Motor Wire Size Calculator
Motor circuit conductor sizing is governed by NEC Article 430, which has specific, different rules from general branch circuit sizing (NEC 210). The key requirement -- NEC 430.22:...
Formula
Source: NEC Article 430, NFPA 70 | Last reviewed: July 26, 2026
Examples
75 hp
= 0.00299 AWG
- voltage = 460
- material = 1
75 HP 460V 3-phase -> NEC FLC = 96 A -> 96 x 1.25 = 120 A -> #1 AWG Cu
10 hp
= 0.00127 AWG
- voltage = 208
- material = 1
10 HP 208V 3-phase -> NEC FLC = 30.8 A -> min 38.5 A -> #8 AWG Cu (50 A)
200 hp
= 0.01 AWG
- voltage = 460
- material = 2
200 HP 460V -> NEC FLC = 240 A -> min 300 A -> 350 kcmil Al (300 A)
Quick Reference Table
| HP | 208V (A) | 230V (A) | 460V (A) | 575V (A) |
|---|---|---|---|---|
| 1 | 4.6 | 4.2 | 2.1 | 1.7 |
| 5 | 16.7 | 15.2 | 7.6 | 6.1 |
| 10 | 30.8 | 28 | 14 | 11 |
| 15 | 46.2 | 42 | 21 | 17 |
| 20 | 59.4 | 54 | 27 | 22 |
| 25 | 74.8 | 68 | 34 | 27 |
| 30 | 88 | 80 | 40 | 32 |
| 50 | 143 | 130 | 65 | 52 |
| 75 | 211 | 192 | 96 | 77 |
| 100 | 273 | 248 | 124 | 99 |
| 150 | 396 | 360 | 180 | 144 |
| 200 | 528 | 480 | 240 | 192 |
| 300 | -- | 720 | 360 | 288 |
| 500 | -- | -- | 590 | 472 |
| AWG/kcmil | Ampacity (A) | Notes |
|---|---|---|
| #14 | 20 | Min motor ckt after 125% for fractional HP |
| #12 | 25 | Typical for motors up to 1 HP 460V |
| #10 | 35 | Typical for 2-5 HP 460V motors |
| #8 | 50 | Typical for 7.5-10 HP 460V motors |
| #6 | 65 | Typical for 15-20 HP 460V motors |
| #4 | 85 | Typical for 25-30 HP 460V motors |
| #3 | 100 | Typical for 40 HP 460V motors |
| #2 | 115 | Typical for 50 HP 460V motors |
| #1 | 130 | Typical for 60-75 HP 460V motors |
| #1/0 | 150 | Typical for 100 HP 460V motors |
| #2/0 | 175 | Typical for 125 HP 460V motors |
| 350 kcmil | 310 | Typical for 250 HP 460V motors |
| 500 kcmil | 380 | Typical for 300-350 HP 460V motors |
Where is this used?
(1) Single motor branch circuit -- the most common application: a dedicated circuit from a motor control center (MCC) bucket or combination starter to a single motor.
Conductor sized per NEC 430.22 at 125% of NEC table FLC.
For a 25 HP 460V 3-phase motor (NEC FLC = 34 A), minimum ampacity = 34 x 1.25 = 42.5 A.
Per NEC 310.16 75 degree C copper column, #8 AWG is rated 50 A -- acceptable.
This is the minimum; if the run exceeds 100 feet, voltage drop should be checked per NEC 210.19(A) Informational Note No.
4 (recommending less than 3% feeder voltage drop and 5% total to the load).
(2) Multiple motors on one feeder (NEC 430.24) -- common in HVAC equipment with multiple fans, pumps, and compressors on one rooftop unit supply.
Conductors sized at 125% of the largest motor FLC + 100% of all other motor FLCs on the same feeder.
Example: a rooftop unit with a 15 HP supply fan (21 A FLC), a 5 HP exhaust fan (7.6 A FLC), and two 3 HP condenser fans (4.8 A each).
Minimum feeder ampacity = 1.25 x 21 + 7.6 + 4.8 + 4.8 = 43.45 A.
