How to Size Cable From kW (Load to Cable Size)

When a load is specified in kilowatts rather than amps, the first step is converting that power figure into a current. Current is what the cable actually carries, and ampacity tables and voltage-drop formulas all work in amps. The kW-to-amps conversion is straightforward, but it requires knowing the supply voltage and the load's power factor.

Why kW is not the same as kVA

Power factor is the ratio of real power (kW) to apparent power (kVA). A purely resistive load like an electric heater has a power factor of 1.0, so 10 kW equals 10 kVA. A motor typically has a power factor of 0.8 to 0.9. At 0.85 power factor, a 10 kW motor actually draws about 11.8 kVA.

Cable current rating is governed by apparent power (kVA), not real power (kW). That is why power factor appears in the conversion formula. If you ignore power factor and size the cable for the kW figure alone, you will undersize it for the actual current the cable must carry.

If you do not know the power factor, a value of 0.85 is a common conservative assumption for motor loads. For resistance heating, use 1.0.

Converting kW to amps: single-phase

For a single-phase supply:

I = P / (V × pf)

Where:

Example: a 4 kW single-phase load at 230 V with a power factor of 0.90:

I = 4,000 / (230 × 0.90)
I = 4,000 / 207
I ≈ 19.3 A

Converting kW to amps: three-phase

For a balanced three-phase supply:

I = P / (√3 × V × pf)

Where V is the line-to-line voltage. √3 is approximately 1.732.

Example: an 11 kW motor load at 400 V three-phase, power factor 0.87:

I = 11,000 / (1.732 × 400 × 0.87)
I = 11,000 / 602.9
I ≈ 18.2 A

Worked example: sizing cable for an 11 kW, 400 V three-phase motor

Starting from the 18.2 A calculated above:

Step 1: Apply the continuous-load factor.

Motors typically run continuously, so NEC 430.22 requires conductors sized at 125% of the motor nameplate current. (The NEC motor article governs; general conductor sizing is in NEC Article 310, but motor branch-circuit conductors follow Article 430.)

I_design = 18.2 × 1.25 = 22.8 A

Round up to 25 A for conductor selection purposes.

Step 2: Select minimum AWG by ampacity.

From NEC Table 310.12 at 75°C, copper conductors:

10 AWG passes the ampacity check at 25 A.

Step 3: Check voltage drop.

Assume a 40-metre (approximately 131-foot) run. At 400 V three-phase, a 3% target means a maximum drop of 12 V.

10 AWG copper has a cross-section of 10,380 circular mils. Using the three-phase voltage-drop formula (see The Cable Size Calculation Formula):

V_drop = (√3 × K × I × L) / A
V_drop = (1.732 × 12.9 × 22.8 × 131) / 10,380
V_drop = (1.732 × 12.9 × 22.8 × 131) / 10,380
V_drop = 66,922 / 10,380
V_drop ≈ 6.45 V  →  1.6% of 400 V

Well under 3%. 10 AWG copper passes both checks for this circuit.

Step 4: Use the larger result.

Ampacity requires 10 AWG minimum. Voltage drop also permits 10 AWG. Final answer: 10 AWG copper.

What changes with longer runs

The motor in the example above is relatively close to the supply. A longer run, say 80 metres (about 262 feet), changes the picture:

V_drop = (1.732 × 12.9 × 22.8 × 262) / 10,380
V_drop ≈ 12.9 V  →  3.2% of 400 V

That exceeds 3%. You would need to solve for the required conductor area and step up to the next AWG size. The voltage-drop check, not ampacity, drives the decision on longer runs. The complete procedure is in How to Size a Cable: A Step-by-Step Walkthrough.

Aluminum instead of copper

If the circuit uses aluminum conductors, substitute K = 21.2 in the voltage-drop formula and note that aluminum ampacity ratings are lower than copper for the same AWG. You will generally need to go two AWG sizes larger than copper to match the same ampacity. Re-run both checks with the aluminum values.

Using a calculator

Running kW-to-amps and then the two cable checks by hand is straightforward for a single circuit. For multiple loads or when comparing conductor materials, a cable sizing calculator converts the power input directly and applies both checks in one step.

Summary

  1. Determine power factor (use 0.85 for motors if unknown, 1.0 for resistance heating).
  2. Convert kW to amps using the appropriate single-phase or three-phase formula.
  3. Apply a 1.25 multiplier for continuous loads.
  4. Find minimum AWG by ampacity from NEC tables.
  5. Check voltage drop; step up conductor size if needed.
  6. Use whichever check gives the larger conductor.

Conductor sizing should be verified against the current NEC, NEC Article 430 for motor circuits, and the code adopted by your local jurisdiction. These calculations are for planning purposes. A licensed electrician should review all permanent installations.