Motor circuit size calculator

How the current, the wire, the device and the overload limit are worked

Four numbers come out of a motor circuit. An NEC table gives the full-load current by horsepower and voltage, and the nameplate is not used for it. Conductors are sized at 125 percent of that table current. A breaker or fuse is limited to a percentage of the table current that depends on the device, with a rule that lets the next standard size up be used and a second rule for a motor that will not start. Overload protection is limited by a percentage of the nameplate current instead. Each result is a maximum, and a smaller device is allowed when the motor starts and runs on it.

Full-load current

I = Table 430.248 or 430.250 value for hp and voltageA 10 hp motor at 460 V, three phase, is 14 A in Table 430.250, and a 1 hp motor at 115 V, single phase, is 16 A in Table 430.248. The value is for a motor at usual speed with normal torque. 430.6(A)(1) uses it for the conductor and the short-circuit device in place of the nameplate current.

Branch conductor

ampacity at least 1.25 x I, Table 310.16430.22 asks for 125 percent of the table current for one continuous-duty motor. 10 hp at 460 V gives 1.25 x 14 A = 17.5 A, which is copper 12 AWG on the 60°C (140°F) column or 14 AWG on the 75°C (167°F) column. On a circuit of 100 A or less the 60°C (140°F) column applies unless every termination is rated 75°C (167°F), the motor by marking or by design letter B, C or D, and the controller and disconnect by marking (110.14(C)(1)(a)); above 100 A the 75°C (167°F) column applies (110.14(C)(1)(b)).

Short-circuit device, Table 430.52

maximum rating = percent x I, then the next standard size upThe percentages for a single-phase or squirrel-cage motor are 250 for an inverse time breaker, 175 for a dual-element fuse, 300 for a nontime-delay fuse and 800 for an instantaneous-trip breaker, 1100 for a Design B energy-efficient motor. A 25 hp motor at 460 V draws 34 A, so the figures are 85 A for the breaker, 59.5 A for the dual-element fuse and 102 A for the nontime-delay fuse. Exception No. 1 takes each up to the next standard size in Table 240.6(A), 90, 60 and 110 A.

Motor will not start

Exception No. 2, largest standard size at or below the limitIf the Table 430.52 size trips on starting, the device may be raised, never above 400 percent for a nontime-delay fuse of 600 A or less, 225 percent for a time-delay fuse, and for an inverse time breaker 400 percent up to 100 A of full-load current and 300 percent above it. A 50 hp motor at 460 V has 65 A, so the breaker goes from 175 A to at most 250 A. A 100 hp motor at 460 V is over 100 A at 124 A, and its limit stays 350 A. Instantaneous-trip settings go to 1300 percent, or 1700 percent for Design B energy-efficient motors.

Overload device

at most 1.25 x nameplate FLA, or 1.15 x430.32(A)(1) allows 125 percent of the nameplate current for a motor marked with a service factor of 1.15 or more or a temperature rise of 40°C (72°F) or less, and 115 percent for any other motor. A 20 A nameplate gives 25 A in the first group and 23 A in the second. If that setting will not start the motor or carry the load, 430.32(C) allows up to 140 percent and 130 percent, 28 A and 26 A here.

Eight motors through the same formula

The 7-1/2 hp, 230 V row is the EC&M example in the 2017 NEC, 22 A, 10 AWG copper in the 60°C (140°F) column and a 60 A breaker. The 25 hp, 460 V row is the 34 A motor in the IAEI article, with 8 AWG copper at 75°C (167°F). Breakers are inverse time. The last column is the largest breaker if the motor will not start on the first one.

Copper only. n/a in the 60°C column: above 100 A the 75°C (167°F) column applies (110.14(C)(1)(b)). Aluminum, the other devices and the overload limit are in the result above for the motor you pick. Sizes are the smallest in Table 310.16 for the conductor ampacity shown, with no ambient or conductor-count correction.

