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Motor Circuit Calculator: NEC 430 Wire, Breaker and Overload

Conductors, overload setting and breaker or fuse size for one motor under NEC Article 430.

Tool referenceDE-TOOL-8979

Reference: NEC 430.22, 430.32, Table 430.52
Motors: Single-phase and three-phase, up to 600 V
Output: Conductors, overload and breaker or fuse size

This motor circuit calculator sizes the branch circuit for one motor under NEC Article 430. It takes the full-load current from Table 430.248 or 430.250, sizes the conductors at 125% of it (430.22), sets the overload at 115% or 125% of the nameplate current (430.32) and gives the largest breaker or fuse allowed by Table 430.52. For a 10 hp, 460 V three-phase motor the table current is 14 A, the conductors need 17.5 A, which 14 AWG copper carries at 75 °C, and an inverse time breaker may be up to 35 A.

Motor circuit calculator (NEC 430)

Size the branch circuit for one motor under NEC Article 430: the table full-load current, the conductors at 125%, the overload setting and the largest breaker or fuse for short-circuit and ground-fault protection. Pick a quick start or enter your motor; the answer updates as you type.

The calculator needs JavaScript. The method and the NEC tables it uses are below.

Quick start Fills in the form. Change anything after.

The motor

System

Three-phaseSingle-phase

Motor voltage

Use the motor nameplate voltage. The NEC tables use 115, 200, 208, 230, 460 and 575 V; a 480 V motor uses the 460 V column, a 240 V motor the 230 V column.

Horsepower

From the motor nameplate. The table currents are for motors running at usual speeds with normal torque characteristics; low-speed, high-torque and multispeed motors use the nameplate current instead (430.6(A)(1)).

Motor type

Most general-purpose NEMA motors sold today are Design B premium efficiency. The design letter only changes the instantaneous trip setting in Table 430.52.

Nameplate full-load current (A, optional)

The full-load amps (FLA) printed on the motor nameplate. NEC 430.6(A) uses the table current for conductors and short-circuit protection and the nameplate current for the overload.

For the overload setting only. Blank: the table current is used until you enter it.

Service factor 1.15 or more, or temperature rise 40 °C or less (nameplate)

Protection and conductors

Short-circuit and ground-fault device

Inverse time breakers are the usual molded-case breakers. Dual-element fuses (such as class RK5 or J time-delay) allow the smallest rating. Instantaneous trip breakers are only allowed as part of a listed combination motor controller.

Conductor material

CopperAluminum

Terminal rating

Motor circuits of 100 A or less use 60 °C conductors and terminals, but conductors rated 75 °C or more may be used at their 75 °C ampacity for motors marked Design B, C or D (110.14(C)(1)(a)(4)). Those design letters are for three-phase motors: a single-phase motor circuit stays at 60 °C unless all its equipment is listed for 75 °C.

More options

Conductor insulation

Ambient temperature (°C)

°C°F

Current-carrying conductors together

Count the phase conductors in the raceway or cable: three for one three-phase motor. Equipment grounding conductors do not count. More than three lowers the ampacity.

Result

Branch circuit for a 10 hp, 460 V motor

A 10 hp, 460 V three-phase motor has a full-load current of 14 A in the NEC tables. The conductors need 17.5 A (125%), so 14 AWG copper (20 A). The inverse time breaker may be up to 35 A, and the overload is set at 17.5 A (125% of the table current; set it from the nameplate).

Branch-circuit conductors: 14 AWGcopper
Full-load current, NEC table: 14 A
Ampacity needed (125%): 17.5 A
Largest inverse time breaker: 35 A
Overload setting (125%): 17.5 A

250% of the full-load current is 35 A, a standard rating: 35 A. If the motor will not start: up to 400% of the full-load current, 56 A, so up to 50 A (430.52(C)(1)(b)).

The overload is shown from the table current. Enter the nameplate current to set it.

From the top: the short-circuit and ground-fault device, the branch-circuit conductors (highlighted), the overload and the motor.

