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NEC Table 430.250 & NEC 430.22Worked ExampleElectrical

Service Demand Calculation for a 3-Phase 20 HP 230V HVAC Motor Unit

Calculate conductor ampacity and full-load current (FLC) for a 20 HP, 230V, 3-phase motor using NEC Table 430.250 and NEC 430.22.

Direct Answer
67.5 A Conductor Ampacity (54 A Running FLC)

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

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Given Scenario Parameters

horsepower20
voltage230
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 FLC54 ARunning Full-Load Current
Conductor Ampacity (125%)67.5 A54A × 1.25 per NEC 430.22
Minimum Conductor Gauge (75°C):4 AWG copper
Max Inverse Time Breaker (250%):125 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
Conductor Ampacity = FLC * 1.25
Citing NEC Table 430.250 & NEC 430.22
1

Obtain Full-Load Current (FLC) from NEC Table 430.250 for 20 HP at 230V, 3-Phase: 54 Amperes.

2

Multiply by 125% for branch circuit conductor sizing per NEC 430.22: 54 A * 1.25 = 67.5 Amperes.

3

Select minimum conductor gauge: 4 AWG copper wire rated at 75°C.

Conclusion: 67.5 A Conductor Ampacity (54 A Running FLC)
NEC Table 430.250 & NEC 430.22