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I am using 6.5 ft (2 meters) BELDEN 9154 shielded twisted pair cable for a sub 1 MHz application and I wanted to make a model of the cable for some simulations. The datasheet of the cable lists two capacitance as shown below: enter image description here How to interpret these capacitance? For a 1 ft segment what values of capacitance is observed across the conductors and across shield and one of the conductors?

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The specification you're refering to must be interpreted in the following way:

  1. You take one conductor and connect it to shield.
  2. Then you take the other conductor and measure the capacitance across it and the other one.

Now take a look at this diagram:

Partial capacitances

Source of image: this paper. Ignore capacitances \$C_1\$, \$C_2\$, \$C_E\$, and assume that \$C_{S1}=C_{S2}\$.

From the diagram we can see that, because you're shorting one of the conductor to shield, the capacitance measured must be then the parallel of the conductor-to-conductor capacitance \$C_{12}\$ and the conductor-to-shield capacitance \$C_{S1}\$.

Capacitances in parallel add together, so we can calculate the conductor-to-shield capacitance \$C_{S1}\$ from the specification, just subtracting. In this case, the conductor-to shield capacitance \$C_{S1}\$ is just 100 pF/ft - 60 pF/ft = 40 pF/ft. That's a 40 pF/ft from each conductor to shield (two capacitances) in your model. Exactly like analogsystemsrf has outlined in his answer.

This is only for the capacitance. If you want a complete lumped model don't forget the rest of parameters (inductance and resistance per length unit) that you can find in the datasheet.

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I'd draw the circuit like this

schematic

simulate this circuit – Schematic created using CircuitLab

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