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Installing an electrical circuit

How to choose and install the circuit breaker and wire for the power tool of your dreams

Last update: Apr 26, 2011

WARNING! I am not a professional electrician. I actually was a licensed residential contractor, but that was 25 years ago. Furthermore, I am not aware of all the various electrical codes around the country. This page merely conveys the way I do things in my own shop, which may be horribly wrong and result in my eventual injury or death! If you base your wiring plan on this information, you should have a professional electrician at least review it before installation. Your building inspector would probably catch any mistakes, but it's better to install it right that have to re-do it. Uh, you ARE getting a building permit, right?

OK, now we have that warning out of the way. Seriously, folks, I really don't know all the ins and outs of wiring. I just know what I think I know and so far it works.

What I think you know

I'm assuming that you're already somewhat familiar with electrical wiring, this is not an absolute beginner's course. I assume you know how to open up your panel box, pop in a new breaker, and attach a wire to it. If not, STOP HERE AND HIRE AN ELECTRICIAN.

"Normal" power

I'm only going to address the type of power typically found in an American home, namely Alternating Current (AC) at either 120 or 240 volts, single phase, 60 hertz. If you need help with other stuff like 3-phase 440, you need to hire someone who knows what they're doing.

Quick Synopsis

This page started out as a quick and dirty guide to sizing a circuit. As I got more and more questions, it's expanded to the montrosity you see today. For those of you who want the quick but potentially misleading version ...

Get the load you need based on the device(s) you're supplying power to.

Use 14-2 wire for up to 15 amps, 12-2 for up to 20 amps, and 10-2 for up to 30 amps.

If you want a 240V circuit, use a double-pole breaker. Connect the black and white wires to the breaker and the ground wire to the ground bar.

For a 120V circuit, use a single-pole breaker. Attach the black wire to the breaker, the white wire to the neutral bar, and the ground wire to the ground bar.

If you didn't understand that or you just want to learn more about the ins and outs of this, read on ...

Three parts of an electrical circuit

An electrical circuit consists of three parts: First, we have a circuit breaker, which I'm assuming you know what that looks like. Second, we have a wire, if you don't know what that looks like go hire an electrician. Finally, we have a load, which is the device (like a table saw) or devices (like shop lights) that are drawing electrical power through the circuit breaker and wire.

How big is the load?

The first step in sizing a circuit is finding out how much electricity your device is going to pull. Every electrical device operates at a particular voltage and type of electricity and draws that electricity at a rate measured in amps. Every device should have a label of some kind that will say something like "120V, 60Hz, 2.2A". What that's saying is "this device uses 120 volt power that alternates 60 times per second and pulls it at 2.2 amps"

The voltage and Hz will always be steady, but some devices draw a burst of power at startup and then use a lot less when just running. Your device may specify a startup amperage and a run amperage. Generally, a circuit breaker will not trip on a momentary burst of high current and the wire will definitely not have a problem unless the burst is ridiculously high (I've never seen anything like that). However, I'll usually take the wire I'm planning to use and temporarily connect the tool to the circuit breaker, just to see if it's OK. If the circuit breaker trips, check with a real electrician on how to handle it.

Some things, like some motors, can be run on either 120 or 240 volts, you just have to change some wires on the motor (it's easy). Generally, you want to leave machinery at 120V. See "Should you bother with changing your motors from 120 to 240?' for more details on why it's not worth the trouble.

If you're wiring a device that's specified in watts, divide the wattage by the voltage to get the amps. For example, a 500 watt halogen light running on 120V power is drawing about 500/120 or about 4 amps.

Isn't 240 cheaper than 120?
This is a common misunderstanding, because 240 has half the "draw". However, the electric meter is measuring watts, not amps. So whether you're running 120 volts at 4 amps or 240 at 2 amps, 120Vx4A = 240Vx2A = 480W. Now, having said that, I think that 240 will be slightly cheaper because of greater transmission efficiency at higher voltages, but I'm sure the difference is miniscule.
Why do many people talk about 110V and 220V?
The actual voltage in your house can vary depending on the power company. The "low" voltage on a device might be 110, 115, or 120, whereas the high voltage will be 220, 230, or 240. Devices are always specified with one of these voltages. A more accurate name might be "one-leg" and "two-leg", as you'll see in a later section. The United States has been standardized on 120V and 240V for several decades at least, but the phrases "one-ten" and "two-twenty" are still very commonly used.
So, my "low-voltage" landscape lights are 120V?
No, landscape lights are almost always Direct Current (DC) which is usually around 12 volts, which is safer and lets you use things like vampire taps. But that's a topic for another day

Next, pick the wire

We're going to simplify wire sizing by limiting ourselves to the three wire sizes you're most likely to use and mostly likely to find in your local hardware store. In a home shop, it's unlikely you'll have loads requiring extremely heavy wiring.