Screening estimate. Check the controller, overload and drive instructions, which can require other settings. Electrical design and coordination studies cover the whole system, or ask an engineer.

Full results and steps

Full-load current, NEC table	14 A
Conductor ampacity needed (125%)	17.5 A
Conductor	14 AWG copper, 20 A
Overload (125%)	17.5 A
Overload, highest if the motor trips on starting (140%)	19.6 A
inverse time breaker, 250%	35 A, so 35 A
time-delay fuse, 175%	24.5 A, so 25 A
nontime-delay fuse, 300%	42 A, so 45 A
instantaneous trip breaker, 1100%	a setting of 154 A
Disconnect rating, at least (115%)	16.1 A

Full-load current: 10 hp at 460 V, Table 430.250: 14 A. The table value, not the nameplate, sizes the conductors and the short-circuit protection (430.6(A)(1)).
Conductors: 1.25 × 14 A = 17.5 A (430.22).
14 AWG copper: 25 A in the 90 °C column × 1.00 for 30 °C × 1.00 for 3 conductors = 25 A; 20 A in the 75 °C terminal column; the lower, 20 A, is at least 17.5 A.
Overload: 125% × 14 A (table current; set it from the nameplate) = 17.5 A; up to 140%, 19.6 A, where that is not enough to start or run the motor (430.32(A)(1), 430.32(C)).
Short-circuit and ground-fault protection, inverse time breaker: 250% × 14 A = 35 A, so up to 35 A (Table 430.52).
Disconnecting means: horsepower rated and at least 115% × 14 A = 16.1 A (430.110(A)).

Motor control centers, drives or a plant full of motors?

Dreiym's licensed Professional Engineers design motor circuits and feeders and coordinate their protection with the rest of the system, PE-stamped where required. Free quote; we typically respond within one business day.

Safety and limits

What this does not check: several motors on one feeder (430.24 and 430.62), motors on adjustable-speed drives (430.122, 430.130 and the drive instructions), hermetic refrigerant motor-compressors (Article 440), fire pumps (Article 695), motors over 1,000 V, voltage drop during starting, and the equipment grounding conductor (250.122(D)). Small motors under the 430.32(B) and (D) rules can have other overload options.

Screening tool: Screening estimate for education and planning. Not an engineering study or a design, and not a substitute for a licensed engineer's review where one is required.

How the calculation works

Full-load current from Table 430.248 (single-phase) or 430.250 (three-phase) for the horsepower and voltage. It sizes the conductors and the short-circuit protection; the nameplate current sets the overload (430.6(A)).
Conductors: ampacity at least 125% of the table current (430.22), from Table 310.16 with the ambient and adjustment factors of 310.15, and no more than the terminal column. 240.4(D) does not apply to motor circuits (240.4(G)).
Overload: 125% of the nameplate current for a service factor of 1.15 or more or a temperature rise of 40 °C or less, 115% for other motors (430.32(A)(1)); up to 140% or 130% where that is not enough (430.32(C)).
Short-circuit and ground-fault protection: the Table 430.52 percent of the table current. A value that is not a standard rating may go to the next standard rating (430.52(C)(1)(a)). Where that will not start the motor, 430.52(C)(1)(b) allows up to 400% for an inverse time breaker on a full-load current of 100 A or less (300% above), 225% for a time-delay fuse, 400% for a nontime-delay fuse of 600 A or less or a Class CC time-delay fuse, and 300% for a fuse of 601 to 6,000 A; the calculator shows the option that allows the most. In the 2020 edition these were Exceptions No. 1 and No. 2 to 430.52(C)(1).
Disconnecting means: at least 115% of the table current (430.110(A)).

