You're on the right track, but let's break it down step-by-step for clarity and confirmation

1. Determine the Motor Full Load Current (FLC):
You need to look up the FLC of a 20hp, 480V, 3-phase motor in NEC Table 430.250. The FLC from the table is 27 Amps.2. Conductor Ampacity Calculation (430.22)

The conductors supplying a motor must have an ampacity of at least 125% of the motor's FLC. Conductor Ampacity Required = 27 Amps 1.25 =

33.75 Amps.3. Conductor Selection

Based on NEC Table 310.16 (ampacity of conductors), a

#10 AWG copper conductor, with a 75°C ampacity rating, is typically rated for 30 Amps. A #8 AWG conductor is rated at 40 Amps. A #10 AWG conductor will meet the min ampacity requirement for this motor FLC only if it is rated at 75°C. Therefore, based on our calculation of 33.75 Amps, #10 awg will not be acceptable because its ampacity (30 Amps) is lower than 33.75 Amps. We have to use a conductor with an ampacity higher than 33.75 Amps. #8 AWG conductor is rated at 40 Amps and will be an acceptable size.4. Short-Circuit and Ground-Fault Protection (SCGFP) - 430.52

This is where it gets a little more involved. You're selecting the OCPD to protect against short circuits and ground faults, not overload. Overload protection is typically handled separately (see step 6).

Inverse Time Circuit Breaker

According to 430.52(C)(1), the maximum rating for an inverse time circuit breaker is 250% of the motor FLC. SCGFP Maximum = 27 Amps 2.50 =

67.5 Amps. 70A OCPD Selection

You are selecting a 70A breaker, which is slightly above the calculated 67.5A. This is permissible if the calculated value does not correspond to a standard breaker size (as per 430.52(C)(1) Exception No. 1 and 240.6). Since 67.5A is not a standard breaker size, you can round up to the next standard size, which is 70A.

5. 240.4(D) and Conductor Overcurrent Protection

This section addresses the overcurrent protection of conductors independent of motor applications. It typically restricts you from protecting smaller conductors with larger OCPDs (e.g., protecting a #14 AWG with a 30A breaker).

Exception 240.4(G) Motor Applications: This is the crucial exception you identified. It states that 240.4(D) does not apply to motor circuits that comply with Article 430. Since you sized the conductor at 125% of the motor's FLC and are using 430.52 for short-circuit and ground-fault protection, you are within the provisions of Article 430.6. Overload Protection (430.32)

This is separate from the SCGFP. Overload protection protects the motor and conductors from sustained overloads (e.g., a stalled rotor or excessive mechanical load). This is usually handled by overload relays within the motor starter. The overload protection is sized at 125% of FLC for motors with a service factor of 1.15 or more, or a temperature rise not over 40°C. Overload Protection Size = 27 Amps 1.25 = 33.75 Amps. You would select the appropriate overload relay size to provide this protection.

Conclusion



You need to use #8 AWG conductor with a 75°C ampacity rating. You can select a 70A inverse time circuit breaker for short-circuit and ground-fault protection. You MUST provide separate overload protection (usually through overload relays in the motor starter) sized appropriately.

Important Considerations



Voltage Drop

Always calculate voltage drop to ensure it's within acceptable limits, especially for longer conductor runs.

Ambient Temperature Correction

Adjust conductor ampacity if the ambient temperature is significantly higher than 30°C (86°F).

Conduit Fill

Ensure the number of conductors in the conduit does not exceed the allowable fill percentage, as this can impact heat dissipation and require further ampacity derating.

Local Codes

Always check with local electrical codes and authorities having jurisdiction (AHJ) for any specific requirements or interpretations.

Therefore, your initial assessment was almost correct, but the conductor must be sized according to its ampacity in NEC Table 310.16. #8 AWG will have an ampacity of 40A which is greater than 33.75A that we have calculated. Flag for review