One of the challenges of renovating buildings is the capacity of existing electrical service and distribution equipment. Additions and remodels often add or replace equipment with higher electrical demand than what the system was originally designed for. The question of how the facility managers can determine the electrical system spare capacity frequently arises.
National Electrical Code art.220.87 describes the permitted methodology to determine the existing loads when detail individual load information is not available.
1. Utility company records.
Most of the facilities have electrical demand meters that allow the utility company to read the highest average load in kilowatts (kW) during any fifteen-minute interval during the time between regular meter readings. This information can be requested from the Utility company construction services department with a signed released from the facility representative. This option is preferred because it’s fast, it’s accurate, it’s exactly what the NEC prescribes and it’s free.
2. 30-day load reading
Where the 1-year demand data is not available, NEC allows the load calculation to be based on a continuous 30-day period. A licensed electrical contractor shall install and remove the recording meter equipment and interpret the data. The recording shall include the highest seasonal or periodic loads by measurement or calculation. This option presents the disadvantages of waiting 30 days for the information, incurs the cost of labor and equipment and could lack in accuracy because of seasonal and periodic loads that might be overlooked.
3. Less than 30-day load readings
Although not specifically permitted in NEC, in some instances a reduced period of load recording might be acceptable to the Authority Having Jurisdiction. Each facility has to be analyzed from the perspective of electrical load profile which include identification of daily, weekly and annual peak times of use of the facility, seasonal and periodic loads. The load reading shall be performed under the supervision of an electrical engineer who shall validate that sample rates and recording results reflect the true maximum load.
EDM Inc. provides electrical engineering services including existing buildings assessments, electrical load analysis, power and lighting design, and energy efficiency recommendations. Please contact us if you would like to discuss how we can assist with your next project.