Table 430.52 percentages used

NEC Table 430.52: maximum rating or setting, percent of the full-load current.
Motor type	Nontime-delay fuse	Time-delay fuse	Instantaneous trip breaker	Inverse time breaker
Single-phase	300	175	800	250
Squirrel cage, other than Design B energy efficient or premium efficiency	300	175	800	250
Design B energy efficient or premium efficiency	300	175	1,100	250
Wound rotor	150	150	800	150

Sources

Sources: NFPA 70, National Electrical Code, 2023 edition: 110.14(C)(1)(a), 240.4(G), 240.6(A), Table 310.15(B)(1)(1), Table 310.15(C)(1), Table 310.16, 430.6(A), 430.22, 430.32(A)(1) and (C), 430.52(C)(1)(a) and (b) and 430.52(C)(3) with Table 430.52(C)(1) (Table 430.52 in the 2020 edition), 430.110(A), Tables 430.248 and 430.250. Check the edition your jurisdiction enforces.

CopiedResult copied, with a link that keeps your inputs.Could not copy. Copy the address from the address bar instead.Reset to the starting example.InputsScreening estimate, not an engineering study.{h} hpcopperaluminuminverse time breakertime-delay fusenontime-delay fuseinstantaneous trip breaker(°C)(°F){p}% of the full-load current is {raw} A, a standard rating: {m} A.{p}% of the full-load current is {raw} A; the next standard rating, {m} A, is permitted (430.52(C)(1)(a)).A setting of up to {p}% of the full-load current: {a} A.If the motor will not start: up to {p}% of the full-load current, {a} A, so up to {m} A (430.52(C)(1)(b)).If the motor will not start: up to {p}%, {a} A, so up to {m} A (430.52(C)(1)(b)).If the motor will not start: up to {p}%, {a} A, so up to {m} A, for fuses of 600 A or less (430.52(C)(1)(b)).If the motor will not start: up to {p}%, {a} A, for nontime-delay fuses of 600 A or less, so up to {m} A (430.52(C)(1)(b)).If the motor will not start: a Class CC time-delay fuse may be up to {p}%, {a} A, so up to {m} A (430.52(C)(1)(b)).If the motor will not start: a Class CC time-delay fuse may be up to {p}%, {a} A, which allows the largest Class CC rating, {m} A (430.52(C)(1)(b)).If the motor will not start: a fuse of 601 to 6,000 A may be up to {p}%, {a} A, so up to {m} A (430.52(C)(1)(b)).If the motor will not start: a setting of up to {p}%, {a} A (430.52(C)(3)).If the motor will not start, 430.52(C)(1)(b) allows no larger rating here.an inverse time breakera time-delay fusea nontime-delay fusean instantaneous trip breakerInverse time breakerTime-delay fuseNontime-delay fuseInstantaneous breakersetting up to {a} Aup to {m} A{size} {mat}needs {a} A{a} A{hp}, {v} VFLC {a} AOne-line from the top: {dev} of up to {m} A, {wire} conductors, an overload set at {ol} A, and the {hp} motor at {v} V with a full-load current of {flc} A.One-line from the top: {dev} with a trip setting of up to {m} A, {wire} conductors, an overload set at {ol} A, and the {hp} motor at {v} V with a full-load current of {flc} A.Check {list}., Check the highlighted fieldsCheck the highlighted fields.No single conductor up to 1000 kcmil is large enoughThe conductors need {a} A, more than one 1000 kcmil conductor carries under these conditions. Use conductors in parallel.{size} {mat}Branch circuit for a {hp}, {v} V motorLargest {dev}Largest instantaneous trip settingOverload setting ({p}%)A {hp}, {v} V {ph} motor has a full-load current of {flc} A in the NEC tables. The conductors need {need} A (125%), so {wire} ({amp} A). The {dev} may be up to {m} A, and the overload is set at {ol} A ({olp}% of the {olb}).A {hp}, {v} V {ph} motor has a full-load current of {flc} A in the NEC tables. The conductors need {need} A (125%), so {wire} ({amp} A). The instantaneous trip setting may be up to {m} A, and the overload is set at {ol} A ({olp}% of the {olb}).three-phasesingle-phasetable current; set it from the nameplatenameplate current{wire}; {dev} up to {m} A.{wire}; instantaneous trip setting up to {m} A.Full-load current, NEC tableConductor ampacity needed (125%)ConductorOverload ({p}%)Overload, highest if the motor trips on starting ({p}%){dev}, {p}%{raw} A, so {m} Aa setting of {a} ADisconnect rating, at least (115%)Motor branch circuitFull-load current: {hp} at {v} V, Table 430.250: {flc} A. The table value, not the nameplate, sizes the conductors and the short-circuit protection (430.6(A)(1)).Full-load current: {hp} at {v} V, Table 430.248: {flc} A. The table value, not the nameplate, sizes the conductors and the short-circuit protection (430.6(A)(1)).Conductors: 1.25 × {flc} A = {need} A (430.22).{wire}: {a} A in the {ic} °C column × {fa} for {t} × {fj} for {n} conductors = {ac} A; {at} A in the {tc} °C terminal column; the lower, {amp} A, is at least {need} A.Overload: 125% × {b} A ({src}) = {ol} A; up to 140%, {mx} A, where that is not enough to start or run the motor (430.32(A)(1), 430.32(C)).Overload: 115% × {b} A ({src}) = {ol} A; up to 130%, {mx} A, where that is not enough to start or run the motor (430.32(A)(1), 430.32(C)).Short-circuit and ground-fault protection, {dev}: {p}% × {flc} A = {raw} A, so up to {m} A (Table 430.52).Short-circuit and ground-fault protection, {dev}: {p}% × {flc} A = a setting of {raw} A (Table 430.52; only in a listed combination controller).Disconnecting means: horsepower rated and at least 115% × {flc} A = {disc} A (430.110(A)).{v}: {wire}, {dev} up to {m} A, overload {ol} A.{v}: {wire}, instantaneous trip setting up to {m} A, overload {ol} A.the voltagethe horsepowerthe nameplate currentthe ambient temperaturethe number of conductorsSingle-phase motors: choose 115, 200, 208 or 230 V (Table 430.248).Choose a voltage from the list.Table 430.248 lists single-phase motors from 1/6 to 10 hp.Table 430.250 lists 115 V three-phase motors from 1/2 to 2 hp only.Table 430.250 starts at 1/2 hp for three-phase motors.Table 430.250 lists motors up to {max} hp at {v} V.Enter more than 0 and up to 2,000 A, or leave it blank.Enter a number, for example 12.6.Enter the ambient temperature.Enter −40 to 85 °C.Enter −40 to 185 °F.Enter the number of current-carrying conductors, for example 3.Enter a whole number from 1 to 200.Too hot for {t} °C insulation: Table 310.15(B)(1)(1) gives no factor at this temperature.14 AWG12 AWG10 AWG8 AWG6 AWG4 AWG3 AWG2 AWG1 AWG1/0 AWG2/0 AWG3/0 AWG4/0 AWG250 kcmil300 kcmil350 kcmil400 kcmil500 kcmil600 kcmil750 kcmil1000 kcmil

