For circuits with multiple motors, the ampacity rating The NEC requires circuits supplying single motors to have an ampacity rating of greater than or equal to 125% of the full-load current rating of the motor. See Maximum Allowable Ampacities for Conductors in Raceway, Cable or Earth (30☌) for other insulation ratings. Note: The wire size in this table is based on 75☌ terminationsĪnd insulation. Full-Load Current and Circuit Protection Data for Three-Phase AC Motors Motor Horsepower After converting to equivalent short circuit MVA use the calculator provided below in case 2 to obtain the utility impedance in R+jX format.Ĭase2: Given Short circuit MVA, X/R ratio To get the short circuit parameters when the short circuit MVA and X/R ratio is provided use the calculator below.The chart below displays information for motor wire size, breaker size, thermal overload ratings, and other data for Use line-line voltage and three phase bolted fault current for MVA sc3 ø and line-neutral voltage and line to ground short circuit current for MVA sclg. Short Circuit Current Conversion Calculators Case1: Given Short circuit current (kA), X/R ratio If data is available in this format, convert the short circuit current to equivalent short circuit MVA using the equations below. Refer to Sequence components if you need additional information on positive, negative and zero sequence parameters. The various equations relevant to X/R ratio calculations are: The hypotenuse of the triangle so formed gives the total impedance (Z) of the circuit. X/R can be plotted on an impedance plane with R on the x-axis and X on the y-axis. If the X/R ratio is 10, it means the inductance of the system is 10 times more than the resistance of the system. At the tail end of long distribution lines and for low voltage systems the ratio will be lower. Near to large generating stations and large substations, this ratio will be high. X/R ratio is the ratio of inductance to resistance of the power grid up to the point of fault. Where, V ll is the line-line voltage and V ln is the line-neutral voltage at which short circuit value is provided. If the impedance is desired in actual ohms, the following formula can be used: Now that the base parameters are defined let’s see how the per unit parameters are defined: KVA base, I B base current (A) and Z B base impedance(Ω) are given by following equations: But it is possible to select any other base if the operator chooses so. Voltages are always line-line or phase-phase voltage.īase MVA or Base kVA: A widely used base is 100MVA. If the power company delivery voltage is 13.2kV, the base voltage will likely be 13.2kV unless otherwise noted. They are base voltage and base MVA.īase Voltage (kV B): Often the supply voltage is used as the base voltage. The key is to understand that there are two main parameters that need to be known when dealing with per unit quantities. Here are some of the basics in per unit concept. Calculators are provided at the end of this article.
This article details how to convert from one format to another and provides calculators that can handle this operation. Often times we need to convert from data in one format to another. Data from power grid (utility) operators is often provided in one of the following formats at a given system voltage: