Okay, let's break down how to use Figure B.3 and your given Rho and LF values to determine the ampacity of a 500 kcmil conductor in six underground duct banks. Also, we'll address the use of the 0.74 ratio.

Understanding the Problem & Variables Rho (ρ)

Soil thermal resistivity, measures how well soil resists heat transfer. Your value is 60.

LF (Load Factor)

The ratio of average load to peak load over a period of time. You have 100%, meaning the average load is always at the peak (continuous load).

500 kcmil Conductor

The conductor size.

6 Underground Duct Banks

This significantly impacts the overall thermal resistance and ampacity. More ducts = more heat generated.

Figure B.3

Ampacity adjustment factors for varying Rho and LF values in underground installations.

I2/I1 Ratio

This is used to calculate the adjusted ampacity based on the Load Factor (LF).

Steps to Determine Ampacity Using Figure B.31. Locate Figure B.3

Find Figure B.3 in the relevant section of the NEC (National Electrical Code) or the specific standard you are referencing. You need the actual chart to perform the interpolation.2.

Find the Base Ampacity for 500 kcmil

Before you can use Figure B.3, you need the base ampacity of a 500 kcmil conductor. This base ampacity will be found in Table 310.16 (or a similar table based on the conductor insulation type and temperature rating you are using) assuming you're using 3 conductors in a raceway or cable.

This base ampacity assumes a specific set of standard conditions (usually 90°C conductor temperature, 30°C ambient temperature, and specified soil thermal resistivity).3. Determine the Number of Conductors Per Duct

You need to know how many current-carrying conductors are in each duct. Figure B.3 is sensitive to the total number of conductors in the bank and is dependent on conductor spacing and arrangement.4.

Locate Appropriate Curve

Figure B.3 will likely have multiple curves, each corresponding to a specific number of conduits and the number of conductors in each conduit. If the exact number of conduits/conductors for your installation does not appear, use the curve with more conduits/conductors.5.

Find Adjustment Factor for Rho

On the horizontal axis of Figure B.3, find your Rho value (60). Follow a vertical line up from 60 until it intersects the curve corresponding to your installation configuration. Read the adjustment factor from the vertical axis.6.

Find Adjustment Factor for Load Factor

Enter the LF (100%) on the horizontal axis, and find the curve that corresponds to the ratio (0.74). Then read the adjustment factor on the vertical axis.
7.

Calculate the Adjusted Ampacity



Adjusted Ampacity = (Base Ampacity from Table 310.16) (Adjustment Factor for Rho) (Adjustment Factor for Load Factor)Is the I2/I1 Ratio Still Considered 0.74?Yes, but with important clarification. The I2/I1 ratio is still the basis for determining the adjustment factor based on the LF. However, the curve in Figure B.3 is based on the assumption that the relationship between load factor and ampacity reduction follows the following equation:

Ampacity_Adjusted = Ampacity_Base  sqrt(LF/100)

Where sqrt(LF/100) is approximately .74 when LF is at 55. This factor is built into the graph.

Important Considerations and Cautions



Figure B.3 Accuracy

Figure B.3 is an approximation. The actual ampacity may vary depending on specific installation details.

Conductor Temperature Rating

Make sure you are using the correct base ampacity from Table 310.16 for the temperature rating of your conductor insulation (e.g., 75°C, 90°C).

Ambient Temperature

Table 310.16 base ampacities are based on a specific ambient temperature (usually 30°C). If your actual ambient temperature is higher, you'll need to apply an additional temperature correction factor from Table 310.15(B)(1) or similar table.

Complete NEC Compliance

This guidance is based on typical NEC practices. Always consult the latest edition of the NEC and any applicable local codes for complete and accurate requirements.

Professional Engineering Advice

For complex or critical installations, it's always best to consult with a qualified electrical engineer to ensure proper and safe design.

Duct Arrangement

Figure B.3 is based on specific duct bank configurations. If your duct bank arrangement is significantly different, the adjustment factors may not be accurate.

In Summary:1. Find the base ampacity of your 500 kcmil conductor.
2. Obtain Figure B.3 from the NEC or relevant standard.3. Determine the appropriate curve based on the number of ducts and conductors per duct.
4. Find the adjustment factors for Rho and LF from the chart.5. Multiply the base ampacity by the adjustment factors to get the final adjusted ampacity.
6. Make sure your math and reasoning are correct.Without having access to the actual Figure B.3 chart, I cannot provide specific numerical values for the adjustment factors or the final ampacity. You'll need to do the interpolation from the chart itself. Flag for review