Choosing between NYM-J, SWA, and NYY-J cables

NYM-J is a standard unarmoured power cable with PVC insulation, intended for stationary installation inside buildings and in cable structures. SWA / NYBY armoured cable provides the same functionality but includes steel strip armour and a protective sheath, making it suitable primarily for direct burial in the ground. The specific version is selected based on the suffixes: low-smoke/oxygen-limited versions are used for internal electrical installations, fire-resistant versions (FE180 / PH90) are used for fire protection systems, and specific low-toxicity/low-smoke versions are used in schools and medical facilities.

A cable designation is not a brand name, but a description of its construction. Each letter represents a specific component, while the suffixes denote fire performance. The structure of these designations follows harmonised standards for power cables with polymeric insulation. There is no single universal table of all possible markings; rather, the specific construction of a particular cable is defined by the manufacturer's technical specifications and compliance with EN standards. Consequently, while markings follow a systematic logic, the exact composition is governed by the manufacturer's technical documentation.

How to read the designation

A cable designation is analysed from left to right: conductor material, insulation, protective screen, sheath, and finally, fire performance indices.

Letter/Suffix	Meaning	Standard/Reference
A at the beginning	Aluminium conductor; copper conductors are the default and are not explicitly marked	Harmonised construction rules
V	PVC insulation or sheath	Harmonised construction rules
XLPE	Cross-linked polyethylene insulation	Harmonised construction rules
H	Halogen-free polymer compound	Harmonised construction rules
G	Without protective armour	Harmonised construction rules
B	Armour made of galvanised steel strips	Harmonised construction rules
K	Armour made of galvanised steel wires	Harmonised construction rules
S / P	Protective sheath made of PVC / polyethylene	Harmonised construction rules
E	Screening	Harmonised construction rules
-P at the end	Flat cross-section	Harmonised construction rules
LS, HF, FE180, PH90, etc.	Fire performance (reaction to fire and circuit integrity)	CPR Regulation (EU) 305/2011 / EN 50200

Let's examine N2XH FE180 3×2.5 mm²: PVC-insulated cores, PVC sheath, non-armoured, screened, flame retardant (Euroclass A), maintains circuit integrity during a fire (to EN 50200), low smoke and acid gas emission, with three 2.5 mm² cores.

Limits defined by the standard

The following technical parameters are derived from harmonised standards—the specific figures used to resolve disputes during site inspections and commissioning:

• Number of cores: 1, 2, 3, 4, or 5;
• Cross-sections: from 1.5 mm² to 1000 mm², with multi-core cables not exceeding 400 mm², and 0.6/1 kV cables not exceeding 50 mm²;
• Conductor class: Class 1 or 2, meaning the conductor is intended for stationary installation rather than highly flexible applications;
• Operating temperature range: −50 °C to +50 °C, or −60 °C to +50 °C if equipped with a polyethylene sheath;
• Installation without heating: not lower than −15 °C for PVC and halogen-free (N2XH) sheaths, and not lower than −20 °C for XLPE-insulated cables with a polyethylene sheath;
• Bending radius: at least 7.5 times the outer diameter for multi-core cables and 10 times the diameter for single-core cables;
• Maximum continuous conductor temperature: 70 °C for PVC, 90 °C for XLPE;
• Service life: typically 25, 30, 35, or 40 years, with the specific value defined in the manufacturer's technical data sheet; the warranty period is typically 5 years.

The bending radius should be memorised exactly as specified: 7.5 D and 10 D. The "10–15 diameters" rule often cited in informal practice is not supported by the standard.

NYM-J series: non-armoured PVC-insulated cables

N2XH is the workhorse for internal wiring: buildings, cable ducts, trays, conduits, and chases. This is the round version. According to standard application guidelines, N2XH (halogen-free) is intended for use in internal electrical installations, buildings, structures, and enclosed cable enclosures.

N2XH with fire resistance (FE180 / PH90) — provides the same properties as standard halogen-free cables, plus circuit integrity during a fire. These are used in fire protection systems: fire alarms, warning and evacuation control, emergency lighting along escape routes, smoke extraction ventilation, and fire pumps. This requirement is based on mandatory safety standards for fire protection systems.

N2XH with low toxicity (LTx) and N2XH with fire resistance and low toxicity (FRLSLTx) — versions featuring the LTx index. LTx regulates the toxicity of combustion products, limiting the equivalent index to 120 g/m³. This index is unrelated to smoke density. According to safety guidelines, the primary application for these cables is in nurseries, schools, elderly care homes, hospitals, and residential dormitories; FRLSLTx versions are used for fire protection systems within these same buildings.

Flat N2XH (halogen-free) — a flat-profile version designed for concealed installation in grooves (chases) and structural elements. It differs from the round version only in shape; the electrical characteristics remain identical.

