Elevator Demand Factors: Feeder Reduction
Updated: Jun 21, 2021
Whether it be in high-rise offices, industrial facilities, or hospitals, elevators have become a necessity for all modern-day buildings. Elevators provide a simple solution for many applications including patient transportation, product movement, and means of egress. While elevators have become mainstays for many of these applications, the costs for installing them are not always a welcome addition to any construction project. Luckily, there are a few code requirements that can help minimize the financial burden when including elevators within a project.
When sizing electrical services or panelboard feeders, there are a number of different demand factors that can be used to help minimize the size of the conductors required, which in turn reduces the initial material and labor costs associated with the elevator installation. The scale of the cost reduction can be increased, depending on the number of elevators a project may have.
Motor Feeder Sizing
When sizing feeders or branch circuits to serve 3-Phase motor loads, the size of the feeder is based upon the full-load current values found in Table 430.250 of the National Electrical Code (NEC). Typically, when sizing feeders that serve a single motor, the feeder is sized to accommodate 125% of the full-load current of the single motor. If the feeder serves multiple motor loads, the size is increased by 100% of each additional motor full-load current, once 125% of the largest motor is accounted for based on Article 430.24 of the NEC. When sizing the feeder for an elevator bank, a different set of demand factors can be used as found in Table 620.14 of the NEC.
Elevator Feeder Sizing
When serving a single elevator motor, a feeder does not need to account for 125% of the full-load current of the elevator motor and instead only needs to account for 100% of the full-load current due to the intermittent nature of elevator motors. Once more elevators are added to the feeder, the applicable demand factor of the elevators further reduces the calculated load seen by the feeder. Based on Table 620.14 of the NEC, the number of elevators being served by a single feeder can significantly decrease the ampacity requirements of the feeder. The demand factor can be reduced from 100% down to 72% if ten or more elevators are served by the same feeder.
While combining all elevator loads into a single panelboard can help reduce construction costs, it can also reduce the reliability of the elevator system as well. If the panelboard serving all elevators is ever taken down for maintenance or due to an unexpected fault, it is possible for a building to lose access to all elevators for the duration of the repair. One solution provided in the National Electrical Code for fault conditions requires selective coordination of breakers when two or more elevators are fed from the same distribution board. This mitigates fault conditions eliminating power to an entire distribution board serving multiple elevators and only affects one elevator. Another solution is to include two distribution panelboards to serve the elevators, depending on the number of elevators within a project. Many times, facilities will feed one bank of elevators from one location and a second bank from another to ensure that one bank remains operational should the other go down for any reason.
Elevator Branch Circuit Sizing
It is also good to keep in mind that Table 620.14 of the NEC applies only to the feeders serving the elevators. When sizing elevator branch circuits you have to refer to Table 430.22(E) of the NEC. Using this table, you can reduce or upsize the elevator branch circuit cabling based on the motor classification.
Utilizing these tables found in the NEC, it is possible to minimize a project’s material and labor costs by reducing the ampacity requirements of various feeders throughout the project. Drawing from their depth of experience, Fitzemeyer & Tocci works with building owners and construction managers to utilize national and local codes to help minimize costs while still providing the best overall solution for any construction project.
Timothy Piehl, PE
Electrical Project Engineer