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Should you bother with changing your motors from 120 to 240?

Is there really any benefit? For the most part, the answer is no.

Published 1/10/2011

One very frequent question on woodworking forums is 120 vs 240 volt motors. Or, more specifically, how can I convert my wimpy 120 volt powertool into a manly 240 volt beast?

A common mistake
You may be thinking "my motor has a kilowatt (kW) rating next to the horsepower and it's nowhere near as much as the volts times amps". Be very careful here, because now we're getting into energy used vs work produced. The kW rating on your motor tells you how much power comes out, not what goes in. That kW rating is just the horsepower converted to thousands-of-watts (746 watts per horsepower), just another way to measure work. To match a motor to the job, you need to look at horsepower/kW rating. But when talking about the power flow, the kW rating doesn't matter. Without getting into too much detail, an inductive load like a motor doesn't actually consume all of the electricity it pulls, as much as 80-90% goes right back to the power company. When a motor is just spinning, this "give-back" or "power" factor is right at zero and rises as the load on the motor increases. That's why, if you merely look at the wattage input vs the horsepower output, you might think your motor is horribly inefficient. In truth, much of the power you have to through the wires is just "borrowed" from the power company.

Most of the time, this question is driven by a misunderstanding of power tool motors. Which is understandable, because we're used to cordless battery-powered tools where more voltage equals more power. But standard AC (alternating current) motors are a different animal altogether. Converting to 240V may be a good idea, but most likely is a waste of time.

To understand your options, let's get some background on electric power. In a typical home setting, your power arrives on two wires that we'll refer to as L1 and L2. You also have a neutral wire. Basically, a circuit formed by connecting one of the L's and the neutral wire gives you 120V power. A circuit that connects the L1 and L2 together gives you 240V power.

You also need to understand loads. A load is an amount of energy pulled by a device, in our case a motor. That load is measured in watts. Watts are a combination of voltage and amperage. You can think of voltage as the amount of force being used to push the electricity though the load, while the amperage how fast that voltage is flowing. Watts (or wattage) is simple a multiple of volts times amps. So, a 120V motor that's using 1200 watts of power is pulling 10 amps. (see "A common mistake" in the sidebar).

OK, now that we have a basic understanding of the power coming into your motor, let's address some specifics ...

Can you do it?

Not all motors can be converted. You need to check the motor, it should have a metal plate with the relevant information. Here's the motor plate on my bandsaw, click on it for a larger view

As you can see, the voltage has two numbers: 115 and 230, which means that this motor has been designed to work with two different voltages. Don't worry that they're not quite what you have, 115 vs 120 or 230 vs 240 isn't important.

If your motor only has one voltage rating, there's nothing you can do.

How do you do it?

Motor designs are different, this is just an example. To convert your motor, you need the directions for your specific motor!

In every motor I've seen, there's a bundle of wires inside a metal housing attached to the motor. How these wires are attached to the power lines and to each other determine the voltage for the motor. The directions are usually printed inside the housing or right on the motor plate. Here's the part of the wiring diagram from another motor:

What are L1 & L2???

As I said before, there are two "hot" lines coming into your house. These lines go into your panel box. When you install a 240V circuit breaker, one side connects to L1 and the other to L2, so the two wires coming from the circuit breaker are also L1 and L2. See "Installing an electrical circuit" for more details.

I've only shown the "high voltage" part of the plate. As you can see, the L1 and L2 lines are shown coming in. If you look at the motor, the wires leading into the motor are labled P1, T8, etc. In this case, for 240V operation, you connect the L1 wire to the P1wire and the L2 to both T4 and T5. T3, T8, and T2 are connected to each other and P2 is capped off (not connected to anything, just put a wire nut on it).

Why not do it?

There are several reasons that you shouldn't bother converting your motor ...

You won't save money
A common myth surrounding 240V vs 120V is that the machine now uses only half as much power. Remember, a motor draws a certain number of watts, which is volts multiplied by amps. So if we double the voltage, we cut the amps in half. Unfortunately, the power company measures your electrical usage (and charges you) in watts, not amps.
You won't get more power
There are some motors that do produce more power at higher voltages. Your power tools don't use those motors. Your motor produces the horsepower it's rated for. Really, can you imagine that a company's tool can produce a higher power rating and they wouldn't tell you about it? Your buddies who tell you their saw is more powerful, starts up quicker, etc, are engaging in wishful thinking unless their tool is long way off from the panel box (See "why would you do it" below).
There's no point in "balancing" the legs.
One of the stranger justifications I've seen for converting to 240 is that, because it uses both legs, the load is "balanced". The idea is that a 10-amp 120v load puts 10 amps on one leg and nothing on the other. On the other hand, a 5-amp 240V load puts that load of 5 amps on each leg, using a smaller portion of each leg. However, keep in mind that a 100 amp panel allows you 100 amps on each leg, for a total of 200 amps. Since a 240V load uses both legs, you're putting that 5A load on each one. So a 10-amp 120V load uses 5% of your panel's capacity (10/200) while the 5amp also uses 5% of the capacity (5*2/100).
You will cut down on your panel's physical capacity
Each 240V circuit breaker takes up two slots in your panel, whereas a 120V breaker only takes one. In most shops, where only one or two high loads run at once, slots for circuits are more of a premium than amps.
You (probably) won't increase your motor's life
Yes, a higher-amp motor runs hotter which cuts the expected lifetime. But we rarely run our equipment long enough for heating to be an issue. And if we cut our motor's service life from 30 years to just 25, is that really worth getting worked up over?

Why would you do it?

There are really only a couple reasons to bother with converting

You can use smaller wires in the circuit
The amperage your motor draws determines the kind of wire you need to run to it. Too high of an amp load will cause the wire to overheat. But larger wires are more expensive, harder to work with, and may not be available. For example, if your motor draws 25 amps @ 120V, rewiring to 240 will drop the load to only 12.5 amps. That means you can use 14/2 wire instead of 10/2, although you should probably use 12/2 to be safe. Of course, that only matters if you're installing a new wire. If you have an existing wire, rewiring for 240V means you can fit a bigger motor onto the same existing wire.
You will lose less power along the way (and you might notice it)
If your tool is located quite a ways from the panel box, higher voltages experience less drop during the voyage. Let's look at a fairly typical example. Let's say you have a 120V motor that draws 10 amps. The total wire length to the tool is 50' of 14/2 wire. The voltage drop for that situation would be 3 volts or 2.5 percent. Changing that motor to 240V would reduce the drop to 1.25 percent, in other words completely and totally not noticable.

If, however, we quadruple that length to 200' we also quadruple the loss, i.e. 10% versus 5%, which now might be noticable.

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