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Conduit Fill Calculator

Calculate conduit fill percentage per NEC

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NEC Fill Limits

# ConductorsMax Fill
1 wire53%
2 wires31%
3+ wires40%

Per NEC Chapter 9, Table 1

Why Fill Limits Matter

• Prevents conductor overheating
• Allows easy wire pulling
• Prevents insulation damage
• Leaves room for future wires

Related

Wire GaugeVoltage Drop

Understanding Conduit Fill Calculations

Conduit fill is the percentage of the conduit's internal area occupied by conductors. The NEC limits fill percentages to prevent overheating, allow for proper heat dissipation, and make wire pulling practical without damaging conductor insulation.

NEC Fill Requirements

Per NEC Chapter 9, Table 1:

  • 1 conductor: 53% maximum fill
  • 2 conductors: 31% maximum fill
  • 3 or more conductors: 40% maximum fill

How to Calculate Conduit Fill

1. Look up wire cross-sectional area from NEC Chapter 9, Tables 5 and 5A

2. Multiply by number of conductors

3. Look up conduit internal area from NEC Chapter 9, Table 4

4. Divide total wire area by conduit area

5. Multiply by 100 for percentage

Important Notes

Equipment grounding conductors count toward fill. Different wire types (THHN, XHHW, THW) have different outside diameters even at the same gauge. Always use the correct table for your specific wire type.

Frequently Asked Questions

Per NEC Chapter 9 Table 1: 1 conductor = 53% fill, 2 conductors = 31% fill, 3 or more conductors = 40% fill. These limits apply to all conduit types and help prevent overheating while allowing easy wire pulling.

Conduit fill % = (Total wire area / Conduit internal area) × 100. Sum up conductor areas from NEC Chapter 9 Table 5, then divide by the conduit internal area from Table 4. This calculator does this automatically using NEC data.

Fill limits prevent conductor overheating by allowing air circulation around wires. They also make wire pulling easier without damaging insulation, and leave room for future conductor additions. Overfilled conduit can cause heat buildup and insulation failure.

Yes, equipment grounding conductors must be included when calculating conduit fill. All conductors in the raceway, regardless of purpose, contribute to the total fill percentage and affect heat dissipation.

EMT (Electrical Metallic Tubing) has the thinnest walls and largest internal area. IMC (Intermediate Metal Conduit) is thicker. RMC (Rigid Metal Conduit) has the thickest walls and smallest internal area. For the same trade size, you can fit more wires in EMT than rigid.

No, NEC conduit fill limits are code requirements, not recommendations. Exceeding them can cause conductors to overheat (potentially starting fires), make wire pulling difficult (damaging insulation), and will fail electrical inspection.

Per NEC Chapter 9, Table C.1: 3/4" EMT can hold up to 16 THHN 12 AWG conductors at 40% fill. For THWN, it's 10 conductors. Always verify with current NEC tables as this varies by insulation type.

Yes, NEC 314.16 covers box fill calculations. Box fill considers conductor volumes, device volumes, and clamp allowances. A separate calculation method is used - boxes don't use the same percentage fill as conduit.