In mission critical facilities, constant power supply is vital — it is the key to protecting data, supporting operations, and maintaining business continuity in the digital age.
However, furnishing a facility with temporary power has its own set of potential pitfalls. That’s why proper temporary cable routing and management is of special importance to ensure the safe, reliable delivery of electricity.
Temporary cable can be used to connect a variety of equipment in mission critical facilities, such as backup generators, load banks, electric heaters, chillers, cooling towers, air handlers, and other electrically powered equipment. This equipment may be needed for building construction, facility commissioning, routine facility maintenance, emergency equipment failures or to restore redundancies in the event of an unexpected outage. At the end of construction, during the commissioning phase of data center facilities, temporary power cabling is necessary for installing load banks to simulate future electrical and heat load that will eventually be in place once servers have been installed. When commissioning a facility, particularly during Level 4 commissioning, installing temporary cables and load banks allows for the testing of individual components, such as power distribution units (PDUs) and backup generators. Load banks also support Level 5 commissioning, integrated systems testing (IST), for data center facilities.
Since temporary power needs can vary greatly from one mission critical facility to another, there are a multitude of additional uses for temporary cables and varying means to route and manage them.
Design and Layout Best Practices
Safety is the top priority when it comes to temporary cable routing and management. Less than
optimal cable routing reduces carrying current capacity and increases the possibility of a short or fire that could result in electrical shock and burns. Following best practices helps meet the necessary safety precautions while simultaneously providing the proper level of constant power.
Typically, temporary power applications require the use of 4/0 type W cable (also called 4/0 AWG cable), and there are three key best practices to consider when routing these.
1) Distance or run length
Minimize the length of the cable runs whenever possible — 50 to 100 feet is optimal. Longer cables pose the risk of voltage drop between the power source and the load. If an application makes it impossible to have shorter cable runs — for example, if a load bank is outside with a connection point inside a facility — it may be necessary to add cables in parallel to decrease the resistance in the cable run. In this example, another option would be to add a larger load bank to apply additional load, thus compensating for the voltage drop.
When possible, route temporary cable in shaded areas outdoors or, if it’s an option, place cable runs inside where the temperature can be controlled. Both options help minimize the risk of the cables overheating by providing a lower ambient operating environment. That is important, since temperature ratings for cables are based on their operation in free air. A typical 4/0 type W cable has a rating of
In addition, consider the temperature of the surface that the cable is on, particularly for high-capacity load-bank testing or power generation applications.
When routing three-phase temporary cabling, the best practice is to leverage the triangular triplex method in laying out the cable. Three-phase power cabling consists of four or five conductors: one for each of the three phases (typically referred to with the letters A, B, and C), one ground, and, in some applications, a neutral. It’s important to note that although temporary power cabling has an insulation jacket protecting the cable from shorts, applying power still creates a magnetic field around the cable. The triangular triplex method places the cable in a fashion that minimizes the magnetic field around a phase cable when grouped with other phases. Grouping A, B, and C cables together in a triangular fashion allows the magnetic field of each individual phase cable to cancel each other out due to their proximity. Never route all the cables of an individual phase together. Although the triangular triplex method is best, is also acceptable and routine to simply triplex the cables in a pattern of A/B/C-A/B/C-A/B/C and so forth, with each of the three cables lying flat across the surface.
In most three-phase applications found in the U.S., the power running through the cable oscillates at 60 times per second, swinging between positive and negative. As that oscillation occurs, one cable is in high voltage as it goes through the sine wave and another is in low voltage. That counteracts the magnetic field. It also supports high carrying capacity and reduces heat, creating a safer cable layout.
Special Routing Situations
In addition to best practices for distance, temperature, and placement for temporary cable routing and management, there are also situations that require specific solutions to ensure safety and proper delivery of power. In an ideal scenario, it’s best to avoid the following; however, in reality, sometimes these circumstances cannot be prevented.
1) High-traffic areas
If a temporary cable is routed through a high-traffic area — whether it’s pedestrian or vehicle crossings — it can degrade over time. Temporary cable differs in its construction from permanent cable. In one 4/0 type W cable, there are as many as 2,000 thinner copper strands inside compared to a permanent cable that has dozens of rigid ones. While the increased number of strands provide flexibility for temporary applications, they are also more prone to damage. When the copper strands break down from traffic, it lowers the current carrying capacity. If routing cables through a high-traffic area is unavoidable, use cable trays, also known as cable ramps, to protect them. Note that cable trays enclose temporary cables so they can no longer cool themselves in free air and could become hot. Take the appropriate precautions to monitor the heat increase.
2) Metal and sharp edges
Routing cable over metal brings the potential for generating a magnetic field, which, in addition to creating a large unintended magnet, can also generate heat and cause the insulation of the cable to break down. Using the triangular triplex method reduces the risk, but it is still best to elevate cables on an insulated mat or wood pallets when routing over any metal surface. Similarly, protect the cable and its insulation by avoiding routes over sharp edges, especially metal ones. The metal can cut the cable, creating an electrical arc that could lead to a fire or injury to personnel.
3) Vertical and overhead routing
Routing cable vertically requires the use of stress relief measures to protect the Camlock connectors that connect the cables end to end. A 50-foot-long temporary cable weighs approximately 50 pounds, so it’s critical to take steps to alleviate downward pressure that can stretch the cable, its insulation, or the connectors. A best practice is to mount a stress relief every 12 feet of cable without mounting it on the connector. If cable needs to be routed overhead, use scaffolding or overhead trays.
4) Wet environments
Standing water poses a serious safety risk for cable routing even though the cable and insulation are waterproof. If the cable is degraded or cut, it can cause arcing or electrocution. Use pallets or scaffolds to raise the cable from the water.
Routing 4/0 type W cable through a conduit reduces its rating from 405 A to 260 A. Without the free air to cool it, the cable could easily overheat. To prevent this issue, route two cables in place of one to halve the amperage on each as to prevent either cable being pushed to the point of overheating.
Keeping It Safe
For those operating or building mission critical facilities, it is imperative to consider ways to reduce risks when routing temporary cable. Always follow best practices for design and layout and avoid situations that could damage the cable and cause injury. Safe practices also help ensure the facility will receive the appropriate power to keep operations up and running.