Introduction: Two Types of Flexible Metal Conduit
Metal conduit serves as a flexible protective sheath for cables and wires. It is used where a cable route passes through movable assemblies, vibrating equipment or requires additional mechanical protection. Manufacturers produce two basic varieties: non-liquid-tight metal conduit, which is a spirally wound galvanized steel strip, and liquid-tight metal conduit, in which a sheath of PVC compound or a sealing layer that does not allow moisture and dust to pass is applied over the metal spiral. Externally they are similar, but the difference in protective properties separates them into different areas of application.
Non-liquid-tight metal conduit is usually marked as RZ-Ts or MG and costs less. It perfectly protects the cable from mechanical damage, but allows water, dust and oils to pass through. Liquid-tight metal conduit is the same steel spiral covered with a polyvinyl chloride sheath that completely isolates the internal space. It is used in damp, dusty, chemically active environments, as well as outdoors. More details about liquid-tight versions can be found in the article liquid-tight metal conduit, and about flexible galvanized and stainless models — in the review flexible metal conduit.
Comparative Table of Liquid-Tight and Non-Liquid-Tight Metal Conduit
| Parameter | Non-liquid-tight metal conduit | Liquid-tight metal conduit |
| Design | Galvanized steel spiral strip | Steel spiral with an outer PVC sheath |
| Protection against moisture and dust | Absent, allows water to pass | Full, IP65–IP67 |
| Resistance to oils and chemicals | Low | High, depends on the sheath material |
| Flexibility | High | Slightly lower due to the outer layer |
| Bending radius | Smaller, up to 3–4 diameters | Not less than 5–6 diameters |
| Temperature range | From –40 to +100 °C | From –20 to +60 °C standard; down to –40 °C low-temperature |
| Sheath material | Absent | PVC compound, polyurethane |
| Presence of pulling wire | Possible | Possible, more often with pulling wire |
| Cost | Lower | Higher |
| Service life | 10–15 years in dry rooms | 15–20 years, including outdoor installation |
| Typical diameters | 6–50 mm | 8–50 mm |
| Main application | Dry rooms, concealed wiring, protection from mechanical damage | Damp, dusty, outdoor conditions, chemical shops |
Design and Materials of Manufacture
Non-liquid-tight metal conduit is produced from steel strip galvanized by the Sendzimir method, which is wound in an overlapping spiral. Micro-gaps remain between the turns, through which moisture, dust and gases penetrate inside. Such a design retains flexibility and provides reliable mechanical protection of the cable from impacts, crushing and abrasion. The walls of the conduit are not solid, therefore it is not intended for installation outdoors or in damp basements without additional protection.
Liquid-tight metal conduit has the same metal base, but on top of it a sheath of PVC compound, polyurethane or thermoplastic elastomer is extruded. The sheath tightly encloses the spiral, covering all gaps. The inner surface remains metallic, which ensures earthing and shielding, while the outer polymer layer protects against corrosion, water, oil and ultraviolet radiation. In a number of models, a special adhesive layer is added between the metal and the polymer, increasing adhesion and excluding delamination during bending. Galvanized steel or stainless strip determines corrosion resistance: stainless versions are used in sea water and at food production facilities.
Liquid Tightness and Protection from External Influences
The key difference is the ability to retain moisture and dust. A non-liquid-tight conduit freely allows water to pass, which, accumulating inside, causes corrosion of the steel strip and damage to the cable insulation. In dry heated rooms this is not critical, but in basements, attics, laundries or outdoors such a conduit becomes the reason for premature cable failure. A liquid-tight conduit completely excludes the penetration of liquid and dust, which is confirmed by the IP65 or IP67 protection degree. It can be laid in the ground, in concrete, under the open sky, in workshops with water cooling of machine tools.
Separately, liquid-tight metal conduit with a pulling wire is distinguished — a steel wire facilitating cable pulling. The pulling wire saves the installer’s time, especially on long straight sections. Non-liquid-tight models are also equipped with a pulling wire, but its anti-corrosion protection is lower. When choosing a liquid-tight version for damp rooms, it is important to clarify whether the sheath is designed for constant immersion: standard PVC withstands splashes and short-term flooding, while special polyurethane sheaths are required for full immersion.
Flexibility, Strength and Bending Radius
Non-liquid-tight metal conduit without an outer coating is more flexible: the bending radius is 3–4 outer diameters. This is convenient when installing in a limited space, behind turns, in corrugated sections of machine tools. Liquid-tight conduit is somewhat stiffer due to the polymer sheath; the minimum bending radius increases to 5–6 diameters. If bent excessively, the PVC layer may separate from the metal, forming a fold where moisture will enter.
In terms of mechanical strength, both types are close: the steel spiral withstands significant crushing and impact loads. But liquid-tight conduit is additionally protected from abrasion by the sheath, which extends its service life in contact with concrete, metal trays or vibrating elements. Under conditions of strong vibration, the liquid-tight one is preferable, since it dampens oscillations and prevents the cable from chafing against the inner edges of the steel strip.
