How to Calculate Cable Size for a 3-Phase Motor Load

By the Solvebility Engineering Team · Reviewed against NEC, IEC 60364, CEC and AS/NZS 3008 practice Last Updated: August 2026 · Next Review: February 2027

Undersize the cable on a 3-phase motor and you get heat, voltage drop, and a nuisance trip on the day you least expect it. Oversize it and you’ve spent money on copper you didn’t need.

Cable size calculation for a 3-phase motor load comes down to three numbers: the motor’s full load current, a safety margin on top of that, and a voltage drop check over the actual cable run. Get those three right and the rest is just reading a table.

In this guide you’ll size a cable step by step, in kW or HP, in mm² or AWG, with a worked example you can copy for your own motor. The guide explains the common engineering checks used in motor-circuit cable selection. The exact conductor ampacity, correction factors, protection rules and voltage-drop limits depend on the electrical code and installation conditions that govern the project.

The Cable Size Calculation Formula for a 3-Phase Motor

That’s the whole calculation in one sentence. Everything below is just the detail behind each piece, because skipping a step is how cables end up undersized on real jobs.

Step 1: Find the Motor’s Full Load Current (FLC)

Check the motor’s nameplate first. Manufacturers print the rated current (FLA or FLC) right on the plate, and that number already accounts for the motor’s real-world efficiency and power factor. Use it.

No nameplate handy, or you’re sizing a cable before the motor arrives? Calculate it:

If exact power factor and efficiency are unavailable, representative values can be used for a preliminary estimate only. Do not use assumed values for final cable selection when manufacturer nameplate or datasheet information is available.

Step 2: Apply the Safety Factor

The 125% requirement is a code-based conductor-sizing rule; it should not be described simply as a generic allowance for every possible load swing or temperature condition. Ambient temperature, grouping, installation method and other correction factors are separate checks.

For a motor feeding more than one load, or a group of motors on a shared feeder, the code math changes (largest motor at 125%, the rest at 100%). That’s outside a single-motor calculation, so treat this guide as covering one dedicated motor circuit.

Step 3: Pick a Cable From the Ampacity Chart

Ampacity, the current a cable can carry continuously without overheating, depends on conductor size, insulation type, installation method, and ambient temperature. The chart below is a typical PVC-insulated copper reference for conduit or trunking at 30°C ambient (Method B1 style). Treat it as a planning tool, not a substitute for your local code table.

Aluminum generally has lower conductivity than copper, so an aluminum conductor often requires a larger cross-sectional area for the same design current. The actual selection depends on the applicable ampacity table, conductor construction, terminations, installation method, cost, weight and project requirements. Do not select copper or aluminum solely from motor kW.

Step 4: Check Voltage Drop Over the Cable Run

Ampacity tells you the cable won’t overheat. It says nothing about the motor getting enough voltage at the far end of a long run. That’s a separate check, and it’s the one people skip.

Long runs can make voltage drop the controlling factor even when ampacity is adequate. Starting voltage drop can also be more severe because motor starting current may be several times full-load current. There is no universal cable-length threshold at which voltage drop suddenly becomes mandatory; calculate it whenever the run length, motor characteristics or project requirements make it relevant.

Step 5: Apply Derating for Heat and Grouping

Two corrections matter most in practice:

• Ambient temperature: above 30°C, ampacity drops. At 40°C ambient, expect roughly a 10–15% reduction on a PVC-insulated cable; at 45°C, closer to 20–25%.
• Grouping: three or more current-carrying cables bundled in one conduit or tray reduce each other’s rating. Four cables together commonly lose 20% of individual ampacity; six or more can lose 35% or more.

Divide your required current by the applicable correction factors, then reselect from the ampacity chart if the number changed the answer.

Worked Example: Sizing Cable for a 15 kW (20 HP) 3-Phase Motor

Here’s the full calculation, step by step, for a motor you’ll actually find on a plant floor.

