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NEC Table 430.250, NEC 430.22, & Table 430.52Worked ExampleElectrical

Branch Circuit Conductor & Breaker Sizing for 15 HP 460V 3-Phase Motor

Calculate minimum conductor ampacity and inverse time breaker sizing for a 15 HP 460V 3-phase motor under NEC 430.22 and 430.52.

Direct Answer
26.25 A Conductor (10 AWG) with 60A Inverse-Time Breaker

Calculated strictly in accordance with NEC Table 430.250, NEC 430.22, & Table 430.52.

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Given Scenario Parameters

horsepower15
voltage460
phase3

Interactive Calculator (Pre-filled)

Adjust inputs to evaluate variations of this NEC code scenario.

Interactive Motor Demand & Conductor Sizer

Pre-loaded with NEC Table 430.250 & NEC 430.22 values

Standard NEMA motor rating

Motor operating voltage

Single or three-phase

NEC 430.6(A)(1) mandates that conductor ampacity and breaker sizing MUST be based on the NEC table values (Table 430.250), not the motor nameplate current.

NEC Calculated ResultsTable 430.250
Table FLC21 ARunning Full-Load Current
Conductor Ampacity (125%)26.3 A21A × 1.25 per NEC 430.22
Minimum Conductor Gauge (75°C):10 AWG copper
Max Inverse Time Breaker (250%):50 Ampere
Code Governing Article:NEC Table 430.250 & NEC 430.22
Select any HP or voltage above to test different motor service and feeder scenarios.

Step-by-Step Solution

Formal engineering derivation citing applicable National Electrical Code articles.

Governing Code Equation
Ampacity = FLC * 1.25 | Breaker = FLC * 2.50
Citing NEC Table 430.250, NEC 430.22, & Table 430.52
1

Locate Full-Load Current (FLC) from NEC Table 430.250: 21 Amperes.

2

Conductor Ampacity: 21 A * 1.25 = 26.25 Amperes (Select minimum 10 AWG copper rated at 75°C).

3

Short-Circuit Protection (Inverse Time Breaker per Table 430.52 at 250%): 21 A * 2.50 = 52.5 Amperes.

4

Select standard breaker size per NEC 240.6(A): Next standard size up is a 60 Ampere circuit breaker.

Conclusion: 26.25 A Conductor (10 AWG) with 60A Inverse-Time Breaker
NEC Table 430.250, NEC 430.22, & Table 430.52