(3) Motor control centers (MCCs) -- the vertical bus and horizontal bus ampacity must accommodate all connected motor loads per NEC 430.24.
An MCC with 20 buckets averaging 50 HP each at 460V requires bus ampacity calculated from the sum of load FLCs.
(4) Fire pump motor circuits -- per NEC 695, conductors must be sized to carry locked-rotor current indefinitely.
For a 100 HP 460V fire pump (NEC FLC = 124 A, typical locked-rotor code letter G giving LRA = 5.6-6.3 x FLC = 694-781 A), the conductors must handle this current without exceeding thermal limits during the potentially hours-long fire event.
(5) Adjustable-speed drive (VFD) circuits -- NEC 430.122 requires the VFD output conductors to be sized at 125% of the VFD rated input current, not the motor FLA.
A 100 HP VFD may have a rated input of 142 A at 460V, requiring minimum conductor ampacity of 177.5 A -- #2/0 AWG copper.
(6) Submersible pump motors -- deep well pumps have special requirements: the long vertical drop cable has different voltage drop and the NEC table FLC values in Table 430.250 still apply, but the cable manufacturer's ampacity for submerged vs.
in-air conditions must be considered.
(7) Hazardous location motors -- NEC Articles 500-516 require additional considerations: motors in Class I, Division 1 locations often use explosion-proof conduit seals, and the conductor insulation must be rated for the classified area temperature.
(8) Motor feeders in high ambient temperature -- industrial facilities in hot climates (foundries, steel mills, desert installations) require ambient temperature derating per NEC 310.15(B)(2).
At 50 degrees C (122 degrees F), the 75 degree C column ampacity of #1 AWG copper drops from 130 A to 130 x 0.75 = 97.5 A -- below the 120 A required for a 75 HP motor, forcing an increase to #1/0 AWG.
Real-World Usage Scenarios
Plant floor motor replacement and conductor verification
A manufacturing plant replaces a 40 HP 460V pump motor that failed after 18 years. The new premium-efficiency motor has a nameplate FLA of 48 A. The existing #6 AWG THHN conductors (75 degree C ampacity = 65 A) in 1-inch EMT have been in service since 1998. The electrical engineer checks: NEC Table 430.250 FLC for 40 HP 460V = 52 A. Minimum conductor ampacity = 52 x 1.25 = 65 A. The #6 AWG conductors at exactly 65 A meet the NEC minimum -- but with zero margin. Voltage drop is checked: 185 ft run at 65 A on #6 copper gives 4.1% drop at 480V -- exceeding the 3% feeder recommendation. The decision is made to pull new #4 AWG conductors (85 A ampacity, 2.6% voltage drop) to provide margin for future motor upsizing and to meet the voltage drop guideline.
VFD-fed motor conductor sizing on a cooling tower retrofit
A chilled water plant retrofits a 150 HP 460V cooling tower fan with a VFD for energy savings. The original across-the-line starter used #2/0 AWG copper (NEC FLC = 180 A, min ampacity = 225 A). The new VFD nameplate lists a rated input of 212 A at 460V. Per NEC 430.122, output conductors must be sized at 125% of 212 A = 265 A minimum. Checking NEC 310.16: #2/0 AWG is rated 175 A at 75 degrees C -- not listed high enough. In fact, 300 kcmil copper at 75 degrees C is rated 285 A -- this becomes the new minimum. Additionally, the 310 ft cable run triggers the VFD manufacturer's requirement to use VFD-rated cable (XLPE insulation, three symmetrically placed ground conductors, overall shield) to prevent reflected wave voltage doubling from damaging the 20-year-old motor's winding insulation. The project cost increases by $4,800 for the larger, special-purpose cable.
Multi-motor compressor skid feeder sizing
A natural gas compressor skid contains three motors: a 200 HP main compressor (240 A NEC FLC), a 15 HP lube oil pump (21 A FLC), and a 7.5 HP cooling fan (11 A FLC), all 460V 3-phase. The skid has a single feeder disconnect and the customer requests a single feeder cable from the MCC. Per NEC 430.24: minimum feeder ampacity = 1.25 x 240 + 21 + 11 = 332 A. Selecting from NEC 310.16 75 degree C copper: 350 kcmil is rated 310 A -- insufficient. 500 kcmil is rated 380 A -- adequate but expensive. The engineer considers running individual branch circuits instead: 200 HP gets #4/0 AWG (230 A minimum, 250 kcmil with 255 A selected), 15 HP gets #10 AWG (26.25 A minimum, rated 35 A), 7.5 HP gets #12 AWG (13.75 A minimum, rated 25 A). The combined cost of three smaller conduits and cables is actually $1,200 less than the single 500 kcmil feeder, and each motor now has independent short-circuit and overload protection at the MCC -- a safer design.
Common Mistakes to Avoid
Using motor nameplate FLA instead of NEC Table 430.250 FLC
This is the single most common and dangerous wiring error for motor circuits. A 50 HP 460V motor may have a nameplate FLA of 60 A, but NEC Table 430.250 lists 65 A. Conductors sized at 60 x 1.25 = 75 A would use #4 AWG (85 A). But the correct calculation using the NEC table is 65 x 1.25 = 81.25 A, still #4 AWG (85 A). While the result is the same in this case, for other HP ratings the difference pushes the conductor size up. The NEC requires the table value because: (1) the nameplate FLA reflects that specific manufacturer's motor at rated voltage, while the table value conservatively covers all manufacturers per NEC 430.6(A)(1), and (2) a future replacement motor, even of the same HP, may draw more current. Plan reviewers and inspectors will reject submittals using nameplate FLA for conductor sizing.
Ignoring voltage drop on long motor feeder runs
NEC 430.22 establishes the minimum conductor ampacity but does not itself mandate voltage drop limits. However, NEC 210.19(A) Informational Note No. 4 recommends feeder voltage drop below 3% and total circuit drop below 5%. A 75 HP 460V motor with NEC FLC = 96 A, min ampacity = 120 A, on a 400 ft run: #1 AWG copper meets the 120 A minimum but produces a 3.8% voltage drop at 480V. At the motor terminals, reduced voltage during starting causes proportionally higher starting current and longer acceleration time -- potentially tripping the overload relay on start. The fix is upsizing to #2/0 AWG (175 A, 1.8% voltage drop) or adjusting the overload relay Class setting. Additionally, NEC 250.122(B) requires the equipment grounding conductor to be proportionally upsized when phase conductors are increased for voltage drop.
Applying standard 310.15(B)(16) ampacities to VFD output conductors without adjustment
VFD output conductors carry pulse-width-modulated (PWM) waveforms with high-frequency harmonic content (switching frequencies typically 2-16 kHz). This harmonic current causes additional I2R heating beyond what the fundamental 60 Hz current would produce. NEC 430.122 explicitly requires VFD output conductors to be sized at 125% of the VFD's rated input current -- which is already larger than 125% of the motor FLC because the VFD input includes losses (typically 2-3%) and harmonic current drawn by the rectifier. Furthermore, for cable runs over 100 ft at 480V, the VFD's rapid voltage rise time (dv/dt) creates reflected wave voltage doubling at the motor terminals (up to 1200V peak from a 480V RMS source), which can exceed the voltage rating of standard THHN insulation (600V) in the conductor as well as the motor winding insulation. Using inverter-duty motor lead cable (rated 2000V peak) is the industry standard practice for runs over 100 ft.
Industry Standards Referenced
Frequently Asked Questions
How do I size wire for a motor?
1) Find FLC from NEC Table 430.250 (not the nameplate FLA). 2) Multiply by 1.25 per NEC 430.22. 3) Select a conductor from NEC Table 310.16 with ampacity >= the result. 4) Verify voltage drop is within limits. 5) For VFD-fed motors, use NEC 430.122 (125% of VFD input current). 6) For multiple motors on one feeder, use NEC 430.24 (125% of largest + 100% of others).