I'm also assuming that you're using relatively short runs, let's say under 100'

Simple case

If you're wiring up a single device, like a table saw, life is easy. Just pick the wire based on the load from the label.

Amps of Load Use this wire
15 or less 14-2
15-20 12-2
20-30 10-2

The naming convention tells you the gauge (thickness) of the wire and the number of conductors, e.g. 2 14-gauge wires (lower number = thicker wire). All the common wires you'll find at your local hardware store will also contain a third ground wire that's bare copper, which you'll need. Here's some 12-2 showing the individual wires:

You'll hear people use the terms "Romex" or "Simpull" but those are specific brand names that are commonly used to refer to this class of wiring, not specific wire sizes. You'll probably buy it in a roll, here's a typical 250' roll of 12-2:

Complex case

Choosing the wire size gets more complicated when wiring multiple devices like banks of lights. Here, we have 5 banks of 4 lights each, where each light draws just 1.4 amps

Although it's not technically correct, you can think of electricity "flowing" from the circuit breaker (purple) to the lights, which take off their part of the power and pass on the rest. At every point, the wire must handle all the power required by all lights "downstream" from it. So the wire lengths on the right are only supplying one light each, which means they're only carrying 1.4 amps. The next segment upstream is feeding two lights, so that part has to handle 2.8 amps. So the wire at the beginning of each bank of lights has a total of 5.6 amps. All these loads are easily handled by a 14-2 wire (green).

However, when we start running the line back to the breaker (purple) by the time we add on the third bank we're at 16.8 amps and need to switch to 12-2 (orange) to handle that current load. The fourth bank pushes us over the limit for 12-2, so we need to switch to 10-2 (red). Since our final amp load is 28, we'll use a 30 amp breaker.

Realistically, if these lights were close enough together, we'd probably just run 10-2 for the entire main run and use 14-2 for each individual bank. However, understanding the amp load allows us to use inexpensive 14-2 for most of this installation instead of expensive, heavy, and hard-to-work 10-2.

As an aside, yes, we could use progressively smaller wire as we move down each bank of lights. However, that would be a major pain and not worth the effort unless these lights where located a long distance from each other. And, in that case, you'd have to start figuring line losses and end up hiring an electrician anyway.

Non-simultaneous use

In a home shop, wiring can be installed a little different than a commercial shop. For example, let's say you have an 18A power saw, a 16A planer, and a 12A jointer. You could run a single 12-2 wire feeding all three of them, since you're unlikely to be using more than one at a time and, as long as you're using a 20A breaker, you can't do any damage. If your shop is used by multiple people, run individual lines to each tool.

Same thing applies to outlets: A single wire can feed multiple outlets, understanding that the TOTAL simultaneous load is limited by the wire (and breaker). Size your wire based on how many things will be plugged in at once into all those connected outlets, remembering things like space heaters, fans, etc. To be safe, I step up one size, e.g. if I think I need 14-2, I'll install 12-2 to be sure.

Outlets are not lighting!

Don't make the mistake of running multiple wire types to outlets. Based on the lighting example, you might be tempted to run 10-2 wire around the shop and 14-2 drops to each outlet. However, outlet loads are variable, e.g. you may think you'll never put more than a 15 amp load on an outlet. But then you plug in a space heater along with your other tools, taking the load to 25 amps. The 30-amp circuit breaker you installed won't pop and the 10-2 wire is fine, but you're overloading the 14-2 and soon you're wondering about that smokey odor ...

Wire outlets with a single wire size all the way around.

Finally, pick the breaker

OK, this is the easiest part. If you're running a 120 circuit, you need a single-pole breaker. For 240, you'll use a double-pole breaker. Here are some breakers in a panel box:

Breakers 1 & 2 are single-pole (SP), i.e. they switch a single wire. Breaker #3, which is twice at tall, is a double-pole (DP) breaker and switches two wires. Notice that the SP breakers have a black wire attached to them, while the DP has both a black and white wire attached. More on that later.

A breaker's amp limit is usually indicated by the number on the toggle. It can be printed, molded, or both. In this case, breakers #1 and #2 have "15" molded into the toggle, meaning they're 15 amp breakers. I hope that, by now, you've figured out that breaker #3 is a 20 amp.

Also notice how the DP breaker (#3) has just a single toggle (the switch flipper)? Some DP breakers will actually have two toggles tied together with a metal clip, same effect.

