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Motor Calculations Practice for the California Exam

Motor wiring sizes off the table full-load current, not the nameplate, and the conductor carries 125% of it. On the California exam, calculations are part of the Determination of electrical system requirements domain. Here is the pattern, one worked example, and a question to try.

Last reviewed June 2026

One worked example

A 10 HP, 230-volt, three-phase motor. Table 430.250 gives a full-load current of 28 amps. Size the branch-circuit conductor.

  1. 1
    Name the problem

    Single motor conductor. Use the table FLC and the 125% rule.

  2. 2
    Pick the rule

    Branch-circuit conductors for a single motor carry 125% of the table full-load current (430.22).

    Conductor = 1.25 × FLC

  3. 3
    Pull the numbers

    Table FLC is 28 amps.

  4. 4
    Run the arithmetic

    Add the 25%.

    Conductor = 1.25 × 28 = 35 A

  5. 5
    Check it

    35 amps. Size the wire for 35, even though the nameplate might read lower.

Now try one

Your turn. A 5 HP, 230-volt, three-phase motor. Table 430.250 gives a full-load current of 15.2 amps.

What's the minimum conductor ampacity at 125%, in amps?

  1. 1
    Run the arithmetic

    Add the 25%.

    Conductor = 1.25 × 15.2 = 19 A

  2. 2
    Check it

    19 amps. Off the table value, always, not the motor's nameplate amps.

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