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NEC Chapter 9 Table 1 & Table 4 (RMC)Worked ExampleElectrical

Conduit Sizing for Four 500 kcmil THHN Conductors in RMC

Size Rigid Metal Conduit (RMC) for four 500 kcmil THHN copper conductors using NEC Chapter 9 Tables 1, 4, and 5.

Direct Answer
3-inch RMC Conduit (Wire fill: 2.8292 sq. in. vs 2.95 sq. in. permitted)

Calculated strictly in accordance with NEC Chapter 9 Table 1 & Table 4 (RMC).

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Given Scenario Parameters

conductors1 conductors
conduit TypeRMC

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Adjust inputs to evaluate variations of this NEC code scenario.

Conduit Cross-Section CAD Blueprint

Trade Size: 3″ RMC

NEAR MAX (WARNING)
1234Fill: 37.73% (NEC Max 40%)Wire Area: 2.8292 in² / Total Area: 7.499 in²
Conductors: 4 Total
Conduit Size3″ RMC
Fill Area2.8292 in²
Fill %37.73%
01

Conduit & Wire Schedule

Quick Presets:
Conductor Wire List (4 Wires)
Conduit Sizing Schedule & Trade Size
37.7% Fill (Limit 40%)
Recommended Trade Size
3RMC
4 Conductors • Total Wire Area: 2.8292 sq in
Raceway Fill Telemetry
Conduit Internal Area:7.4990 sq in
Max Code Fill Capacity:40%
Actual Fill Utilization:37.7%

Step-by-Step Solution

Formal engineering derivation citing applicable National Electrical Code articles.

Governing Code Equation
Total Conductor Area <= 40% of Conduit Interior Area
Citing NEC Chapter 9 Table 1 & Table 4 (RMC)
1

Find area of one 500 kcmil THHN conductor from NEC Table 5: 0.7073 sq. in.

2

Multiply by 4 conductors: 4 * 0.7073 = 2.8292 sq. in.

3

Consult NEC Table 4 RMC 40% fill column: 3-inch RMC allows 2.95 sq. in., satisfying the 2.8292 sq. in. requirement.

Conclusion: 3-inch RMC Conduit (Wire fill: 2.8292 sq. in. vs 2.95 sq. in. permitted)
NEC Chapter 9 Table 1 & Table 4 (RMC)