In my infinite wisdom for preparation to buy an air compressor I have decided to add a 20 amp circuit to my existing breaker box. Can I just install a 20 amp breaker in the box and wire heavier gauge wire to a brand new 20 amp outlet? Or is this something I need to hire an electrician for?
How much headroom does your service have?
Hire an electrician. Way too many things can go wrong if you don't know what you're doing. I know, this is GRM, but there is a point where cheap becomes dangerous.
In reply to cwh:
+1. If you know what you're doing, it's pretty easy. If your existing panel has room it's easy. If you're asking about it on a car forum, it might not be so easy. Do a search on YouTube for how-to videos and see if that makes sense to you. If you have any concerns whatsoever, hire an electrician.
To put it bluntly: in a worst case scenerio, getting it wrong can mean burning your house down.
I design this stuff for a living, so it means nothing to me to add a breaker to a panel. Hell, I rarely even shut the panel off when doing so. That said, I know a lot of people who go white when I say that.
The first thing you need to know is what your breaker box capacity is. Many older houses have 125 amp panels, not really enough extra capacity to add 20 amps comfortably. Later houses generally have 200 or better amp panels, as long as the total capacity allows another 20 amps yet leaves some capacity you are generally OK. Like this: if it's a 200 amp panel, you add the total of all the existing breakers and it's 150 amps that means if you add another 20 amps you still have a 30 amp 'cushion'. I'd be OK with that.
If it's a 125 amp panel and you are already using 110 amps that means if you add 20 amps you are now at 130 amps, over the panel's capacity.
Keep in mind both these examples are assuming that at some point everything in the house is turned on at the same time: HVAC, washer, dryer, water heater etc. While this is not very likely it is possible, overloading the wiring coming to the panel is an excellent way to set something on fire.
As far as the mechanical connections they are really quite simple. At the same time they can be extremely dangerous if you don't know what you are doing. 110v will give you quite a jolt and can be fatal under the right (wrong) circumstances. 220v can be instantly fatal.
There are tools available to check for voltage; I have a neat one which can detect power by placing it against an insulated wire, you don't have to poke a hole in the insulation. It was in a set of Greenlee electrical tools that cost me a whopping $28. If it's placed within 1/2" or so of a 'live' breaker box terminal it will light up and buzz then too.
Consult a code book to confirm this is kosher, but a lot of time you'll find whoever wired the place used 12 gauge wire for a little added safety(in their mind), even though there is a ton of safety factor already built into household wiring. If you can verify the run is 12 gauge all the way from the breaker to the outlet, you can just pop in a 20 amp outlet and breaker.
Curmudgeon wrote: Keep in mind both these examples are assuming that at some point everything in the house is turned on at the same time: HVAC, washer, dryer, water heater etc. While this is not very likely it is possible, overloading the wiring coming to the panel is an excellent way to set something on fire.
I thought this was the whole point of the "main" breaker? E.G. my parents house, which given, is not wired that well, has a 100 amp panel, the 220 breakers alone add up to 110 amps(well dryer garage), then there's a whole 4 bedroom house of 15 amp breakers under them.
That's not exactly right.
The breaker box capacity is irrelevant.
The only thing that matters is the load.
If you have a 125 A breaker box, and 120A of breakers in it, but never pull more than 60A of load, there is no reason you can't add a 20A circuit.
But you also need to derate the circuit and/or system.
The National Electric Code requires 25% margin.
So, if your load is 100A, multiple by 1.25. You would need a breaker of 125A.
You can determine load with a load meter, or calculate the loads of all appliances.
Most houses have over rated circuits. This means that the circuit can handle more load, but never will.
Additionally, the individual circuit breakers should then be swapped so they do not exceed 75% of the load.
Might want to hire an electrician.
SVreX wrote: That's not exactly right. The breaker box capacity is irrelevant. The only thing that matters is the load. If you have a 125 A breaker box, and 120A of breakers in it, but never pull more than 60A of load, there is no reason you can't add a 20A circuit. But you also need to derate the circuit and/or system. The National Electric Code requires 25% margin. So, if your load is 100A, multiple by 1.25. You would need a breaker of 125A. You can determine load with a load meter, or calculate the loads of all appliances. Most houses have over rated circuits. This means that the circuit can handle more load, but never will. Additionally, the individual circuit breakers should then be swapped so they do not exceed 75% of the load. Might want to hire an electrician.