Now, for every rule there's an exception. Here's a wierd little guy ...

It looks like a DP breaker and, technically, it is. It's a breaker with double poles and double toggles, but not tied together. These are actually two half-height SP breakers combined into a single package the same size as a normal SP breaker. These are two separate breakers and NOT the same thing as a DP 240V breaker. It'll become clear later why this is true.

Before heading off the the hardware store, be aware that breakers are not all the same! Panel box makers have their own standards of how a breaker hooks to a panel. For example, a breaker from Square-D will not snap into a GE panel. Some manufacturers make different lines of panels whose breakers are incompatible, so you can't go by brand. Other manufacturers make compatible breakers for multiple brands of panels. The best way is to remove an existing breaker and take it with you. The workers at most hardware stores will be able to help you get the right shape.

So, all you need to do is pick up the correct breaker in the correct size! Always choose the breaker size that matches the MAXIMUM capacity of your wire. So, if your table saw draws 18 amps, you'd choose 12-20 wire (max 20 amps) and a 20 amp breaker.

THE PURPOSE OF THE BREAKER IS TO PROTECT THE WIRE, NOT THE DEVICE! The breaker doesn't feed a certain amperage to your device, it allows your device to draw as much as it wants, up to a certain limit. That limit must be UNDER the capacity of your wire.

Here's a little table for 12-2 wire that may clarify things for you

12-2 Wire (20 amp max) 8 amp load 18 amp load 28 amp load
10 amp breaker OK, the wire will handle this and the breaker will allow it. Won't work. The wire would handle the load, but the breaker is too small and will trip. Safe, the wire would be overloaded but the breaker will trip first
20 amp breaker OK, the wire will handle this and the breaker will allow it. OK, the wire will handle this and the breaker will allow it. Safe, the wire would be overloaded but the breaker will trip first
30 amp breaker OK, the wire will handle this and the breaker will allow it. However, if the load rises, the wire can become overloaded and the breaker will not trip. Do not use oversized breakers! OK, the wire will handle this and the breaker will allow it. However, if the load rises, the wire can become overloaded and the breaker will not trip. Do not use oversized breakers! FIRE! The load is over the wire's maximum capacity, but the breaker will not trip. Do not use oversized breakers!
What about your lighting example? You have a 30 amp breaker feeding 14-2 wires.
By the time you get down to the 14-2 wire, the wire is only feeding a few lamps and the amp load is well below 15 amps. Only in the main circuit feed, where the wire is feeding all the lights, does the the amperage rise. And there we're using 10-2

What if one of the lights gets a short? Won't that overload the 14-2?
A dead short will generate a huge amperage that will trip the 30 amp breaker anyway. A breaker's job is to protect the wire under normal load.

Wiring to your new breaker

TURN OFF THE POWER FIRST!!! Depending on your exact setup, you may or may not have a main breaker in the top of the box you're putting the new breaker into. If you have a really large breaker in the top of your panel, it's marked with a large amperage like 100, and there are no wires running into the side like the regular breakers, that's your main breaker. Most likely, inside your house, that panel will not have a main breaker, there will just be large wires running directly into large lugs on the top, like this:

If you don't have a main breaker, that means you're working on a sub-panel. The large wires are bringing the power from your main panel to this sub-panel. THOSE WIRES AND LUGS ARE ELECTRIFIED!!! If you slip and touch them, you're going to get the biggest and possibly the last shock of your life! Go find your main panel (it's probably right next to your power meter) and flip the really large main breaker:

As you can see, this is a 200 amp breaker! Look for a sticker or note like this one that says "Service Disconnect". It will cut off the power to the entire house. Now your sub-panel is safe to work on.

Let's start with wiring to a 240 breaker first, since that's actually easier!

The power comes into your house on four wires: Two hot wires (referred to as the "legs"), a neutral, and a ground. Here's the image of the no-main-breaker box again:

The two black wires are the legs. The way this works is that a connection between either leg and the neutral wire gives you 120V, while a connection between the two legs gives you 240V. The gray wire is the neutral and the bare silver wire is the ground.

A panel box is built so that the hot wires (legs) connect to two heavy metal bars called buses (or busses). Each adjacent breaker "slot" is has a connector to alternating buses. So, looking back at our first panel box:

Breaker #1 is connected to just bus (and leg) #1 and breaker #2 is connected to just leg #2. But breaker #3, since it's connecting to two adjacent slots, it actually connecting to both legs. This is why breaker #1 & 2 are feeding out 120, but #3 is feeding out 240.

