Class I Locations
Cable Trays have been permitted in the hazardous (classified) locations in the National Electrical Code for Class I (flammable vapor and gases) since the 1978 NEC and have been used extensively in chemical plants, refineries, and other types of facilities.
This article is about code requirements for the “Division” classification system, we will discuss the “zone” system in a future Cablegram.
In the 1978 NEC Cable types PLTC per article 725, type TC, type MI, and type MV were permitted in Class I, Division 2 locations in section 501-4(b). This option had not changed until the 1996 NEC, when the new type ITC cable was added to Division 2 options.
Class II Locations – Division 2
Cable trays were added to the combustible dust section (Class II) of the 1987 NEC in section 502-4 wiring methods. Cable types MI, MC, PLTC, or TC were permitted in ventilated channel tray in a single layer with a space not less than the larger cable diameter between two adjacent cables. In the 1993 NEC there were changes to this section. PLTC cable was permitted in dust locations without being in a single layer or with a cable space between cables. Type MC or TC cable was permitted in ladder, ventilated trough, or ventilated channel in a single layer with a space not less than the largest cable diameter between them.
The 1996 NEC included a number of changes in these sections. In Class I, Division 1 locations Type MC Cable listed for use in Division 1 locations with a gas/vapor tight continuous, corrugated aluminum sheath, an overall jacket of suitable polymeric material, separate grounding conductors in accordance with NEC section 250-95 and provided with termination fittings listed for the application is permitted in industrial establishments with restricted public access, where conditions of maintenance and supervision ensure that only qualified persons will service the installation.
In Class I, Division 2 locations type ITC was added to the types of cable permitted in tray. Type ITC cable is instrumentation Tray Cable per new article 727 that provides an alternate to PLTC cable and the requirements of article 725.
For combustible dust locations, type MC cable, with the same requirements as in Class I, Division 1 locations, was added to the wiring methods permitted in Class II, Division 1 locations. The cable is to be listed for Class II locations. Note, there are no spacing restrictions nor do the cables have to be in a single layer, unless Article 318 requires them to be because of their size.
Type ITC cable was also added with no spacing between cables or single layer requirements for Class II, Division 2.
The acceptability of a cable tray system in a hazardous location (or any location) depends on the cable.
Section 318-3 indicates that cable tray in hazardous locations shall contain only the cable types permitted in sections 501-4,502-4,503-3, and 504-20.
MI, mineral insulated cable, with termination fittings approved for the location, has been permitted in Class I, Division 1 and Class II, Division 1 locations since the 1962 NEC. This cable can be installed in cable trays in Division 1 locations and can also provide fire protection.
Cable tray systems must comply with article 318 with respect to ampacity, grounding, fill, spacing and segregation of cable types. Cables must comply with their respective NEC articles and should be listed but in Division 2 locations it is not necessary that they be listed for hazardous locations.
Intrinsically Safe (IS) cabling can easily be installed in Class I, Divisions 1 and 2 and Class II, Divisions 1 and 2 locations, but the installation must comply with article 504.
IS systems are instrumentation systems that have energy limiting circuitry that will prevent ignition of failed field wiring even under worst case conditions. It is exempt from hazardous area requirements except the requirements of article 504.
Separation of IS from non IS wiring is a major issue. Section 504-30 allows IS and non IS circuits in the same tray if they are separated by at least 2 inches and secured or by a grounded metal partition or approved insulating partition.
IS and non IS circuit conductors can also be run in the same tray without separation if either are in grounded metal sheathed or metal clad cables where the sheathing or cladding is capable of carrying fault current to ground.
A proposed change to the 1999 NEC would add type ITC cable with an armour jacket as acceptable in Class I, Division 1 locations in industrial establishments etc. This cable must have a gas/vapor tight continuous corrugated aluminum sheath, overall jacket and be listed.
Sealing in hazardous locations is always a controversial topic. Section 501-5 is long, complicated, and not clear in some instances. The purpose of sealing is to minimize the passage of gases and vapors and prevent flame propagation.
When a cable tray passes through a boundary of a Division 2 location to an unclassified location, cable seals in general are not needed. Compliance with NEC sections 501-5, d(2) exception, e(3) , and f(3) emphasize that for process connected cables (and only process connected cables) passage of gases through the cable in the unlikely event that a thermowell or process seal should fail is prevented by sealing of the cable at the instrument or device, and not at the boundary.
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