Single Phase vs 3 Phase Power for a Home Shop
Single phase power is one alternating voltage that rises and falls in a repeating cycle. 3 phase power is three of these voltages on three hot wires. Each one runs a third of a cycle behind the one before it. Most US homes get single phase power, and most industrial machine motors are built for 3 phase. A home shop can still run those machines by making 3 phase from the single phase power it already has.
At Built for Production, we sell rotary phase converters, digital phase converters and VFDs (variable frequency drives) with single phase input to shop owners who run 3 phase machines on single phase power. Each of these takes in the single phase power your shop already has and puts out 3 phase power for the machine. Below, you'll see how the two kinds of power compare, how to tell which one you have, and what your options are when your machine needs 3 phase.
Prefer to watch? This 4 minute video covers the same ground.
The difference between single phase and 3 phase power
| Single phase | 3 phase | |
|---|---|---|
| Hot wires | Two hot wires plus a neutral (US home service) | Three hot wires |
| How the power arrives | Power comes in pulses and stops for an instant twice in every cycle | Power flows at a steady rate. When one wave crosses zero, the other two still carry power, so it never drops to zero |
| Common US voltages | 120/240V | 208/120V or 480/277V. Some older industrial buildings run 3 phase at 240V, so 240V alone doesn't tell you the phase |
| Where you find it | Most homes, plus some apartment buildings and small commercial buildings | Factories, industrial plants and larger commercial buildings |
| How motors start | The motor needs extra parts to start, such as a start capacitor or an extra start winding with a switch | The motor starts on its own |
| Typical use | Lighting, heating and household appliances | Large motors and machinery |
What is 3 phase power?
3 phase power is three AC voltages carried on three hot wires. Each one rises and falls like the power in your house. The three are staggered, so each wave runs 120 degrees, or one third of a cycle, behind the one before it.
The single phase power in your house is one wave. It passes through zero twice in every cycle, and at those two instants no power flows. With 3 phase, the waves are staggered, so one of them is always carrying power when another one crosses zero. That means 3 phase power flows at a steady rate and never drops to zero.
The staggered waves also matter for motors. When a 3 phase motor's windings are fed 3 phase power, they set up a magnetic field that rotates. That rotating field turns the rotor, so a 3 phase motor starts on its own, with no start capacitor or extra start winding.
What is single phase power, and what is split phase?
Single phase power is one alternating voltage.
Most US homes get 120/240V single phase service, which is often called split phase. The service has two hot wires, called hot legs, plus a neutral wire. The neutral is center-tapped, which means it sits halfway between the two hot legs in voltage. Either hot leg to the neutral gives you 120V for lights and plug-in loads. The two hot legs together give you 240V for big loads like a water heater or an electric range. Homes are usually single phase, but not always, and we cover the exceptions further down.
Is 240V single phase or 3 phase?
240V in a US house is single phase. The two hot legs come from the same single phase supply. A 240V load connected across both legs gets one alternating voltage, which is what single phase means. The 240V outlet for a dryer or a welder in your garage is single phase power too, because 120/240V residential service is single phase.
Some older industrial buildings have 3 phase power at 240V. So a machine listing that says "220V" or "240V" could need either single phase or 3 phase, and the voltage alone doesn't tell you which. The phase marking on the motor nameplate is what tells you.
Why do shop machines use 3 phase motors?
Most industrial machines use 3 phase motors. Those motors start on their own, and they come in a much larger range of horsepower sizes than single phase motors. A single phase motor has no starting torque by itself, which means it has no force to start turning. It needs a start capacitor, or an extra start winding with a switch, to get it going.
Factories and larger commercial buildings usually run on 3 phase service, so many of the machines built for them come with 3 phase motors. When one of those mills, lathes, table saws or air compressors sells used, it often has a 3 phase motor.
How can you tell which one you have?
You need two answers: what the machine needs, and what your building has. You can get both by reading labels and asking questions.
- The machine. Read the motor nameplate. Look for the line marked "PH" or "Phase." It tells you whether the motor is built for single phase or 3 phase power.
