2 minutes.

Every one of those figures belongs to the clause named beside it. This page reports what ANEP-100 specifies; it is not a substitute for reading the clause itself before any of these numbers are used in a design.

What the document does not cover

ANEP-100 does not describe abnormal system behaviour such as short-circuit faults, beyond loss of generator sets or a malfunctioning voltage regulator or governor (clause 1.1, Annex C.5). It does not apply to user equipment on special-purpose power sources - aircraft supplies, electric propulsion buses, submarine batteries (Annex B.1.e) - and it names no certification scheme: what it describes instead is coordination and approval between the Electrical Power System Designer, the Naval Administration and the equipment supplier or manufacturer.

Standards it references

• ANEP-77Background