SOLUTIONS ELECTRICAL DISTRIBUTION By Garrett Nariman, PE
Install medium-voltage cables in petrochemical plants How to ensure code requirements for hazardous environments are satisfied
M Figure 1: When it comes to operating mediumvoltage equipment, petrochemical facilities come with many unique challenges. Courtesy: Samuel Engineering
edium voltage (MV) equipment is commonly encountered in today’s petrochemical facilities. Electric motors operating at 2.3 kV to 15 kV are often deployed for larger horsepower equipment such as compressors and pumps. Petrochemical facilities come with many unique challenges (see Figure 1). Principal among these is the presence, or possible presence, of flammable concentrations of gas in the atmosphere, or release of flammable liquids due to equipment failure. Electrical systems installed in these demanding and hazardous environments must satisfy the requirements of NFPA 70-2020: National Electrical Code (NEC). Specific requirements for MV wiring methods are often grouped at the end of relevant NEC articles. These MV requirements can either amend, or completely supersede the low voltage requirements. The design process can often require one to completely understand both the low voltage and the MV sections. Add the requirements for classified areas into this mix, and it is easy to see where confusion may set in. This article explains some of the more common aspects of MV cable installations in hazardous environments and how to ensure code requirements will be satisfied.
Conductor sizing
A typical MV motor feeder circuit often originates at a motor control center starter cubicle or a switchgear circuit breaker compartment. These are typically located in a non-hazardous location remote from the motor. From the source, one or possibly more MV cables are usually routed in cable tray to the load (see Figure 2). Because of the thickness of MV cable insulation and required bending radius due to the metallic tape shield, cable tray is most often preferred over conduit for MV applications (see Figure 3). The conductors must be sized in compliance with NEC Articles 430 and 311 with necessary adjustments for cable tray fill and spacing per Article 392. Note that according to Article 430, MV motor conductors are not sized based on motor full load amps (FLA) as is done for low voltage motors, but instead selected based on the motor overload protection setting. Additionally, the grounding requirements for MV circuits are grouped at the end of Article 250. These requirements supplement and modify the previous sections of this long article. Note that cable selection based on ampacity requirements must be done according to the requirements of Article 311 factoring in derating for ambient temperature and mutual heating effects encountered in grouped underground installations rather than relying solely on manufacturers data sheet values.
Classified area considerations
Once the cables and cable tray are properly sized for the load served, requirements for the portion of the circuit that lies within the classified area must be considered. For this discussion, assume this scenario consists of the cable and tray going out to the load and the cable terminations at the load. The discussion s focused ion Class 1 Division 2 (C1D2) Group D environments, which are www.plantengineering.com
PLANT ENGINEERING
April 2021
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