Table 430.52 percentages used

Percent of the table full-load current for single-phase and squirrel-cage motors, as reprinted. The first column is the Table 430.52 maximum, and the second is the largest rating if the motor will not start on it (Exception No. 2). An inverse time breaker is held to 300 percent in the second column when the full-load current is above 100 A, and the 400 percent for a nontime-delay fuse applies up to 600 A; fuses rated 601 to 6,000 A are held to 300 percent. A time-delay Class CC fuse takes the nontime-delay fuse row. Design B energy-efficient applies to three-phase motors only.

Standards and data sources

• NEC 2023, Tables 430.248 and 430.250, 430.6(A), 430.22, 430.32, 430.52 and Table 430.52, Table 310.16, Table 240.6(A) and 110.14(C) - the table full-load currents, the conductor, overload and short-circuit rules and the standard sizes. The NEC text was not read in NFPA's viewer, which needs a login. Wording and numbers come from the reprints below, and in the 2023 edition Table 430.52 is numbered Table 430.52(C)(1) and the two exceptions are 430.52(C)(1)(a) and (b), as Mike Holt Enterprises reports.
• 2011 NEC pages 70-337 and 70-338 (Tables 430.248, 430.249 and 430.250), as scanned by NECA-IBEW and Trask Instrumentation - every full-load current on this page, read cell by cell off two scans of the printed 2011 pages and checked against reuven.tools and electriciancalc.com, which print the same numbers, and against a 2023 table at voltcal.com for the 460 V column. The 200 V columns have fewer independent checks than the rest.
• Mike Holt in EC&M, Stumped by the Code? Code Requirement for Sizing the Overcurrent Protection of Conductors (2018, 2017 NEC) - the worked motor example this page reproduces, a 7-1/2 hp, 230 V, three-phase motor at 22 A, with a conductor of at least 28 A on 10 AWG copper in the 60°C (140°F) column and an inverse time breaker of 22 x 2.50 = 55 A taken up to 60 A.
• IAEI Magazine, Overcurrent Protection Part 2 (2007) - the 34 A motor example, 42.5 A of conductor on 8 AWG at 75°C (167°F), a dual-element fuse of 1.75 x 34 = 59.5 A taken up to 60 A, a nontime-delay fuse of 110 A and an inverse time breaker of 90 A.
• Jade Learning, Understanding Motor Branch-Circuit Overcurrent Protection Devices and Motor Calculations Part IV - the Table 430.52 percentages, the next-size-up exception, the 400, 225, 300 and 1300 percent limits for a motor that will not start, and the 50 A, 60 A and 60 A worked answers for 28 A, 21 A and 27 A motors.
• Mike Holt in EC&M, Motor Calculations Part 2, and the EC&M Code Quiz on overload devices for continuous-duty motors - the 125 and 115 percent overload limits from the nameplate, the 140 and 130 percent limits of 430.32(C), and the 1 hp, 115 V and 5 hp, 230 V single-phase examples.

Limitations of this calculator

• This page gives preliminary numbers for estimating and for checking a drawing or a quote. A licensed electrician or engineer sizes and installs the circuit under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
• NFPA's own viewer was not used to read the NEC text. Table currents come from two scans of the printed 2011 pages and were checked against web reprints, and the rules for the conductor, the short-circuit device and the overload come from EC&M, IAEI and Jade Learning. Fewer independent checks exist for the 200 V columns than for the rest. Section numbers moved in the 2023 edition, and the 2023 table pages were not opened. Check the edition and the local amendments your AHJ uses.
• The page works one general-purpose motor on its own branch circuit, single phase (Table 430.248) or three-phase induction (Table 430.250). Synchronous, wound-rotor, direct-current and two-phase motors, 2300 V motors, 115 V three-phase motors and multispeed motors are not covered. A motor with a variable-frequency drive, a motor in a hazardous location and a hermetic compressor each have their own rules.
• A hermetic refrigerant compressor, as in an air conditioner or a heat pump, is sized from its nameplate under Article 440 and not from these tables. The AC breaker size calculator covers that case.
• Wire sizes are the smallest in Table 310.16 for 125 percent of the table current, for copper and aluminum in the 60°C (140°F) and 75°C (167°F) columns, up to 4/0 AWG. Ambient and conductor-count corrections, voltage drop and conduit are not worked, and a larger motor can need a size above 4/0 AWG, which the page flags. The small-conductor limits of 240.4(D) are not applied, because Table 240.4(G) sends motor circuits to Article 430.
• Short-circuit results are a maximum under Table 430.52 and its exceptions, for single-phase and squirrel-cage motors. Exception No. 2 is for a motor that will not start on the first size, and the page does not decide that for you. A time-delay Class CC fuse takes the nontime-delay fuse percentages (a note to Table 430.52 and Exception No. 2), so pick the nontime-delay fuse for it, and an instantaneous-trip breaker is allowed only in a listed combination motor controller. Wound-rotor, synchronous and direct-current rows of the table are not used.
• Overload limits are a percentage of the nameplate current you enter. 430.32(A) is read in the sources for continuous-duty motors over 1 hp without built-in thermal protection, so a motor of 1 hp or less (430.32(B) when automatically started, 430.32(D) when started by hand) and a motor with a built-in protector follow other parts of 430.32. Percentages from 430.32(A)(1) and (C) are shown, and no relay or heater is chosen.
• Disconnect sizing (430.110), the controller, a feeder or a short-circuit device for more than one motor (430.24, 430.62), a combination starter and a capacitor are outside this page. Table full-load current is not the starting current, and a motor with a high-inertia load or a code letter can need engineering.

Common mistakes with motor circuits

1. Sizing the wire from the nameplate. A 10 hp, 460 V, three-phase motor is 14 A in Table 430.250, and the conductor is 125 percent of that, 17.5 A. The nameplate of an actual motor can read higher or lower. 430.6(A)(1) takes the table value for the conductor and the short-circuit device, and the nameplate current is kept for the overload device.
2. Reading Table 430.52 as the size to install. A 7-1/2 hp motor at 230 V works out to 55 A on an inverse time breaker, and 60 A is the next standard size, which Exception No. 1 allows. These percentages are maximums. A smaller standard breaker is allowed if the motor starts on it, and a larger one is not allowed until Exception No. 2 applies.
3. Raising the device before the motor has failed to start. Exception No. 2 exists for a motor that trips the device on its first size. A 50 hp motor at 460 V takes a 175 A breaker from Table 430.52, and the exception lifts the limit to 250 A. The larger device protects the circuit less, so it is a step taken after the smaller one has been tried.
4. Picking the column from the supply voltage instead of the motor. The tables list rated motor voltages and permit each column for a range of system voltages. A 480 V system uses the 460 V column and a 240 V system the 230 V column. The 208 V column is for motors rated 208 V, so the nameplate voltage picks the column and not the voltage at the panel. The same 10 hp motor is 14 A at 460 V and 28 A at 230 V.
5. Setting the overload relay from the table current. The overload device is sized from the nameplate. For a nameplate of 20 A the limit is 25 A when the motor is marked service factor 1.15 or more, and 23 A for any other motor. The table value of 14 A for the same motor would give 17.5 A, which belongs to the conductor and not to the relay.
6. Using a motor table for an air-conditioner compressor. A hermetic refrigerant compressor in an air conditioner or heat pump has its own nameplate rules in Article 440, with a minimum circuit ampacity and a maximum overcurrent protection marked on the unit. These Article 430 tables do not apply to it. The AC breaker size calculator works from that nameplate.

Common questions about motor circuit sizing

Related calculators

• AC breaker size calculator The breaker for an air conditioner or heat pump compressor, which follows Article 440 and its nameplate and not these tables.
• Wire size calculator Wire for a general load with continuous-load rules, corrections and voltage drop.
• Voltage drop calculator The drop on a long run to the motor, which this page does not work.
• Conduit fill calculator The conduit size for the conductors this page gives.
• Equipment grounding conductor size calculator The ground wire that runs with the motor circuit, sized from the device rating.

Used on the same job

• Voltage drop calculator Whether the conductor is big enough for the length of the run.
• Conduit fill calculator The conduit for the wire this page gives.
• Equipment grounding conductor size calculator The ground wire for the device rating this page gives.