Walk the numbers: I = 15,000 ÷ (1.732 × 400 × 0.85 × 0.90) = 28.3 A. Applying the NEC continuous-duty conductor-sizing factor gives 28.3 × 1.25 = 35.4 A. The illustrative 6 mm² reference is 36 A, so it only narrowly exceeds the design current before any installation correction. That is not enough information for a final selection. The illustrative 10 mm² reference is 50 A, giving more margin. For the voltage-drop example, using R = 2.30 Ω/km and X = 0.08 Ω/km gives approximately 3.1 V over 50 m, or about 0.78% of 400 V. Actual voltage drop must use the cable manufacturer’s impedance data and the actual installation conditions.

Notice the cable didn’t get bigger because of the run length here. It got picked with margin from the start, and the voltage drop check simply confirmed it was fine. On a substantially longer run, voltage drop may become the controlling factor and require a larger conductor even when the smaller cable still passes the thermal ampacity check. The exact result must be calculated from the selected cable’s resistance/reactance and the applicable design criteria rather than assumed from distance alone.

How Motor Voltage, Power and Cable Length Affect Cable Size

Motor power alone does not determine conductor size. Voltage, power factor, efficiency and cable length all affect the calculation. For the same motor output, a lower supply voltage generally means higher current, while a longer cable run can make voltage drop the controlling consideration.

Copper vs. Aluminum Motor Cable

Copper and aluminum can both be suitable conductor materials when the selected cable, terminations and installation comply with the governing requirements. Copper provides higher conductivity for a given cross-sectional area, while aluminum can offer lower weight and lower material cost.

Because aluminum usually requires a larger cross-section for the same current, compare the complete installed cost rather than the conductor price alone. For either material, use the manufacturer’s data and the applicable code table for ampacity, temperature ratings, termination compatibility, installation method and correction factors. Do not assume that a copper-to-aluminum size conversion is a fixed percentage.

Starting Current and Starting Voltage Drop

Motor cable selection is normally based on the applicable running/full-load conductor-sizing requirements rather than simply multiplying the cable size by locked-rotor current. However, starting current still matters because it can produce a temporary voltage drop that affects motor starting performance.

Direct-on-line starting can draw several times full-load current. Star-delta starters, soft starters and variable-frequency drives can change the starting-current profile. For a long feeder, high-inertia load or sensitive motor application, check starting voltage at the motor terminals as well as normal running voltage drop.

Common Mistakes That Undersize a Motor Cable

Sizing from horsepower alone, without checking voltage

A 20 HP motor at 230V draws roughly double the current of the same motor at 460V. Skip the voltage in the calculation and you’ll undersize the cable by a wide margin.

Ignoring starting current

Direct-on-line starting can draw several times full-load current for a short period. The running conductor calculation and the starting/protection calculation are related but not identical. Check the motor starting method, protective-device requirements and starting voltage drop separately where applicable.

Forgetting the voltage drop check on long runs

An ampacity-only calculation on a long feeder can miss excessive voltage drop. The motor may then experience reduced terminal voltage, reduced torque or starting problems even though the conductor is thermally adequate.

Using free-air ampacity for a cable in conduit

The same cable rated 50A in free air might only be good for 38A once it’s bundled with other conductors inside a conduit. Always match the table to the actual installation method.

Frequently Asked Questions

Get It Right the First Time

Cable size calculation for a 3-phase motor load isn’t complicated once you’ve run through it once: find the current, add the margin, match the ampacity table, check voltage drop, correct for heat and grouping.

Where people get burned is skipping step 4 on a long run, or trusting horsepower alone without checking the actual voltage. Run all five steps, every time, and the cable you pull will be the cable that’s still working fine in ten years.

References

• NFPA 70®, National Electrical Code® (NEC), Article 430 — requirements for motors, motor circuits, conductors and protection. Verify the NEC edition adopted by the project jurisdiction.
• IEC 60364-5-52:2009+AMD1:2024 CSV — selection and erection of wiring systems, including cable selection and voltage-drop provisions.
• Canadian Electrical Code (CEC), CSA C22.1, Part I, Section 28 — motor and generator requirements. Verify the edition adopted by the project jurisdiction.
• AS/NZS 3008.1.1:2025 — selection of cables for typical Australian installation conditions, including current-carrying capacity, voltage drop, short-circuit temperature rise and correction factors. Verify the edition and requirements applicable to the project.