Why does NEC mandate 125% for motor conductors?
Motors can operate continuously above their nameplate rating (up to the service factor, typically 1.15). They draw higher current during voltage sags, phase unbalance, and mechanical overload. The 125% factor provides thermal margin for the conductors under these conditions and accommodates future motor replacement with a different (potentially less efficient) model. A conductor running at exactly 100% of its ampacity will reach its rated temperature (60/75/90 degrees C), which accelerates insulation aging. The 125% factor ensures the conductor typically operates at approximately 80% of its rated ampacity under normal conditions, extending its service life.
Does VFD output wiring have different sizing rules?
Yes. NEC 430.122 requires VFD output conductors to be sized at 125% of the VFD's rated input current, not the motor's NEC table FLC. VFDs also generate high-frequency harmonics and common-mode currents that increase conductor heating. Use VFD-rated cable (XLPE insulation, shielded, with symmetrical grounds) for runs over 50 ft -- standard THHN in conduit is acceptable for short runs inside the MCC room. Additionally, the VFD manual may specify a minimum conductor size larger than the NEC calculation to handle reflected wave voltage doubling at the motor terminals (critical for 480V motors with cable runs over 100 ft). Always consult the VFD manufacturer's installation manual for output wiring requirements beyond the NEC minimum.
What is the difference between motor nameplate FLA and NEC table FLC?
The motor nameplate full-load amperes (FLA) is the actual current that specific motor draws at rated voltage and full mechanical load, as measured by the manufacturer during testing. The NEC table full-load current (FLC) is a standardized, intentionally conservative value published in NEC Tables 430.248 (single-phase) and 430.250 (three-phase) that applies generically to all motors of that horsepower and voltage rating. The table values are approximately 5-15% higher than typical nameplate values. NEC 430.6(A)(1) explicitly requires using the table values for conductor sizing because: (a) the table covers the worst-case motor that could be installed now or in the future, (b) a replacement motor may draw more current even at the same HP, and (c) the table provides a uniform basis for plan review and inspection. The nameplate FLA is used only for overload protection device selection and setting (NEC 430.32).
How do I size conductors for multiple motors on one circuit?
Per NEC 430.24, the minimum feeder conductor ampacity for two or more motors is: 125% of the full-load current rating of the highest-rated motor, plus the sum of the full-load current ratings of all other motors in the group. This applies to motor control center horizontal and vertical bus, feeder breakers in distribution panels, and any junction point where multiple motor circuits combine. For example, a feeder supplying a 50 HP (65 A), two 20 HP (27 A each), and a 10 HP (14 A) motor -- all 460V: min ampacity = 1.25 x 65 + 27 + 27 + 14 = 149.25 A. If the motors have different duty cycles (intermittent, varying), NEC 430.26 allows a demand factor to be applied for motors that do not operate simultaneously -- but this requires careful documentation of the control interlocking scheme.
Does conduit fill derating affect motor conductor sizing?
Yes. When more than 3 current-carrying conductors are installed in a single raceway, NEC Table 310.15(B)(3)(a) requires derating: 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%. For motor circuits, only the phase conductors count as current-carrying (neutrals carrying only unbalanced current from nonlinear loads may count in some cases). The equipment grounding conductor does not count toward the derating. If derating drops the conductor ampacity below the 125% minimum required by 430.22, the conductor must be upsized. Example: three 3-phase motor circuits in one conduit (9 conductors, 70% derating). A #10 AWG copper at 75 degrees C is 35 A base, derated to 24.5 A -- too small for a 5 HP 460V motor requiring 7.6 x 1.25 = 9.5 A minimum. It still passes, but a larger motor may fail. Always calculate both the NEC 430.22 minimum and the derated ampacity, and use the larger of the two.
Reviewed for accuracy
Reviewed against NEC 2023 Article 430 (Motors, Motor Circuits, and Controllers), NEC Table 430.250, NEC Table 310.16, and NEMA MG 1 motor standards · Last reviewed: July 26, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.