Determining Conductors Per Phase: 451 Amps/ Ampacity

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SUMMARY

The discussion focuses on determining the number of conductors per phase for a motor circuit, specifically addressing a 300 HP, 3 Phase, 60 Hz, 480V motor with a full load amperage of 361 Amps. The calculated ampacity for overload protection is 451 Amps, derived from multiplying the full load amperage by 125%. The participants emphasize the importance of considering starting surges in wire sizing, rather than solely relying on full load current. The use of 600 MCM/kcmil wire is questioned in relation to the calculated amperage.

PREREQUISITES
  • Understanding of NEC (National Electrical Code) standards
  • Knowledge of motor circuit protection calculations
  • Familiarity with ampacity ratings for conductors
  • Basic concepts of electrical motor specifications
NEXT STEPS
  • Research NEC guidelines for motor circuit conductor protection
  • Learn about ampacity calculations for different wire sizes
  • Explore the impact of starting surges on conductor sizing
  • Investigate the differences between MCM and kcmil wire sizing
USEFUL FOR

Electrical engineers, electricians, and professionals involved in motor installation and circuit design will benefit from this discussion, particularly those focused on conductor sizing and protection methods.

Dwa
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How do you determine the number of conductors per phase?

From my understand, the full load amperage divided by the desired wire size's ampacity rounded up if there is a remainder.

An example: If I have a 361 Full load Amp motor from the NEC table and I calculated the motor circuit conductor protection, which the specs of the motor 300 HP, 3 Phase, 60 Hz, 480V. 125 % x 361 Amps is 451 Amps. Which Amp would I chose to divide my desire wire? I would assume to use the 451 Amps for overload protection, but the calculations don't look right if I use 600MCM/kcmil wire.
 
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Wire sizing is often dictated by starting surges, not full load current
 

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