Reducing electrical system costs

03/04/2013


Use the load as an advantage

For derating conductors in underground installations and for voltage drop, derate based on actual load instead of circuit-breaker rating. The NEC recommends that conductors be derated for underground installations and voltage drop. However, if the required voltage is not available at the load, the load will fail to operate properly. 

The most conservative approach is to use the circuit breaker ampacity to determine the load size for derating calculations. However, this can cause the conductors to be significantly oversized, especially when the conductors are servicing a dedicated load. For dedicated loads, use the actual maximum current-based nameplate rating of the device or the manufacturer’s data sheet as the load size for derating calculations. This results in smaller, less expensive conductors to install. In no case shall derated conductor sizes be less than nonderated conductors for the same device, but in some instances the conductor size may not need to be increased. For panels, switchboards, or similar types of equipment, it is recommended to base derating calculations on the equipment’s full-load capacity because the loads may change over time due to facility retrofits or upgrades. 

When underground is not an option

Figure 2: Cable tray can be used instead of conduit in situations that prevent underground feeder installation, especially if many conductors must be routed. Courtesy: CH2M HillIn cases where feeders can’t be installed underground, consider using cable tray instead of conduit (see Figure 2). While the cost and installation of cable tray is greater than that of conduit, overall cost-savings can be achieved if there is a need to route a significant quantity of conductors. The cost break point will vary depending on the facility type, width, and material of the tray itself along with the quantity of feeders being installed. This cost break point should also incorporate the difference in labor hours to pull the conductors through the conduit versus the reduced hours for laying conductors in the tray. An added benefit is that cable tray easily allows future flexibility in adding, changing, or removing conductors if the loads change. 

Another option is to use busway for the large feeders between switchgear, switchboards, and panelboards. This can be a cost-effective solution compared to using conduit and cable for a large installation. 

Battery packs

For facilities that need emergency power for life safety only (i.e., emergency lighting and fire alarm panels), consider using battery-backed devices rather than an emergency generator. Emergency generation systems are expensive to purchase, install, operate, and maintain. Even if the design is already using a standby or backup generator for other loads, battery-backed life-safety equipment can still be a cost-effective solution. 

The NEC requires that where generators are supplying any combination of life safety (emergency), standby, and backup loads, they must have the capability of selective load pickup and load shedding to prioritize the loads and ensure adequate power for the life safety loads. Boxes, enclosures, transfer switches, and panels must be permanently marked to identify these devices as part of the emergency system. The NEC further requires that wiring from an emergency source or panel to the load be independent from all other wiring and equipment. Material costs for a generator load management system, permanent identification of all systems, and independent emergency wiring—in addition to the labor to install these systems—can exceed the costs of a life-safety system designed using battery-backed devices.

Offsite prefabrication

Specify and design equipment, components, and assemblies such that they can be prefabricated offsite. Prefabrication is a technique that divides complex electrical installations into manageable subassemblies. These subassemblies can be designed and constructed at offsite manufacturing facilities. Advantages to this approach include reduced on-site labor costs, improved quality control, and improved schedule performance. Often, prefabrication will occur concurrently with on-site preparation, thereby improving the speed of construction for the entire project and potentially reducing the impact of critical-path items. 

Prefabricated components undergo quality control procedures and functional testing, and can be partially commissioned prior to shipment to the site. Prefabrication also allows an increased level of material utilization controls, which reduces material waste, improves overall sustainability of the installation, and reduces the environmental impact of construction. Additional advantages of prefabricated assemblies include enhanced worker safety, minimized delays due to weather or labor shortages, and improved site security. 



TOM , TX, United States, 03/06/13 09:06 AM:

Some utilities require a 4 wire incoming service even when there are only 3 wire motor loads.
PRASAD , GA, United States, 03/06/13 10:25 AM:

Interesting article !
JASON , SC, UNITED STATES, 12/10/14 03:47 PM:

In all of my design implementations, I found that the cost of going overhead is more expensive. The addition of supports do not typically outweigh the de-rating and the labor that is required to install the underground duct with the conduits and the spacers and the bare ground in a duct. The overhead cable trays also cuts down in pulling the cable from point "A" to "B". I am of course speaking from an industrial sector. In a commercial sector this comment may not be valid. Also the aluminum cables are typically bigger than their copper conterparts so the raceway sizes will have a cost impact.

The Al vs Cu is very contraversial. I have some clients that are so set on copper they would request a new lead engineer for even proposing the idea. To date I have not been associated with an Al conductor project, but I have only been in the industrial industry (18 years). Commercial my have more applicable scenarios. 250HP 460V Motor require big Cu wire and Al would need to be even bigger so labor would play a factor.
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