Okay, let's break down your plan and analyze the considerations for combining those branch circuits into fewer conduits running from the MDP to the sub-panel.

Understanding the Situation You have seven separate 60A circuits you need to run from the MDP to a subpanel. Each run is about 80 feet long. You intend to run these circuits in RGS (Rigid Galvanized Steel) conduit. You're considering combining circuit pairs to reduce the number of conduits needed. Your primary concern is conduit fill and ampacity derating.

Assumptions (Crucial for Calculations) Conductor Size

#6 AWG copper conductors.

Conductor Insulation

THHN/THWN-2 (or similar, rated for 90°C). This is the most common for conduit runs and allows you to use the 90°C ampacity rating for derating purposes.

Neutral

You're assuming a neutral conductor is required for each circuit.

Ground (Equipment Grounding Conductor - EGC)

You are assuming a ground conductor is required for each circuit.

Analysis of Combining Two Circuits

Let's focus on combining two 60A circuits in a single conduit, as you proposed.1. Conductors per Conduit

Two hots (#6 AWG) Two neutrals (#6 AWG) One ground (Assuming #10 AWG, per NEC 250.122 for a 60A circuit)

Total: 5 conductors2. Ampacity of #6 AWG Copper (90°C Rating)

According to NEC Table 310.16, #6 AWG copper THHN/THWN-2 is rated for

75A at 90°C.3. Derating Factor

NEC 310.15(C)(1) requires derating the ampacity when there are more than three current-carrying conductors in a raceway. For 4-6 current-carrying conductors, the derating factor is 80%. In this case, the hots and the neutrals are considered current-carrying. The ground wire is not considered a current carrying conductor.
4.

Derated Ampacity

75A 0.80 =

60A5. Conduit Size

You'll need to calculate the appropriate conduit size.

Conductor Area

Consult NEC Chapter 9, Table 5A for conductor dimensions. Approximate values for THHN: #6 AWG: 0.0507 sq. in. #10 AWG: 0.0211 sq. in.

Total Conductor Area

(4 0.0507 sq. in.) + (1 0.0211 sq. in.) = 0.224 sq. in.

Conduit Fill

For over two conductors in a conduit, the maximum fill is 40% (NEC Chapter 9, Table 1). Consult NEC Chapter 9, Table 4 for RGS conduit dimensions. A 1" RGS conduit has an internal area of 0.864 sq. in. 40% Fill Area: 0.864 sq. in. 0.40 = 0.346 sq. in.Since 0.224 sq. in. (total conductor area) < 0.346 sq. in. (40% fill), a

1" RGS conduit is sufficient for the two circuits.

Important Considerations Voltage Drop

At 80 feet, voltage drop may not be a significant issue with #6 AWG for a 60A load, but it's crucial to

calculate and verify. Use a voltage drop calculator (many are available online) to ensure it stays within acceptable limits (typically 3% for branch circuits). If voltage drop is excessive, you might need to upsize the conductors, which affects conduit fill calculations. Ground Fault Protection

Make sure your ground fault protection scheme is appropriate for the combined circuits.

Local Codes

Always check and adhere to your local electrical codes, which may have stricter requirements than the NEC.

Summary and Recommendation

Combining two 60A circuits using #6 AWG conductors in a 1" RGS conduit, with the appropriate derating, seems feasible based on the assumptions made. HOWEVER, it's essential to


Confirm Conductor Type

Ensure you are using THHN/THWN-2 or similar conductors with a 90°C rating.2.

Perform Voltage Drop Calculations

Voltage drop is critical. Calculate it precisely using your actual load characteristics.3.

Verify Local Codes

Your local electrical codes take precedence.4.

Document Everything

Keep detailed records of your calculations and decisions.5.

Consult with a Qualified Electrician

It's

strongly recommended to have a licensed and qualified electrician review your plan and perform the installation. They can assess the specific conditions of your installation and ensure compliance with all applicable codes.By combining the circuits, you'll reduce the number of conduits, but you'll increase the conductor count within each conduit. Careful planning and adherence to the NEC are essential for a safe and compliant installation. Flag for review