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How to Calculate Conduit Fill

The purpose of this article is to describe conduit fill and explain in detail how to calculate conduit fill. The step by step workflow in accordance with NEC will be presented.

• Overview

Conduit fill is the percentage of a conduit’s cross-sectional area that is occupied by electrical wires. The wire outside diameter and the conduit inside diameter can be read from manufacturer data or from National Electrical Code typical value.

• What is allowable conduit fill ?

Table 1 of NEC Chapter 9 gives general guidelines for conduit fill percentage.


It should be noted from the allowable conduit fill that equipment grounding or bonding conductors shall be included when calculating conduit or tubing fill.

• Conduit fill calculation example

Let’s examine how to calculate conduit fill apply to a simple example of 2-#6 AWG THHN and 3-#3 AWG THWN wire in a 11/4 inch metal conduit RMC type.

Refer to table 4 and table 5 of NEC for conduit inner area and conduit outer area respectively:

  • 1 1/4 inch RMC Conduit: 1.53 in2
  • 2 nos. of #6 THHN wire: 2 * 0.0507 in2 = 0.1014 in2
  • 3 nos. of #3 THWN wire: 3 * 0.0973 in2 = 0.2919 in2

Therefore, conduit fill is : (0.1014 + 0.0973) / 1.53 = 25% which is well below allowable factor for more than 2 wires (40%).

There is a tool here to verify this calculation or just simply reduce time spending on doing the math.