Pros and cons of common branch circuit wiring installation methods in data centers
When it comes to wiring a data center, there are many ways to go. Because you can complete this project using a number of different methods, deciding on a particular wiring strategy depends on several factors that are specific to your facility, such as available raised floor area, vertical space above the data center floor and ceiling space, and coordination with the air distribution system. The next time you take on a wiring job in a data center environment, consider using one of the six alternatives discussed below. Because each of these branch circuit wiring options has positive and negative attributes, it’s your job to weigh the advantages and disadvantages — and provide your customers with the distribution system that’s most beneficial and cost effective for their facility.
- Liquidtight flexible conduit system — The liquidtight flexible conduit system, which tends to be the lowest initial cost option, is the most common type of method for branch circuit wiring. It is installed under the raised floor in ladder racks or anchored directly to the subfloor. There are many critical features that must be attended to when using this option.
First, the layout of the flex under the floor needs to be carefully planned in order to avoid bundles of flex being looped together, as these can quickly get to the point of never being untangled. Additionally, circuit labeling where the flex enters the power distribution unit (PDU) is vital for the future tracing of the flex — because as the flex is added, the abandoned flex needs to be traced and removed. Most PDUs are constructed for underfloor connections, and overhead accessible PDUs may consist of larger cabinets and changes in clearances. Finally, air flow models should also be included in the design to assure that the bundles of flex are not restricting air flow in critical areas of high-density racks.
The negative consequence of this system is that the work must be performed on your hands and knees on the ground. Most users prefer an overhead solution, as this option is more visible and accessible.
- Tray cable — A tray cable can be a thing of beauty, being that it is installed in ladder racks as an overhead distribution system, consisting of a ladder tray installed in a dedicated layer above the racks. Per NEC requirements in 392.11(A)(3), the cable must be installed in a single row across the bottom of the ladder tray and with appropriate spacing between cables.
The advantage of this system is that the power receptacles for each rack can be installed on the vertical face of the ladder tray, resulting in a branch circuit system that is completely visible and accessible, leaving the raised floor area void of any possible restrictions from the power cables.
The tray cable system also requires a data center space with high ceilings so that the power tray system can be on a separate plane from the data tray system. Although the initial installation is labor intensive and more expensive than other solutions, the result is an excellent solution for visibility. No longer is the system out of sight and out of mind.
- Flexible conduit installed in cable trays — Similar to the tray cable option, this approach consists of flexible conduits installed in a cable tray system above each row of racks. Each circuit out of the PDU is a separate flexible conduit with branch circuit wiring and does not have to be mounted in single rows like the tray cable. However, don’t bundle the flexible conduit to the point where it cannot be untangled if a circuit needs to be removed.
This system will take a dedicated space above the racks independent from the data cabling. Overall, installing flexible conduit is favorable — as it is one step below the installation cost of the tray cable system, and the labor is not as intense.
- Enclosed wireways installed overhead — This option involves enclosed wireways that are mounted overhead with standard building wire within the wireways. It is labor intensive and occupies overhead space, similar to option three. The number of wires installed in the wireway is an additional limitation.
In 376.22, the NEC limits the number of current-carrying conductors to 30 wires without derating; therefore, if more than 30 wires are installed, a derating factor must be applied, which typically requires the conductors to be “up-sized.” With 3-phase circuits to each rack, the wireway would be limited to serving six to seven racks.
Although this is an overhead solution (which is favorable), the initial costs will be high and the benefits minimal.
- Manufactured bus duct distribution system — A manufactured bus duct distribution system consists of an overhead bus duct with plug-in circuit breakers along the entire distance of the duct and also includes cord drops to receptacles above the racks. This bus duct distribution system is favorable as it provides flexibility and ease for changing out circuit drops and allows the circuit breakers to be located directly above each rack, making it obvious where the rack is fed from (as opposed to hunting through a panelboard in a PDU). The typical ratings of the bus ducts are 60A, 100A, 160A, 225A, and 400A, but the devices that plug into the 400A bus duct are not interchangeable with the other ratings.
With the new high-density blade servers, a 225A bus can only feed seven to eight racks with two blade servers in each rack. Therefore, if you have an “A” and “B” system, you will need to install two rows of bus duct above each row of racks. Again, additional space above the racks will be required and needs to be coordinated with the data cabling.
(Typically the bus duct needs about 12 in. of dedicated space above the racks). There is also the potential that a short in the bus duct could cause the racks or an entire row to shut down. Fortunately, the buses inside the enclosure are protected so that no one can touch a live bus.
Although this is a more expensive system, owners who have this system seem very satisfied with the results — even with the potential of a short taking out a row of racks.
- Using remote power panels — The final option is to use remote power panels that are installed within the rack layout. From a power engineer’s standpoint, the first rule of an efficient power system is to get the distribution system as close to the load as possible. When you use rack power centers, you are about as close as you can get. The remote power panels open up options of short branch circuit feeds overhead or underfloor — and are within the footprint of the rack row.
The rack power centers typically have two 42-circuit panels in the front and two 42-circuit panels in the back. This allows the option for the two front panelboards to be fed from the “A” power system and the two back panelboards to be fed from the “B” power system. The main challenge you will find with these remote panels is getting the data center manager to give up the amount of rack space needed for power distribution.
This system is more expensive when it comes to the cost of the PDUs and remote panels, but it’s an extremely efficient power system.
Korstad is a licensed professional engineer with a LEED AP certification. He currently serves as vice president at Swanson Rink in Denver, and can be reached at firstname.lastname@example.org.