Single-phase AC (1 phase AC) is an alternating current power distribution system where all voltage waveforms change in unison, typically delivered via one live conductor, one neutral conductor, and one equipment ground. If you are wiring a standard home outlet, installing a window air conditioner, or powering a benchtop oscilloscope, you are working with 1 phase AC. Understanding this waveform dictates your maximum continuous power envelope per circuit and forces specific hardware choices—most notably, the requirement for start capacitors in induction motors, because a single alternating waveform produces a pulsating magnetic field rather than a naturally rotating one.

The most common point of failure for DIYers and junior technicians is confusing true 1 phase AC (like Europe's 230V Line-to-Neutral) with North American split-phase 240V. Split-phase is technically two 120V legs 180 degrees out of phase sharing a neutral, derived from a center-tapped utility transformer. While it behaves like single-phase for basic load math, the wiring topology, breaker requirements (2-pole), and neutral current cancellation rules are entirely different.

The Math: Real-World 1 Phase AC Load Calculations

Let's look at what 1 phase AC changes in a real installation by sizing a circuit for a 2,400W baseboard heater. Because a heater is a continuous load (expected to run for 3 hours or more), the NEC requires us to derate the circuit to 125% of the actual load to prevent breaker nuisance tripping and terminal overheating.

Scenario A: 120V True Single-Phase (Line-to-Neutral)
  • Base Current: I = P / V = 2400W / 120V = 20A
  • Continuous Derating (125%): 20A × 1.25 = 25A
  • The Pick: You must use a 30A single-pole breaker and 10 AWG copper wire (rated for 30A at 60°C/75°C). You cannot use a 20A breaker; it will trip thermally.
Scenario B: 240V Split-Phase (Line-to-Line, no neutral needed for pure 240V loads)
  • Base Current: I = P / V = 2400W / 240V = 10A
  • Continuous Derating (125%): 10A × 1.25 = 12.5A
  • The Pick: You can use a 15A 2-pole breaker and 14 AWG copper wire. Doubling the voltage halves the current, allowing you to use significantly less copper and reducing voltage drop over long runs.

This math scales up to heavy residential loads like Level 2 EV chargers. A 40A continuous EV charge rate on a 240V 1 phase AC circuit requires 40A × 1.25 = 50A. You must install a 50A 2-pole breaker and pull 6 AWG THHN wire in conduit. For detailed code compliance on continuous loads, always cross-reference NFPA 70 (NEC) Article 210.

Where You Meet 1 Phase AC in Practice

You will encounter single-phase and split-phase systems in specific environments where the total power demand does not justify the infrastructure cost of 3-phase utility drops.

  • Residential Branch Circuits: Standard 120V/15A and 120V/20A receptacles powering lighting, electronics, and small appliances.
  • Residential Heavy Appliances: Electric ranges, dryers, and heat pumps utilizing 240V split-phase (often requiring a neutral for 120V control boards).
  • Light Commercial HVAC: Rooftop units and condensing units up to 3 to 5 tons typically run on 208V or 240V single-phase before stepping up to 3-phase for larger buildings.
  • Workshop Machinery: Table saws, lathes, and milling machines under 3HP. (Anything larger usually requires a phase converter).

Decision Tree: Sizing Breakers and Wire for 1 Phase AC

Use this decision path to select your hardware. Always verify local AHJ (Authority Having Jurisdiction) requirements, as local amendments can override baseline NEC guidance.

If Your Load Is... And the Voltage Is... Then Calculate... Concrete Hardware Pick (Copper, 75°C Column)
Under 1,440W (Standard Plug-in) 120V 1-Phase Max 80% of 15A (12A continuous) 14 AWG NM-B, 15A 1-pole breaker
1,440W - 1,920W (Kitchen/Bath) 120V 1-Phase Max 80% of 20A (16A continuous) 12 AWG NM-B, 20A 1-pole breaker
1,920W - 3,840W (Hardwired Heat) 240V Split-Phase Watts / 240V, then × 1.25 10 AWG THHN, 20A 2-pole breaker
3,840W - 9,600W (EV Charger/Range) 240V Split-Phase Watts / 240V, then × 1.25 6 AWG to 3 AWG THHN, 30A-50A 2-pole
Over 10,000W or 3-Phase Motors N/A Stop. 1-Phase is inefficient here. Upgrade to 3-Phase service or install a VFD

The Motor Problem: Why 1 Phase AC Needs Capacitors

When you apply 3-phase power to a motor stator, the 120-degree phase offset naturally creates a rotating magnetic field. The rotor simply chases the field and spins.

1 phase AC cannot do this. A single alternating waveform creates a magnetic field that merely pulses back and forth along a single axis. If you apply 1 phase AC to a standard induction motor, it will hum, vibrate, and draw locked-rotor current until it burns out, but it will not spin.

Bench Fix: To make a 1 phase AC motor self-starting, you must artificially create a second phase. This is done using an auxiliary start winding wired in series with a start capacitor (typically 50µF to 200µF for fractional HP motors). The capacitor shifts the current phase angle in the start winding by roughly 30 to 40 degrees, creating an elliptical rotating field just long enough to get the rotor spinning. A centrifugal switch then cuts the start winding out of the circuit at roughly 75% of rated RPM to prevent the capacitor from exploding. If your compressor hums but won't spin, test the start capacitor with a multimeter's capacitance setting before replacing the motor.

FAQ: Clearing Up Single-Phase Confusion

Is US residential 240V actually single-phase?

Technically, it is split-phase. The utility transformer secondary has a center tap (the neutral). Measuring from either end to the center gives you 120V. Measuring across the two ends gives you 240V. Because the two 120V legs are exactly 180 degrees out of phase, they are mathematically a single sine wave stretched across a center tap. It is not 2-phase, and it is not true 240V single-phase (which would be Line-to-Neutral, as seen in Europe). For breaker sizing and wire ampacity, treat it as a single-phase 240V load, but always use a 2-pole breaker with a common internal trip.

Can I run a 3-phase motor on a 1 phase AC supply?

Yes, but not by wiring it directly. You have two concrete options. First, use a Variable Frequency Drive (VFD). You wire 1 phase AC into the VFD's input terminals (L1, L2), and the VFD's internal rectifier and inverter stage synthesize a 3-phase PWM output for the motor. A 2HP 240V VFD costs around $150-$200 and gives you soft-start and speed control. Second, use a rotary phase converter, which uses an idler motor to generate a synthetic third leg. For modern workshop builds, the VFD is the default recommendation due to lower cost and better motor protection.

Why does my 120V circuit measure 114V at the outlet?

Voltage drop. On a 1 phase AC circuit, the wire itself has resistance. If you pull 15A through 100 feet of 14 AWG copper, you will lose roughly 6 volts (calculating at 1.98 ohms per 1000ft for 14 AWG). 120V nominal minus 6V drop leaves you with 114V. While most electronics tolerate 114V-126V, induction motors will draw higher amperage to compensate for low voltage, leading to overheating. If your voltage reads below 115V under load, upsize your wire gauge.

When designing or troubleshooting 1 phase AC systems, always default to the 125% continuous load rule, respect the physical limitations of single-phase motor starting, and use a VFD when your horsepower requirements outgrow the single-phase envelope.