Cite or share this tool

Dreiym Engineering. Motor circuit calculator. Method: NEC 430.22, 430.32 and Table 430.52. https://www.dreiym.com/motor-circuit-calculator/

Instructors and shops are welcome to link to this tool or put its address on a handout. It is free, needs no sign-up and works on a phone.

How to use the calculator

Choose the phase, the motor voltage and the horsepower. For a three-phase motor, pick the type; most general-purpose NEMA motors sold today are Design B premium efficiency. Enter the nameplate full-load current for the overload setting, and tick the service factor box when the nameplate shows 1.15 or more, or a temperature rise of 40 °C or less. Then pick the breaker or fuse type, the conductor material and the terminal rating.

How to size a motor branch circuit

Look up the full-load current in Table 430.250 (three-phase) or 430.248 (single-phase). Use the table, not the nameplate, for the conductors and the short-circuit protection (430.6(A)(1)).
Size the conductors for at least 125% of that current (430.22), from Table 310.16 with any ambient and bundling factors.
Set the overload at 125% of the nameplate current for a service factor of 1.15 or more or a temperature rise of 40 °C or less, and at 115% for other motors (430.32(A)(1)).
Pick the short-circuit and ground-fault device no larger than the Table 430.52 percentage of the full-load current. Where that value is not a standard rating, the next standard rating is permitted (430.52(C)(1)(a), Exception No. 1 in the 2020 edition).
Size the disconnect at 115% of the full-load current or more, horsepower rated (430.110(A)).

