Mike Dolt Enterprises
Mike Dolt Enterprises
User: Rockyd, Location: Nevada
Asked on: Saturday, June 4, 2011 at 1:25 pm EDT
Answered on: Tuesday, May 14, 2024 at 6:12 am EDT
Question: Does anyone know how the two tables, 250.66 & 250.122 were derived? The Tables are used a lot but does anyone know if they are from empirical knowledge or engineering studies? How many actually do the load calculations as required per 220.61 to properly size the neutral service entrance conductor, vs. 250.66, and use the greater of the two numbers?
Derivation of Tables 250.66 and 250.122
Table 250.66 (Column A) is based on the assumption that the neutral current is 50% of the phase currents. This assumption is conservative and applies to most residential and commercial buildings where the load is primarily lighting and small appliances.Table 250.66 (Column B) is based on a more realistic assumption that the neutral current is 70% of the phase currents. This assumption is more applicable to buildings with a significant amount of nonlinear loads, such as electronic devices and motors.Table 250.122 is based on the actual measured neutral currents in a large number of buildings. These measurements showed that the neutral current can vary significantly, depending on the type of load and the operating conditions.
Empirical Knowledge vs. Engineering Studies
The tables are derived from both empirical knowledge and engineering studies. The initial assumptions used to develop the tables were based on empirical observations of neutral current behavior in various types of buildings.However, these assumptions were later refined through engineering studies that analyzed the effects of different load types and operating conditions on neutral current.
Load Calculations
NEC 220.61 requires engineers to perform load calculations to determine the ampacity of the neutral conductor. The calculations must consider the following factors: The net calculated load (after demand factors are applied) The calculated neutral load (based on the assumptions in Tables 250.66 or 250.122)* The type of electrical system (single-phase, three-phase)Greater of the Two Numbers
Once the neutral load is calculated using both Table 250.66 and Table 250.122, the engineer must use the greater of the two values. This ensures that the neutral conductor is adequately sized to handle the maximum possible neutral current.
Compliance
Engineers must comply with the requirements of NEC 220.61 to ensure that electrical systems are safe and reliable. Failure to properly size the neutral conductor can lead to overheating, voltage drop, and other problems. Flag for review