Key Factors to Consider in Cable Routing
Once you have selected the most suitable cable for a generator or load bank application, you must consider three aspects of successful cable routing: run length, temperature, and placement. This careful consideration and planning can help minimize premature cable failure and associated consequences.
When running cable to a load bank or generator, shorter runs are preferred. Shorter cable runs will ensure against voltage drop and keep equipment supplied with the correct voltage for optimal efficiency.
Most cable runs measure 50 to 100 feet, which is ideal. Longer runs, such as a 4/0 type W cable exceeding 200 feet, could experience substantial voltage drop. Check equipment specifications to ensure the cable from the load bank or generator will be able to deliver sufficient power to meet application requirements.
Sometimes longer cable runs are unavoidable. For example, when the distance of a large outdoor load bank is located from an indoor connection point, the load bank cannot be moved. Instead, a new way must be found to overcome the voltage drop. Similarly, when water flow decreases because of friction loss in a pipe and a larger pipe or larger pump must be used, analogous solutions exist for electricity. Another cable can be added, which will provide more overall power. Or a larger load bank can be implemented at the end, pulling more power through. The key is to be aware that the voltage may drop more than the application will allow, especially for load bank testing.
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Ambient temperature, surface temperature, and the heat a cable will generate must be considered when running cable for a generator or load bank. Temperature can affect cable performance.
Cable has a temperature rating based on operation in free air. Per NEC/NFPA70, 4/0 type W cable is rated at 405 amperes with a 90°C rise (194°F) over a 30°C (86°F) ambient. The cable is most efficient at that ambient temperature. But in real-world applications, the actual ambient temperature varies. If you run 100 feet of cable in August when the ambient temperature is 100°F, the carrying capacity of the cable will be less than if you run that cable in December when the ambient temperature is 40°F.
The ambient temperature around the cable may also be affected by the temperature of the surface where the cable rests. Bare ground in spring may be cooler than the ambient temperature of 77°F, thus making the ambient temperature nearest the cable cooler. However, asphalt in the sun could reach 125°F, making the ambient temperature nearest the cable much hotter.
When running cable, it’s important to understand how the heat the cable generates can affect performance. Most 4/0 low voltage cable carried by Sunbelt Rentals has a 2-kV insulating jacket and a 405-amp capacity. But, the closer to the rated capacity the amperage is, the more heat the cable will generate.
A cable running at 400 amperes will, therefore, generate a significant amount of heat and must be in free air for the insulating jacket to remain intact. In free air, the cable will dissipate heat up to a point. However, if you run the cable in conduit, heat can build up that can melt the insulation and cause a short circuit. All cables have a lower rating when used in conduit.
Three-phase electric power is a common form of power transmission. It relies on three cables that carry three alternating currents of the same frequency. The current runs in three phases, providing constant power transfer and
canceling the magnetic field around the cable which eliminates the resistance. This allows the cable to run much cooler. But this works only if the cables are routed A/B/C-A/B/C-A/B/C and so on in a triangular pattern.
Cables should never be placed with all A phase, B phase, and C phase cables together or all cables piled on top of each other. This reduces the current carrying capacity of the cables and could damage the insulation, possibly resulting in electrical shock, as well as increase resistance at the cable ends, causing them to overheat.
It’s important to run cables in a triangular placement to ensure that the cables will operate at the highest carrying capacity, generate the least heat, and create the lowest risk.
If you have any questions about the best procedures for cable routing and connections in an application or would like to learn more about optimizing your load bank testing projects, contact our team today! Give us a call at 866-249-9747 or have us reach out to you.