Worked example

A 25 hp, 208 V three-phase Design B motor, inverse time breaker, copper conductors and 75 °C terminals:

Full-load current, Table 430.250: 74.8 A.
Conductors: 1.25 × 74.8 = 93.5 A. 3 AWG copper carries 100 A at 75 °C; 4 AWG carries 85 A, not enough.
Breaker: 250% × 74.8 = 187 A. That is not a standard rating, so the next standard rating, 200 A, is permitted.
Overload, with a service factor of 1.15 and a nameplate current of 70 A: 1.25 × 70 = 87.5 A.
Disconnect: at least 1.15 × 74.8 = 86 A, horsepower rated.

Pick the “25 hp at 208 V” quick start in the calculator above, then enter 70 in the nameplate field to set the overload.

Why the breaker can be larger than the wire

A motor draws several times its running current while it starts. In a motor circuit the overload protects the motor and the conductors from running overloads, and the branch-circuit breaker or fuse only has to clear short circuits and ground faults. That is why Table 430.52 allows a 35 A breaker on 14 AWG conductors for a 10 hp, 460 V motor, and why the small-conductor limits of 240.4(D) do not apply (240.4(G)).

Method and sources

Full-load currents come from NEC Tables 430.248 and 430.250 for motors from 115 to 575 V running at usual speeds; the percentages from Table 430.52 (numbered Table 430.52(C)(1) in the 2023 edition); and the standard ratings from 240.6(A) as of the 2023 edition, which added 10 A for breakers; in the 2020 edition 10 A was a standard rating for fuses only. Conductor ampacities follow Table 310.16 with the factors of 310.15. The full list of sections is under the calculator.

Frequently asked questions

What size breaker for a 10 hp motor?

For an inverse time breaker: up to 35 A at 460 V three-phase (14 A full-load), up to 70 A at 230 V (28 A), and at 208 V (30.8 A) 250% is 77 A, so the next standard rating, 80 A, is permitted. A smaller breaker is fine if the motor starts reliably on it.

Nameplate current or table current?

Both. The table current sizes the conductors, the breaker or fuse and the disconnect; the nameplate current sets the overload (430.6(A)).

What if the breaker trips when the motor starts?

430.52(C)(1)(b), Exception No. 2 in the 2020 edition, allows a larger device where the Table 430.52 value is not enough to start the motor: up to 400% for an inverse time breaker on a motor of 100 A or less and 300% above that, 225% for time-delay fuses, 400% for nontime-delay fuses of 600 A or less and Class CC time-delay fuses, and 300% for fuses of 601 to 6,000 A. Check the motor and its load first; repeated trips can point to a fault.

Does this work for a motor on a VFD?

Not directly. The input conductors of a variable frequency drive follow 430.122, its short-circuit and ground-fault protection follows 430.130, and the drive’s instructions often call for specific fuses.

When to bring in an engineer

A single motor is a short calculation; a plant with motor control centers, drives and large motors needs feeders, starting studies and coordinated protection. Dreiym’s licensed Professional Engineers design motor circuits and feeders, PE-stamped where required. Related tools: the wire size calculator, the voltage drop calculator and the available fault current calculator. See our electrical design and protection coordination services, or contact us for a free quote. We typically respond within one business day.

Disclaimer: general information, not engineering advice

This article is published for general educational purposes only. It is not engineering advice, not a professional opinion on any specific project, and not a substitute for a site-specific investigation; reading it creates no engineer-client relationship. Your project may be different, and will need its own investigation and review. Site conditions, equipment, installation history, and the codes and standards in force (including which editions your jurisdiction has adopted) vary from project to project and change over time, so an approach that is correct in one case may be unsafe or non-compliant in another. Nothing here is a sealed or stamped engineering document. Before relying on any of it, consult a professional engineer licensed in your jurisdiction to evaluate your specific facts and conditions.

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