Phase current is the actual electrical current flowing through a single individual winding or phase element inside a three-phase source or load. If you are wiring a 3-phase motor, sizing a Variable Frequency Drive (VFD), or reading a transformer nameplate, confusing this internal winding current with the current flowing in the external supply cables (line current) will lead to undersized conductors, tripped breakers, or burnt windings. While they are identical in some configurations, they diverge significantly in others, and knowing exactly which one you are dealing with dictates how you size your overloads and branch circuits.
The Core Difference: Phase Current vs. Line Current
To troubleshoot or design a polyphase system, you must separate the transmission path from the load internals. Line current (IL) is the current flowing through the external transmission lines (L1, L2, L3) connecting the power source to the load. This is what your clamp meter reads when you clamp around a feeder wire. Phase current (IP) is the current flowing through the actual internal coils or windings of the generator, motor, or transformer.
Think of a three-lane highway (line current) splitting into local roads (phase currents) to feed a large factory; depending on how the factory's internal roads are routed (Wye vs. Delta), the traffic volume on the main highway doesn't always equal the traffic volume on the internal roads.
The mathematical bridge between these two values is the square root of 3, or approximately 1.732. How this multiplier applies depends entirely on whether the system is wired in a Wye (Star) or Delta configuration. As detailed in All About Circuits' breakdown of three-phase connections, the vector sum of the three phases shifts the relationship between line and phase values by exactly 30 degrees, yielding the 1.732 factor.
| System Configuration | Nominal Line Voltage | Measured Line Current (IL) | Calculated Phase Current (IP) | Phase Voltage (VP) |
|---|---|---|---|---|
| Wye (Star / Y) | 480V | 30.00 A | 30.00 A (IL = IP) | 277V |
| Delta (Δ) | 480V | 30.00 A | 17.32 A (IL / 1.732) | 480V |
| Wye (Star / Y) | 208V | 50.00 A | 50.00 A (IL = IP) | 120V |
| Delta (Δ) | 240V | 50.00 A | 28.87 A (IL / 1.732) | 240V |
Worked Numeric Example: Sizing a Delta Motor Circuit
Let’s look at what phase current changes in a real installation. Suppose you are wiring a 10 HP, 460V, 3-phase AC motor that is internally wired in a Delta configuration. According to NEC Table 430.250, the Full Load Current (FLC) for this motor is 14A.
Crucially, motor nameplates and NEC tables list Line Current. The 14A is flowing through your external THHN branch circuit conductors. But what is happening inside the motor peckerhead?
Because the motor is Delta-connected, the line current is 1.732 times the phase current.
IP = IL / 1.732IP = 14A / 1.732 = 8.08A
What this changes in practice: The internal copper windings of this motor only need to be manufactured to handle 8.08A continuously. This allows the manufacturer to use thinner magnet wire inside the stator, reducing cost and physical size. However, as an installer, you do not size your wire or breakers based on the 8.08A phase current.
Your branch circuit conductors must be sized for the 14A line current. Per NEC 430.22, motor conductors must be rated for at least 125% of the FLC. 14A × 1.25 = 17.5A. You would pull 14 AWG THHN (rated 20A at 75°C) or 12 AWG depending on your terminal temperature ratings. Furthermore, if you are setting the dial on a Square D Class 9065 Type F solid-state overload relay, you set it to the 14A line current value, completely ignoring the 8.08A phase current. Sizing your external protection based on phase current in a Delta system will result in catastrophic overload and fire risk.
Where You Meet Phase Current in Practice
While you rarely measure phase current directly, it dictates the physical design and diagnostic behavior of several common jobsite components:
- Transformer Secondary Sizing: When sizing a 480V Delta primary to 208Y/120V secondary step-down transformer, the primary windings carry phase current. If the transformer is delivering 100A of line current on the 480V Delta side, the internal primary windings are only enduring 57.7A (100 / 1.732). This thermal reality dictates the transformer's internal cooling design and physical footprint.
- VFD Output Stages: Variable Frequency Drives rectify AC to a DC bus, then invert it back to simulated 3-phase AC. The IGBTs (Insulated-Gate Bipolar Transistors) on the output stage switch the phase current. When reading a VFD datasheet, the "Output Current" rating refers to the line current delivered to the motor, but the internal thermal limits of the IGBT modules are calculated based on the phase current waveforms they must switch.
- Clamp Meter Diagnostics: As Fluke's guide to 3-phase power measurements emphasizes, a standard clamp meter measures the magnetic field around a single conductor. You can only ever measure line current with a clamp meter. If you need to know the phase current of a Delta load for a forensic engineering report, you must measure the line current and calculate it mathematically. You cannot clamp a phase current without physically breaking the internal Delta connection, which destroys the circuit's operation.
Common Confusions and Diagnostic FAQs
Is the current printed on my motor nameplate phase current or line current?
It is almost universally line current. NEMA and IEC standards require motor nameplates to display the current drawn from the supply lines (Line Current / FLC). You will not find the internal phase current printed on the outside of the casing. Always use the nameplate amperage for sizing your wire, contactors, and overloads.
Do I need to multiply by 1.732 when calculating 3-phase power (Watts)?
Yes, but only if you are using Line values. The standard 3-phase power formula is P = √3 × VL × IL × Power Factor. If you happen to know the actual internal Phase voltage and Phase current, the formula simplifies to P = 3 × VP × IP × Power Factor. Both formulas yield the exact same total wattage, but mixing line voltage with phase current will give you a wildly incorrect number.
Why did my Wye-connected motor trip the breaker when my Delta motor didn't?
In a Wye (Star) connection, Line Current equals Phase Current (IL = IP). There is no 1.732 reduction. The supply lines must carry the full brunt of the winding current. If you reconfigure a motor from Delta to Wye without adjusting your breaker or overload settings, the line current will increase by a factor of 1.732 for the same mechanical output, instantly tripping your correctly-sized Delta protection.
What happens to phase current during a single-phasing fault?
If one external supply line (L1, L2, or L3) drops out, the motor single-phases. In a Delta motor, the phase current in the winding directly across the remaining two lines spikes to 1.732 times the normal line current, while the other two windings share the remainder. This unequal internal phase current distribution is why solid-state phase-loss monitors are critical; standard thermal overloads might not trip fast enough to save the winding carrying the massive phase current spike from melting.