Temperature Limitations and Climatic Design
Non-liquid-tight galvanized conduit operates in the range from −40 to +100 °C without loss of properties, therefore it is installed in unheated workshops and on open trestles, if there is no direct water ingress. Liquid-tight conduit with a PVC sheath is limited by an upper limit of +60 °C, and in frost below −20 °C the compound hardens and may crack when bent. For cold regions, low-temperature modifications with a sheath of special PVC compound that retains elasticity down to −40 °C are produced. Such series are designated by the index “KhL” or “Low Temperature”. When laying in hot workshops, near furnaces or steam lines, even non-liquid-tight conduit should be protected from heating above +100 °C, otherwise the zinc coating burns out and the steel loses strength.
Installation, Pulling and Compatibility
Both types are installed using special connectors — couplings that are fixed on the thread. For liquid-tight conduit, liquid-tight couplings with sealing rings providing an IP protection degree are used. Non-liquid-tight conduit allows the use of simple clamping couplings without rubber seals, since liquid tightness is not required. When laying in corrugated pipe or tube, liquid-tight conduit is not needed, but if the route passes through a wall with a risk of flooding, it is better to take a liquid-tight one.
The presence of a pulling wire accelerates cable pulling. In non-liquid-tight models, the pulling wire is attached to one end and is easily removed after installation. In liquid-tight versions, the pulling wire is often built in and fixed in the coupling, which is convenient on long sections. Diameters are selected to the outer cable diameter with a 30–50% reserve for free passage. The standard range: 8, 10, 12, 15, 20, 25, 32, 40, 50 mm. For connection to metal enclosures of panels and machine tools, liquid-tight conduit ensures continuous earthing contact if the coupling is made of brass or steel.
When Non-Liquid-Tight Metal Conduit Is Sufficient
- Dry heated rooms: offices, apartments, workshops without moisture and aggressive media.
- Concealed installation in walls, behind plasterboard, in floor screed, where there is no contact with water.
- Protection of cable from mechanical damage in distribution boards and consoles.
- Connection of movable equipment, where maximum flexibility is important and the environment is dry.
- Budget projects with minimal requirements for moisture protection.
When Liquid-Tight Metal Conduit Is Needed
- Installation outdoors, under rain, snow, in conditions of ultraviolet radiation.
- Damp rooms: basements, laundries, baths, car washes, food production with wet cleaning.
- Chemical shops, where vapours of acids, alkalis, oils and solvents are present.
- Underground installation in the ground or in concrete without an additional pipe.
- Facilities with IP65 and higher requirements: pumping stations, treatment facilities, sea vessels.
Typical Mistakes in Selection and Laying
- Using non-liquid-tight conduit outdoors. The first rain starts corrosion and cable destruction.
- Saving on the liquid-tight version in a car wash or at food production, where moisture is inevitable.
- Bending liquid-tight conduit with a radius smaller than permissible. The sheath cracks and liquid tightness is lost.
- Ordering non-liquid-tight conduit without a pulling wire for a long route. Installation drags on for hours.
- Selecting the diameter too tightly. The cable does not pass, especially taking into account insulation irregularities and turns.
- Ignoring the low-temperature design during winter installation. Standard PVC sheath cracks in the frost.
Frequently Asked Questions
How does liquid-tight metal conduit differ from non-liquid-tight?
Liquid-tight conduit has an outer polymer sheath that protects against moisture, dust and oils. Non-liquid-tight is only a steel spiral without coating, allowing water to pass and not intended for damp places.
Can non-liquid-tight conduit be used in the ground?
It is not recommended. Water and aggressive soil components will quickly destroy both the conduit and the cable. For the ground, liquid-tight metal conduit with a PVC sheath or a plastic corrugated pipe is used.
What is metal conduit with a pulling wire?
It is a conduit with a steel wire inserted inside, facilitating cable pulling. After pulling, the wire is removed or left as a reserve for future replacements.
How is the marking MG and RZ-Ts deciphered?
MG — liquid-tight metal conduit, RZ-Ts — protective galvanized conduit. MG has a polymer sheath, RZ-Ts — without it.
Which metal conduit to choose for a machine tool with vibration?
Liquid-tight with a reinforced sheath is preferable: it dampens vibration and protects the cable from chafing. If the environment is dry, a non-liquid-tight one can be taken, but with a flexibility reserve.
Does liquid-tight conduit hold water pressure?
Standard IP67 means protection from short-term immersion to a depth of up to 1 metre. Special polyurethane models with a higher degree are required for constant water pressure.
Conclusion
The choice between liquid-tight and non-liquid-tight metal conduit is determined by environmental conditions, not by price. Non-liquid-tight conduit is good in dry rooms, where only mechanical protection is needed. Liquid-tight is indispensable outdoors, in damp workshops, in chemical productions and underground. The overpayment for a PVC sheath pays off with the cable service life and the absence of emergency shutdowns. When making a decision, one analyses not only current conditions, but also possible changes: leaks, condensation, moisture migration. The reliability of the cable route directly depends on the correct choice of the protective sheath.
