Okay, let's break down the feasibility of using a single 2 1/2" conduit for twenty 12/2 NM-B cable homeruns. There are several things to consider

1. Conduit Fill Calculation (Most Critical)
NM-B Cable Size

12/2 NM-B cable is not round. Its dimensions vary slightly by manufacturer, but generally, expect it to be roughly equivalent to a cable with an outer diameter of .39 inches. Let's assume 0.39" as a conservative average.

Cross-Sectional Area of Each Cable

The area is calculated by Area = πr², where r is the radius (half the diameter). Area = π (0.39/2)² = π 0.195² = approximately 0.1195 square inches per cable.

Total Cross-Sectional Area of All Cables

20 cables 0.1195 sq in/cable = 2.39 square inches.

Conduit Fill Allowance (Crucial for 2 1/2" EMT)

For more than two conductors or cables, the maximum conduit fill is 40% of the conduit's interior area. A 2 1/2" EMT conduit has an interior area of approximately 3.34 square inches.

Allowable Fill Area

3.34 sq in 40% = 1.336 square inches.

Conclusion on Fill

Your 2.39 square inches of cable area is significantly greater than the allowable 1.336 square inches in a 2 1/2" EMT conduit.

This clearly violates code and is not permitted.2. Derating

Even if you could physically squeeze that many cables into the conduit (which you can't legally do), the heat generated by the conductors would require significant ampacity derating. With 20 current-carrying conductors in a single raceway, you'd need to derate the ampacity of the 12 AWG conductors to a very low value, potentially making them unsuitable for their intended purpose.

3. Practicality

Even ignoring code, trying to pull that many NM-B cables through a 2 1/2" conduit would be extremely difficult. NM-B is not designed for tight conduit installations. It's relatively inflexible and prone to damage during pulling.

Code and Safety Considerations



NEC Article 310.15 deals with ampacity of conductors and derating factors. NEC Chapter 9, Table 1 lists conduit dimensions and fill percentages. (Specifically, refer to Table 4 and associated notes regarding adjustments for more than three current-carrying conductors in a raceway.)Recommendations


Revert to Two Conduits

As originally planned, using two separate conduits is the only code-compliant and practical solution here.2.

Increase Conduit Size (Still Not Recommended)

You could theoretically calculate the necessary conduit size that would meet the 40% fill requirement, but it would likely be larger than a 3". Even if you calculate the correct size, the difficulty in pulling so many NM-B cables remains a significant issue. This is not the best approach.3.

Consider THHN/THWN-2 Conductors

If you absolutely must use a single conduit, consider using individual THHN/THWN-2 conductors instead of NM-B cable. These wires are designed for conduit installations and have a smaller overall diameter than NM-B. You would need to use proper junction boxes and wire nuts at each end to transition to NM-B wiring in the wall cavities. Even with THHN, you'd need to calculate the correct conduit size and derate ampacity as necessary. This would require a complete change of wiring strategy. However, this will require more labor and material cost.

In summary, using a single 2 1/2" conduit for 20 12/2 NM-B homeruns is a bad idea, violates the NEC, and is impractical. Use two conduits as you originally planned. Flag for review