This should also make clear why the little half-height breakers we saw earlier weren't a 240 breaker. Since they were both crammed into a single slot, they're both connected to the same leg, so they're feeding out 120.

OK, so now that we understand why a normal DP breaker produces 240V power, it should be obvious how we connect our wire to the breaker. That's right, put the black wire into one side of the breaker and the white into the other. Since you're forming a connection between the two legs, it doesn't matter which one goes where.

Why do some 240 breakers have one switch and others have two?
They don't. A 240 breaker has always has two switches (poles) inside, but sometimes a breaker is designed with one toggle to flip them both, other times the breaker has one toggle for each pole and they're hooked together to they can't move independently.

Why does the black wire have white spots?
Out of control painters. Just look at the breaker!

Couldn't you run each wire to a different SP breaker and still get 240?
In theory, yes, as long as each breaker was on a different leg. I wouldn't do that, partly because I'm not positive if there are any bad side-effects but mainly because it could be really confusing to someone else trying to understand your wiring job.
Hmmm, what about the third wire, the bare copper one? We'll get to that after I explain wiring 120

Wiring 120

Now that we understand how to wire 240, wiring 120 is easier to understand. Since the SP breaker is feeding out 120, we hook our black wire to it. But where does the white wire go? To form a 120 circuit, it has to connect to the neutral wire, but how?

Remember the neutral wire coming into the panel box? Well, just like the hot lines connect to bus bars, the neutral line connects to a neutral bus. Look into your panel and see what the neutral wire connects to, you should see it run to a connector on a heavy metal bar filled with holes and screws, like this:

The lug clamping the neutral wire is connected to the metal bus bus bar below it. There are holes running crossways through the bar. Pick any of the holes that are conveniently near your breaker and slide the end though, then clamp it in place with the screw (as a side note, with the breaker out of the way, you can clearly see the heavy copper hot bus).

Remember, a 120 circuit is formed by a connection between a hot leg and the neutral line. By connecting your black wire to a breaker and the white wire to the neutral bus, you've created that 120V circuit.

And now, for some Nakedness ...

No, not that kind of nakedness. Let's address that bare third wire.


Relax, that's the ground wire. Under normal circumstances, there's no electricity running though it. It's a safety valve that connects to the outside case of equipment, so that if the hot wires inside accidently come in contact with the case, the case doesn't become "live". If the ground wire wasn't there, the case would become electrified and you would be providing the ground, i.e. the electricity would flow though you. Which would be a very bad thing, indeed.

So, if we have hot busses and neutral busses, wouldn't we also need a ground bus? Well, you are correct sir! Looking at the other side of the same example panel we find ...

the incoming ground connected to another chunk of metal with holes and screws! How convenient!

So, for both 120 and 240 circuits, just attach the ground wire to this ground bus. Voila!

Neutral is not the same as ground. Usually ...

Since neutral eventually bonds to ground just like the ground wire, there's a school of thought that says it doesn't matter whether you tie neutrals and grounds to the proper busses. This is the School of Future Electrocution Victims, don't listen to them. There are times when the two busses are tied together, but 99% of you will be dealing with a sub-panel and that special situation doesn't apply.

Don't pay too much attention to these examples!

OK, now, this is VERY IMPORTANT: Don't depend on left and right to identify with bus is ground and which is neutral! In the one particular box we've been looking at, that's how it's arranged. It could just as easily be the other way around. Other panels will have neutral and ground busses on both sides:

In this box, the neutral is the gray-stripe wire is coming into the middle, a metal plate is running behind the plastic to the sides and connecting it to the busses marked N on each side. The ground is the green wire coming in on the right and connects to the ground bus on the right. An additional ground bus is on the left.

So, you ask, how is the bus on the left also a ground? It's not connected to anything! Well, in this panel, the right-side bus actually connects electrically to the panel box itself, effectively turning the entire box into a ground bus. So the left-side bus is also ground.

OK, how to tell what's what. Hopefully you're working with an existing panel and the people wiring it knew what they , in that case just see where the white wires and the bare wires hook up. Just follow the example before you.

If you're looking at a bare sub-panel, look at the incoming wires. I'm sure it's not universal, but in every panel I've ever seen the hot lines were solid black. Neutral wires were either black with a gray stripe or solid gray. Ground wires were usually bare wire, but occasionally green or black with a green stripe.

Wrap up

OK, that pretty much gives out all I think I know about sizing a new power circuit. Thank you to all of those who've asked questions and pointed out confusing bits, contradictory bits, incomplete bits, and flat-out typos.

If you have any questions or comments, please feel free to email me using the address at the bottom of every page.

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