Three Phase Motors is based the information found in NEC Article 430. It calculates minimum circuit ampacity, conductor sizes, fuses and breaker sizes, and the sizes of various conduit types you would need for single runs of THHN conductors. The page was designed as a teaching tool. It purposely excludes calculations for parallel runs, and the exceptions for smaller conductor sizes.
Column I: HP, Horse Power
Column II: NEC FLC, Full Load Current, from NEC Table 430.250.
Column III: FLC is multipled by 125%, in accordance with NEC 430.22, to find the minimum circuit ampacity.
Column IV: The FLC is multiplied by a value chosen from NEC Table 430.52. If the fuse or breaker size is over 800A, the program moves the fuse size to column III in order to comply with NEC 240.4(C). The conductor sizes in column VI are then based on the values in column III.
Column V: The value calculated in column IV is used to select a fuse size from NEC Table 240.6(A).
Column VI: The conductor sizes are chosen from NEC Table 310.15(B)(16), based on the values in column III. The program chooses aluminum or copper conductors, and the temperature rating of the conductors, depending on the choices you made on the top of the page.
Column VII: Equipment grounding conductor sizes are chosen from NEC Table 250.122. Please note that NEC 250.122(A) states that "in no case shall they (the equipment grounds) be required to be larger than the circuit conductors suppling the equipment".
Column VIII: Conductor fill is based on the area three current carrying conductors and the equipment grounding conductor. The table only calculates single runs of conductors.
Column VII: The conduit sizes are selected from NEC Chapter 9, Table 4.
You might be wondering why there are some cells that only show a "-". It simply means that there isn't any useful information to put in that cell. NEC Table 430.250 only shows 115 VAC motors up to 2 HP, so the rest of the cells contain a "-" when 115 VAC motors are selected. The NEC only gives the the approximate area of THHN up to 1000 kcmil, see NEC Ch. 9, Table 5. So, if the calculated wire size is bigger then 1000 kcmil, you get a "-". In reality, most installers start using parallel runs of conductors when the wire size is larger than 250 kcmil. That's a topic for another web page! Try my motor conductor sizing page if you want to calculate parallel runs based on the FLC value of a motor.
Comments are welcome!
Sincerely,
Lorn J. MacIlravie
lorn@lornmac.com