Cable to Amps Chart: AWG Sizing & Load Conversion Guide

For a standard 5,000W (5kW) resistive load, the current draw is 41.67 amps at 120V, 21.74 amps at 230V, and 13.88 amps at 208V 3-phase (assuming a Power Factor of 1.0). To safely wire this, you consult a cable to amps chart (ampacity table) and select 8 AWG copper for the 120V circuit, or 10 AWG copper for the 230V circuit, based on the 75°C column of NEC Table 310.16. The fundamental formula for single-phase conversion is I = P / (V × PF), which substitutes as I = 5000 / (120 × 1.0) = 41.67A. For three-phase systems, the formula shifts to I = P / (V × √3 × PF), substituting as I = 5000 / (208 × 1.732 × 1.0) = 13.88A.

The Master Cable to Amps Chart (NEC Ampacity)

Technically, a cable does not "convert" to amps; rather, it has a maximum thermal limit called ampacity. The load dictates the current, and the chart dictates the minimum cable size required to carry that current without melting the insulation. The table below details copper wire ampacities based on NFPA 70 (National Electrical Code) Table 310.16.

Critical Jobsite Rule: Even if you pull 90°C rated THHN wire through conduit, NEC 334.80 dictates that standard NM-B (Romex) cable is strictly limited to the 60°C column for final overcurrent protection sizing. You only use the 90°C column as a baseline before applying temperature and bundling derating factors.

Converting Load (kW) to Amps: The Math & Variables

Before you can use the ampacity chart above, you must convert your appliance or load wattage into amps. The exact answer shifts dramatically based on three fixed assumptions: Voltage, Phase Count, and Power Factor (PF).

• 120V Single-Phase: Standard US residential outlets. High current draw requires thicker cables.
• 230V/240V Single-Phase: US dryers, ranges, and EU standard mains. Doubles the voltage, effectively halving the current and allowing for smaller AWG wire.
• 208V/480V Three-Phase: Commercial and industrial. The √3 (1.732) multiplier in the denominator drastically reduces the amperage per leg.

Below is a neighboring values table showing how the current draw shifts for loads within a ±20% range of our 5kW baseline, assuming a purely resistive load (PF = 1.0).

When the Conversion is Meaningless: The Power Factor Trap

If you are sizing a cable for a large inductive load—like an uncorrected 5HP induction motor or a massive bank of switching ballasts—and the Power Factor (PF) is unknown, the standard wattage conversion becomes dangerously meaningless. Inductive loads cause current to lag voltage. If a 5kW motor has a poor PF of 0.65, the actual current draw isn't 41.67A; it's 5000 / (120 × 0.65) = 64.1A. Sizing your breaker and cable for the resistive math will result in tripped breakers and overheated conductors. Always check the manufacturer nameplate for Full Load Amps (FLA) or the specific PF rating. For deeper diagnostics on this, refer to Fluke's guide on power factor correction.

Jobsite Realities: Derating and Voltage Drop

The ampacity chart provides ideal, free-air, 30°C ambient baselines. Real-world installations require adjusting those numbers downward.

Furthermore, for runs exceeding 100 feet, voltage drop becomes the governing factor rather than thermal ampacity. According to standard wire gauge engineering data, pushing 40A through 100 feet of 8 AWG copper at 240V yields a roughly 2.5% drop, which is acceptable. However, pushing that same 40A through 200 feet of 8 AWG drops the voltage by 5%, which will cause motors to overheat and lights to dim. In long-run scenarios, you must step up the cable size (e.g., to 6 AWG) purely to maintain voltage, even if the thermal ampacity of the smaller wire was technically sufficient.