N2XH with screen (YSLY/LiYCY equivalent) — featuring a screen. Screening is required where power lines run adjacent to sensitive measuring or digital circuits, or for motor power supplies driven by variable frequency drives (VFDs). If such proximity does not exist, screening is an unnecessary expense and increases the cable diameter, requiring larger cable entries.

SWA Series: Armoured Cables

SWA halogen-free (N2XY/N2XH) and SWA fire-resistant (N2XY/N2XH with FE180/PH90) — feature steel wire armour (SWA) and a PVC or LSZH sheath. Armour is primarily used for direct burial in the ground, as it absorbs mechanical stress and protects against impact during excavation work.

The specific construction of armoured cables is defined by technical standards and manufacturer specifications. While various designations exist in older documentation, modern specifications should focus on the functional construction. For a correct technical specification, use the descriptive construction requirements (e.g., "halogen-free, low smoke, SWA armoured cable") in accordance with the manufacturer's technical data sheets.

Regarding underground installation: standards for cable products intended for installation in buildings and structures do not apply to products intended for installation in the ground or water. The fire performance requirements for an underground section are determined by the point where the cable emerges to the surface and enters the building.

Fire performance: what each index regulates

Fire performance parameters are defined by the relevant European standards for cable reaction to fire and circuit integrity.

Index / Classification	What it regulates	Reference
Flame retardancy (e.g., Euroclass B2ca, Cca, Dca, Eca)	Non-propagation of flame during group installation; the Euroclass letter defines the amount of combustible material during testing	CPR Regulation (EU) 305/2011
Flame retardancy (general requirement)	Length of the charred part of the specimen is limited to prevent flame spread	Standard requirement
LS (Low Smoke)	Smoke production: reduction in light transmittance (smoke density)	Standard requirement
HF (Halogen-Free)	Stricter smoke requirements than LS (higher light transmittance), plus absence of halogens: HCl content, conductivity, and pH levels	Standard requirement
LTx (Low Toxicity)	Toxicity of combustion products: equivalent index for gas toxicity	Standard requirement
FR (Fire Resistant)	Maintenance of circuit integrity under fire conditions; specific duration is defined by the manufacturer's technical specifications	EN 50200

Regarding the "180 minutes" for fire-resistant cables

"A cable with an FR index maintains functionality for 180 minutes" is a common error in technical specifications. According to EN 50200, the specific fire resistance rating (the duration the circuit remains operational) is established by the manufacturer for each specific cable type. The rating scale typically follows standard increments: 180, 150, 120, 90, 60, 45, or 30 minutes. An FR-rated cable must meet a minimum performance level, which is at least 30 minutes. If a specific regulatory document does not mandate a duration, EN 60331 recommends a test duration of 90 minutes, and such marking is specified as "IEC 60331 (90)".

In other words, 180 minutes is not a property inherent to the "FR" designation, but rather the highest level on a scale that must be verified by batch-specific documentation. The required duration is determined by the design: it must be sufficient for the system to perform its intended functions. Before purchasing, check the technical specifications and the marking on the cable drum, rather than relying solely on the brand name in the specification.

Where the requirement for compliance originates

The statement "N2XH is mandatory according to electrical regulations" is frequently made, but it is incorrect: standard electrical installation regulations do not contain such a specific requirement. The regulatory chain is different:

• Fire safety legislation — specifies that cables installed openly must be non-flammable. This provides the basis for the "low flame spread" (LS) requirement.
• System integrity requirements — cable lines and wiring for fire protection systems, fire detection, evacuation warning and control, emergency lighting along escape routes, smoke control, automatic fire suppression, and internal fire hydrants must maintain operational integrity under fire conditions for the duration required to perform their functions. This provides the basis for the "fire-resistant" (FR) requirement.
• Technical codes and standards — translate legal requirements into specific technical executions (such as FE180 or PH90). These standards define the necessary performance levels for various safety-critical systems.
• Standardised application tables — often list specific cable types for certain applications as recommended rather than mandatory. These serve as guidance for designers rather than absolute prohibitions.

Designations such as LS, HF, or FR are not defined in the primary safety legislation itself. Instead, specifications refer to the specific design solution and the applicable technical standards, using general application tables only as a reference.

Type	Primary application area (based on CPR Euroclasses)
Standard	Single installation in cable ducts and industrial premises
Flame retardant	Outdoor cable structures — trestles, galleries
LS (Low Smoke)	Indoor electrical installations, buildings, structures, and enclosed cable ducts
HF (Halogen-free)	Buildings and structures with high occupancy, including multi-purpose high-rise buildings and complex structures
FR (Fire Resistant)	Fire protection systems and other systems required to maintain functionality during a fire (e.g., FE180 / PH90)
LS + High Occupancy	Pre-schools, educational institutions, nursing homes, hospitals, and residential wings of children's institutions
FR + LS + High Occupancy	Fire protection systems within the high-occupancy buildings listed above

NYM-J

NYM is a designation used within the German harmonised system. Manufacturers produce NYM-J according to DIN VDE 0250-204:2000-12 for 300/500 V (with the right to use the VDE mark), or as 0.6/1 kV versions according to specific manufacturer standards. On-site, the difference compared to standard PVC power cables is immediately apparent: NYM-J features an intermediate filler between the cores, the cable has a circular profile, and it is easier to strip during installation.

