CP Calculations Practice Questions

These questions test basic cathodic protection calculations used in field interpretation, design checks, and certification study.

Study Focus

Focus on unit control. Most CP calculation errors come from mixing amps and milliamps, inches and feet, percent and decimal, or ON and instant-off values.

Practice Questions

  1. A circuit has 24 volts and 6 ohms resistance. What is the current?

    1. 4 amps
    2. 144 amps
    3. 0.25 amps
    4. 30 amps

    Answer: A. I = V ÷ R = 24 ÷ 6 = 4 amps.

  2. A rectifier operates at 40 volts and 8 amps. What is the effective circuit resistance?

    1. 320 ohms
    2. 5 ohms
    3. 0.2 ohms
    4. 48 ohms

    Answer: B. R = V ÷ I = 40 ÷ 8 = 5 ohms.

  3. A bond carries 3 amps through 0.2 ohms. What is the voltage drop?

    1. 15 volts
    2. 3.2 volts
    3. 0.6 volts
    4. 0.067 volts

    Answer: C. V = I × R = 3 × 0.2 = 0.6 volts.

  4. Convert 750 milliamps to amps.

    1. 7.5 amps
    2. 75 amps
    3. 0.075 amps
    4. 0.75 amps

    Answer: D. 750 mA = 0.75 A.

  5. A 6-inch diameter pipe is 1,000 feet long. Approximate external surface area is:

    1. 1,571 ft²
    2. 18,850 ft²
    3. 500 ft²
    4. 6,000 ft²

    Answer: A. D = 6 ÷ 12 = 0.5 ft. Area = π × 0.5 × 1,000 = 1,571 ft².

  6. A coated structure has 10,000 ft² total area and 2 percent coating breakdown. What is the exposed area?

    1. 20 ft²
    2. 200 ft²
    3. 2,000 ft²
    4. 5,000 ft²

    Answer: B. 10,000 × 0.02 = 200 ft².

  7. Using 200 ft² exposed area and 3 mA/ft², what current is required?

    1. 600 mA
    2. 60 mA
    3. 6,000 mA
    4. 0.006 mA

    Answer: A. 200 × 3 = 600 mA, or 0.6 amps.

  8. A galvanic anode system has 30 amp-years usable capacity and outputs 1.5 amps. What is estimated life?

    1. 45 years
    2. 20 years
    3. 0.05 years
    4. 31.5 years

    Answer: B. Life = 30 ÷ 1.5 = 20 years.

  9. A polarized potential is −820 mV and the depolarized potential is −690 mV. What is the polarization change?

    1. 130 mV
    2. 690 mV
    3. 820 mV
    4. 1,510 mV

    Answer: A. The magnitude of the change is 130 mV.

  10. A tank has a 100-foot diameter. Approximate bottom area is:

    1. 314 ft²
    2. 7,854 ft²
    3. 10,000 ft²
    4. 31,416 ft²

    Answer: B. Radius = 50 ft. Area = π × 50² = 7,854 ft².

Exam-Aligned Calculation Areas

The uploaded exam preparation guides confirm that CP calculation practice should not stop at Ohm's law and surface area. Strong preparation also requires shunt current, soil resistivity, rectifier efficiency, Faraday's Law, reference electrode conversion, and anode groundbed resistance logic.

  • Use shunt millivolt drops to calculate current and identify current direction.
  • Use Wenner soil resistivity calculations to support anode-bed and field-test interpretation.
  • Use rectifier efficiency calculations to connect DC output with AC input power.
  • Use Faraday-type reasoning to connect current, time, and metal consumption.
  • Use reference electrode conversion carefully when readings are not on the same scale.

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