- A machine you haven't bought yet. Ask the seller for a clear photo of the motor nameplate. The voltage in the listing title won't tell you the phase.
- A nameplate that says 460V or 480V only. Ask the converter or drive maker and a licensed electrician about that machine before you buy anything, because a house usually gets 120/240V service.
- Your building. Call your utility. The utility sets the service at each address and can tell you whether it is single phase or 3 phase.
- Anything you can't answer from a label. Ask a licensed electrician to look at your service in person.
Can you plug a 3 phase machine into single phase?
No. A 3 phase motor fed single phase power can't make the turning magnetic field it needs, so it won't start on its own. Running a 3 phase motor on only one phase is called single phasing, and a motor left in that state heats up a lot. If a 3 phase motor hums, won't turn or starts to heat up, shut off the power right away.
Your options for running a 3 phase machine at home
There are six common ways to run a 3 phase machine in a home shop. Four are pieces of equipment that go between your single phase service and the machine. The other two are bringing in 3 phase from the utility and replacing the motor.
| Option | How it works | What it suits | Source |
|---|---|---|---|
| 3 phase service from the utility | The utility runs 3 phase power to your building. | A property near the utility's 3 phase lines, if the utility agrees to serve it. | Your utility (see the utility section below) |
| Static converter | It makes a third phase for a moment, only to start the motor. After that, the motor runs on single phase. | One motor, if the job only needs about two thirds of the motor's rated horsepower. It is not for CNC machines. | Phoenix Phase Converters' comparison guide; American Rotary's article on converters for CNC |
| Rotary converter | An extra motor, called the idler, runs without a direct mechanical load. Together with a control panel, it makes 3 phase power. | Several machines from one unit. | Phoenix's comparison guide |
| Digital converter | Electronic circuits build the third phase. | Phase Technologies rates its Phase Perfect units under 50 hp to start a motor as large as the converter's own horsepower rating. | Phoenix's comparison guide; Phase Technologies' Phase Perfect Gen4 manual |
| VFD with single phase input | A drive takes single phase power in and puts 3 phase power out to one motor. | One machine with one motor. It also lets you change the motor's speed. | Yaskawa's application note on single phase input; Phoenix's guide to converters for CNC machines |
| Single phase motor swap | You take off the 3 phase motor and fit a single phase motor. | One machine with one motor, when a single phase motor bolts into its place and your utility accepts its starting current. ABB's Baldor-Reliance line lists single phase motors up to 15 hp. | ABB's Baldor-Reliance catalog; our post Can a 3 phase motor be converted to single phase? |
If you have one machine with one motor, and the motor fits inside the drive's single phase rating, start by looking at a VFD with single phase input, since a VFD runs one motor. If you want to run several machines from one unit, start with a rotary converter, since one rotary converter can run several machines.
A transformer isn't one of the options. It changes voltage, and it can't make 3 phase from single phase. Our guide Can a Transformer Make 3 Phase from Single Phase? explains why.
For a full walk-through of each option, read How to Run 3 Phase Machines on Single Phase Power. If you're choosing between a rotary converter, a digital converter and a VFD, Rotary vs Digital Phase Converter vs VFD compares them side by side.
Can a house get 3 phase from the utility?
Sometimes. Homes are normally served single phase, but some homes that run machinery do have 3 phase service. Whether you can get it often depends on where your utility's nearest 3 phase lines are. The utility decides which service it provides at your address. Call your utility and ask whether 3 phase is available where you live. If it is, ask the utility for an estimate of the work to bring it to your building.
What to do next
If your motor nameplate says 3 phase and your house has single phase, pick the type of unit first, using the options above. Then choose the size. Our phase converter sizing calculator takes the numbers from your motor nameplate, applies each maker's published sizing rule, and names the maker's document so you can check the number yourself. When you know the size, you can browse our phase converters.
If you'd like to talk it through first, call (855) 923-2610 or send us a note through our contact page.