Parameter	N2XY	N2XH	NYM-J
Rated voltage	0.66; 1 or 3 kV	0.66; 1 or 3 kV	300/500 V according to DIN VDE; 0.66 kV versions per manufacturer specifications
Flame retardancy (for multi-core installation)	Yes, according to the Euroclass rating	Yes, according to the Euroclass rating	According to manufacturer documentation
Low smoke emission	No	Yes	Not specified by standard classification
Conductor shape	Round; flat versions available as separate types	Round; flat versions available as separate types	Round, with filler
Standard compliance	Standardised construction and marking	Standardised construction and marking	German designation system (DIN VDE) and manufacturer specifications
Typical application	Industrial premises, cable installations	Buildings and internal electrical installations	Internal wiring where ease of stripping is required

When a project requires a specific fire performance rating, it is easiest to verify compliance using the Euroclass system (e.g., B2ca, Cca). NYM-J is a practical choice for installations where specific fire-classification requirements are not mandated, due to its ease of installation.

Copper or aluminium: NAYY and SWA

The notion that "only copper may be used in buildings" is outdated. Current electrical installation standards (such as HD 60364) no longer mandate copper for all building applications. Previous restrictions that required copper conductors for all building wiring and specific minimum cross-sections have been superseded by modern design principles based on load requirements and installation types.

Current requirements state:

• Circuits within main switchgear, distribution boards, and group distribution boards must use copper conductors. Copper is mandatory for connections inside the switchgear itself;
• For supply and distribution networks, aluminium conductors may be used provided the design cross-section is 16 mm² or greater (subject to specific installation conditions and protection against mechanical damage);
• The cross-section of the PEN conductor must be at least 10 mm² for copper and 16 mm² for aluminium.

Aluminium equivalents are indicated by the prefix A: NAYY, NAYY-P, or NYBY armoured cable. The allowable continuous current for aluminium is lower than that of copper with the same cross-section (HD 60364-5-52). In practice, this does not only result in larger cross-sections; it also changes the tray size, conduit diameter, and cable entry requirements, which ultimately impacts the cost estimate for cable management systems.

Selecting the Cross-Section

The cross-section is selected based on heating, economic current density, and corona discharge conditions; if different criteria yield different values, the largest value must be adopted (HD 60364-5-52). Allowable continuous currents are provided in standard tables for wires and cords, and for cables, with separate values for air and underground installation. These values are typically based on a conductor temperature of +65 °C, an ambient air temperature of +25 °C, and a ground temperature of +15 °C. For reference: a 3-core 2.5 mm² copper cable (such as NYM-J 3×2.5) installed in air is rated for 25 A.

A full calculation—accounting for load current, installation method correction factors, voltage drop, and short-circuit thermal stability—is detailed in a separate article: How to correctly select the wire cross-section for electrical circuits. We maintain a single "cross-section vs current" reference table there to ensure consistency across our technical materials.

Common Mistakes in Selection

• FR (Fire Resistance) is expected to guarantee 180 minutes. The duration is taken from the manufacturer's technical specifications; the FR index typically corresponds to a rating of no less than 30 minutes in standard testing.
• The requirement for LS (Low Smoke) is often misattributed to outdated wiring regulations. This requirement stems from EU safety standards and building codes; specific technical tables define preferred application areas rather than universal mandates.
• LTx is often mistaken for smoke density. LTx regulates toxicity (per standard testing protocols), whereas smoke density is regulated by LS and HF indices.
• Customers often order "flat NYM-J" without specifying the flat profile. A flat design is a specific construction (e.g., flat NYY-J) and is listed as a separate item in product catalogues.
• Screened versions are often ordered "just in case." Screening is justified when running cables adjacent to sensitive signal circuits or when powering motors via variable frequency drives (VFDs); in other cases, it unnecessarily increases the line cost and the overall cable diameter.
• Armouring is mistaken for fire protection. Armour provides mechanical protection; fire performance is determined by the reaction to fire (Euroclasses) and fire resistance (e.g., FE180 / PH90) indices. Furthermore, certain standards for surface installation do not apply to direct burial in the ground.
• Power cables are sometimes substituted with control cables. Control cables have a different intended purpose and different cross-sections — see Control cables YSLY and LiYCY.
• H05VV-F or other flexible cords are used in fixed installations. These are different product classes intended for different applications — see Wires H07V-U, H07V-K, and H07V-K N2XH.

Selecting cable from the catalogue