Okay, let's break down this wire ampacity calculation problem step-by-step, considering the specific conditions you've described. It's good you're being thorough with the ambient temperature and conduit fill calculations. Here's a structured approach

1. Determine the Starting Point: Ampacity of the Conductors
You're using THWN-2 wire, rated for 90°C. This is good because you can start your calculations based on the 90°C ampacity, even though you'll likely need to derate later. Consult NEC Table 310.16 (Ampacities of Insulated Conductors Rated Up To and Including 600 Volts) for the ampacity of #10 and #12 copper conductors at 90°C. #10 Copper (90°C)

Generally, this will be 40 amps.

#12 Copper (90°C)

Generally, this will be 30 amps.

2. Account for Ambient Temperature Correction This is the crucial step where you're considering the soil temperature and its impact. Understanding the Issue

Conduit buried in concrete, especially a slab-on-grade without insulation, can be significantly affected by the surrounding earth temperature. Heat generated by the wires cannot dissipate easily.

Soil Temperature Information

You've referenced a source for soil temperature maps (Syngenta GreenCast).

You must get an accurate local soil temperature for your specific location and depth. The average daily soil temperature at the depth of your conduit run is the value you need. Do not simply use air temperature or a generic map. This is critical.

NEC Table 310.15(B)(2)(a) Correction Factors

Once you have your accurate local soil temperature, compare it to the 30°C (86°F) listed at the top of Table 310.15(B)(2)(a). If your soil temperature is higher than 30°C, you'll need to apply a derating factor. You've mentioned a 104% correction factor.

This is incorrect. Correction factors will only be greater than 100% when the ambient temperature is lower than 30°C.

Example

Let's say your local soil temperature is 40°C (104°F). Look at Table 310.15(B)(2)(a). Find the temperature range that includes 40°C (in this case, it will likely be the range 31-35°C or 36-40°C). Read across to the 90°C column. You will find a correction factor that is less than 1.0. This will be your derating factor. The same row will list the correction factors for the other conductor insulation temperature ratings.

Apply the Correction

Multiply the initial 90°C ampacity (40A for #10, 30A for #12) by the derating factor you found in the table. This gives you the ampacity adjusted for ambient temperature.

3. Conductor Fill and Derating for the Number of Conductors

NEC Table 310.15(B)(3)(a)

This table dictates the ampacity adjustment factors for more than three current-carrying conductors in a raceway.

Grounded conductors are not typically considered current-carrying conductors for this derating, but ungrounded conductors are.

Counting Current-Carrying Conductors

You have: #10 ungrounded conductors: Count these. #12 grounded conductors:

Do not count these for derating purposes unless they carry current under normal conditions.

Derating Factor

Based on the total number of current-carrying conductors, find the appropriate derating factor in Table 310.15(B)(3)(a). For example: 4-6 Current-Carrying Conductors: 80% derating factor. 7-9 Current-Carrying Conductors: 70% derating factor.

Apply the Derating

Multiply the ampacity (already adjusted for ambient temperature) by the derating factor from Table 310.15(B)(3)(a).

4. Conduit Fill Calculation

NEC Chapter 9, Table 4

This table specifies the maximum percentage of conduit fill permitted. For a single raceway (your 1" PVC), the maximum fill is 40%.

NEC Chapter 9, Table 5A

This table gives the dimensions of conductors and their corresponding area. Look up the area of #10 THWN-2 and #12 THWN-2.

Calculation

1. Multiply the area of each conductor type by the number of conductors of that type.
2. Sum the areas of all conductors.3. Find the internal area of 1" PVC conduit (NEC Chapter 9, Table 4).
4. Divide the total conductor area by the conduit area and multiply by 100%. This gives you the percentage fill.5. Ensure that the percentage fill is less than or equal to 40%. If you exceed 40% you must increase the conduit size or reduce the number of conductors.

5. Final Ampacity and Overcurrent Protection The result of all the derating steps gives you the allowable ampacity of your conductors in this specific installation.

Overcurrent Protection

You must protect the conductors with overcurrent protection (circuit breakers or fuses) rated at or below the final, derated ampacity. Refer to NEC 240.4.

Important Considerations and Cautions



Accuracy of Soil Temperature

This is paramount. Use reliable, local soil temperature data. A probe placed at the depth of the conduit is the most accurate method.

Continuous Load

If the load on the circuit is considered "continuous" (operates at its maximum current for 3 hours or more), you must further reduce the allowable load to 80% of the calculated ampacity (NEC 210.19(A)(1) Note and 215.2(A)(1) Note).

Terminations

Pay close attention to the temperature rating of your terminations (breakers, receptacles, etc.). Even though the wire is rated for 90°C, the termination may only be rated for 75°C or even 60°C. You must use the ampacity that corresponds with the lowest rated component in the circuit. Most terminations are only rated for 75 degrees C. In this case, even if the starting ampacity for #12 wire THWN-2 is 30 amps, the ampacity at 75C is 25 amps. Use the 25 amp value as your starting point.

Metal Conduit Section

The transition to metal conduit is important for grounding and bonding.

Local Codes

Always consult and comply with all applicable local electrical codes and regulations. They may have more stringent requirements than the NEC.

Professional Advice

For complex installations, it's always best to consult with a qualified and licensed electrician. They can provide accurate assessments and ensure a safe and code-compliant installation.

In Summary:1. Get accurate local soil temperature data.
2. Start with the 90°C ampacity of the conductors.3. Derate for ambient temperature (using Table 310.15(B)(2)(a)).
4. Derate for the number of current-carrying conductors (using Table 310.15(B)(3)(a)).
5. Calculate conduit fill.6. Select the appropriate overcurrent protection.
7. Check the temperature ratings of the terminations.8. Consider continuous load requirements.
9. Comply with all local codes.10. Consult a qualified electrician if needed.This detailed breakdown should help you accurately calculate the allowable ampacity for your conductors and ensure a safe and code-compliant electrical installation. Remember that accuracy is crucial, especially when dealing with heat dissipation in buried conduits. Good luck! Flag for review