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    Calculates locally in your browser · motor branch circuits to Article 430
    Electrical / motors

    Motor full-load current and branch protection

    Article 430 is built on table values, not nameplate values. The full-load current comes from Table 430.250 or 430.248, and every downstream decision — conductor, overload, short-circuit protection, controller — is a percentage of that table number. Sizing from the nameplate is the most common error in a motor circuit, and it fails inspection because the code says the table governs.

    Device35 AController17.5 AUtilisation40%
    Full-load current
    14.0 A
    Table 430.250, not the nameplate
    Conductors
    17.5 A
    125% of FLA, §430.22
    Overload setting
    17.5 A
    125% (SF ≥ 1.15), up to 19.6 A
    Branch protection
    35 A
    250% of FLA, rounded up
    Every value in Article 430
    All of them derive from one table number, and none of them from the nameplate.
    14.0 A FLA
    Every value in Article 430
    ItemValueRule
    Motor10 hp, 3-phase, 460 Vas entered
    Full-load current14.0 ATable 430.250
    Branch conductors17.5 A125% of FLA, §430.22
    Overload target17.5 A125% (SF ≥ 1.15)
    Overload maximum19.6 A§430.32(C)
    Controller rating17.5 A125% of FLA, §430.83
    Protection target35.0 A250% of FLA, Table 430.52
    Standard device35 Anext size up, §240.6
    Ceiling if it trips56.0 A430.52 Exception No. 1

    A 10 hp 3-phase motor at 460 V draws 14 A from the table. Conductors are sized at 125% of that, and the overload is set between 17.5 A and 19.6 A.

    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Full-load current against horsepower
    Nearly linear once the magnetising current stops dominating the small sizes.
    Full-load current against horsepower0.00027.7855.5583.33111.10138.881 hp1 hp2 hp2 hp3 hp3 hp5 hp5 hp7.5 hp7.5 hp10 hp10 hp15 hp15 hp20 hp20 hp30 hp30 hp50 hp50 hp75 hp75 hp100 hp100 hpAmps
    The four Article 430 numbers
    Conductors and controller sit at 125%; protection is far above, because it only has to carry starting current.
    The four Article 430 numbers0.0008.2616.5224.7833.0441.3014.00FLAFLA17.50ConductorsConductors17.50OverloadOverload35.00ProtectionProtectionAmps
    Canonical equations
    The exact relationships implemented by this engine.

    Table 430.250 / 430.248 · By horsepower and voltage.

    1.25 × FLA · §430.22.

    1.25 or 1.15 × FLA · By service factor, §430.32.

    multiplier × FLA · Table 430.52, then next standard size.

    engineering methodology

    A clear path from inputs to results.

    Article 430 is built on table values, not nameplate values. The full-load current comes from Table 430.250 or 430.248, and every downstream decision — conductor, overload, short-circuit protection, controller — is a percentage of that table number.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Choose the horsepower, phases and voltage.
    2. Set the service factor, which selects the overload rule.
    3. Pick the branch protection type.
    4. Read the FLA and the four derived ratings.

    The formula

    Icond=1.25 IFLA  ;  Iprot=k IFLAI_{cond} = 1.25\,I_{FLA} \;;\; I_{prot} = k\,I_{FLA}

    Reference: NFPA 70 Article 430: Table 430.250 and 430.248 for FLA, §430.22, §430.32, §430.52 and §430.83.

    Worked example

    10 hp at 460 V, three phase

    Inputs
    • 10 hp, three phase, 460 V.
    • Service factor 1.15.
    • Inverse-time breaker protection.
    Calculation path
    1. 1

      Table 430.250 gives 14 A for a 10 hp motor at 460 V.

    2. 2

      Branch conductors are 125% of that, or 17.5 A.

    3. 3

      With a 1.15 service factor the overload is set at 125%, also 17.5 A, with a ceiling of 140% or 19.6 A.

    4. 4

      A breaker is sized at 250% of FLA, or 35 A, which is already a standard rating.

    5. 5

      The controller is rated at 125%, or 17.5 A.

    Result

    14 A FLA · 17.5 A conductors · 17.5 A overload · 35 A breaker.

    Interpreting the result

    The protection device is sized far above the full-load current because it only has to survive the starting current — typically six times FLA for a few seconds. The overload, by contrast, is set just above the running current, because that is what protects the motor from sustained overheating.

    Assumptions & limitations

    • Table values are for ordinary induction motors at the listed voltages.
    • The service factor selects the 125% or 115% overload rule.
    • The branch device is the standard size; the 430.52 ceiling is reported separately.
    • No motor circuit protector, soft starter or drive is modelled.

    Frequently asked questions

    Why not use the nameplate current?

    Because §430.6(A)(1) says the table governs for conductor and protection sizing. The nameplate is only used when the motor is not listed in the table.

    Why 250% for a breaker?

    Because a motor draws roughly six times its full-load current while starting, and the breaker must not trip on that. The overload handles the running condition.

    Does a VFD change this?

    Yes — a drive limits starting current, so the branch protection is sized to the drive input rather than the motor table, and the rules move to Article 430 Part X.

    What is the difference between overload and branch protection?

    The overload protects the motor from sustained overcurrent; the branch device protects the conductors from short circuit. They are sized from different percentages and neither replaces the other.

    What if the standard breaker trips on start?

    430.52 Exception No. 1 allows the next size up, up to 400% for a breaker or 225% for a dual-element fuse, provided the conductors still protected.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.