Ding,ding, ding!! We have a winner. I bet if everyone went onto their panel box right now, and added up the total of the individual breakers they would come up with more than the total number on the main breaker, doesn't mean anything as the load is not anywhere close to the size of the main. If you have a space in the panel for another breaker I don't see why you couldn't do it, but if you don't know what you're doing hire an electrician. Wiring is not a hobby.
Just this past weekend my BIL asked me about some wiring that he was helping his friend do during the renovation of said friends bathroom. Turns out they didn't know what they were doing, and had I not gone over there as a professional licensed electrician I think we would have been reading about the fire in the house shortly afterwards. Scary what they had done! I didn't even charge the guy for my time, but begged him to call me in the future if he has anymore renovations going on. I will charge him next time.
Wait, you mean people don't use round and cylinder fuses anymore? FML.
I redid my detached garage's wiring last year, I don't have enough juice to run a 220 welder out there, but enough for a compressor and the 3 banks of 4ft florescent lights. Once the box was hooked up, I don't think I ever shut it down again to add breakers/circuits as I went either. Ever since I got buzzed by a 220v panel working on the farm's corn dryer, 110v just tingles. 
yamaha wrote: I don't think I ever shut it down again to add breakers/circuits as I went either. Ever since I got buzzed by a 220v panel working on the farm's corn dryer, 110v just tingles.
It's better to be safe than sorry...more people in the US are killed by 120vac electrocution than any other voltage, I think mainly because they let their guard down.
Again, not quite.
It's not the volts that will kill you, it's the current. The AMPS.
A 220 line has 2 hot legs. Let's say you are running a dual voltage motor- a motor than can be set up to run either 110V, or 220V.
Let's say it's a 10A 110V motor (pretty big). That same motor will pull 5A on each leg if running on 220V.
If you short the motor through your body on 110V, you're gonna get nailed with 10A. If you short it through your body on 220V, only 5A.
110V has twice the amperage of 220V, therefore twice the likelihood of killing you.
Additionally, there are probably 20X as many 110V circuits in residential wiring in the US as there are 220V circuits, so far more people die from 110V
In reply to SVreX:
IDK, granted I'm not arcing 110v stuff, just my hands conducting when wiring outlets. The 220 panel had been hit by lightning, and something inside the box had melted, breakers blown apart, it was a bad feeling when I opened it up. Damn thing had electrified the box itself. My arms hurt after that one. 
In reply to yamaha:
It's pretty unlikely you got hit with 220V. You would have to short BOTH hot legs at the same instant.
A single hot leg of a 22V circuit is only 110V.
But you DID get juiced by high amperage.
The way a motor works is probably not the right analogy for a person being electrocuted. Meat sacks are purely resistive loads to electricity, like a light bulb; twice the voltage will push twice as much current through. Put a 220v light bulb with a fixed amount of resistance into a 110v circuit and it will be dim and draw less current than it did hooked up to 220v and it will draw enough current to immediately blow out if you put it in a 450v circuit.
Well under a single amp is plenty to kill if it goes through the heart, about a tenth of an amp was the number they threw around when I was in training. Worst case for a person who was dehydrated, drunk, tired, and sweating (all things that make a meat sack conduct electricity better) the human body typically still offers up over 300 ohms of electrical resistance along a path that goes from a hand through the heart and out of the body. That makes 30v the limit for how high voltage can be without being able to drive enough current through the body to be fatal, which is why there are different rules for low voltage stuff.
One tenth of an amp can kill you. You can calculate what that will be by holding both leads of an ohm meter, one in each hand. I=E/R. 0.1=E/R_(on the meter.) Algebra... E = 0.1*R. Or, your resistance divided by 10. That's your personal kill voltage if you ground one hand and touch hot with the other.
Its also heavily related to the path through your body. If it goes through your chest cavity 0.1A (yup 100 mA) can kill. It interrupts your heart.
OP: Hire an electrician.
Dr. Hess wrote: One tenth of an amp can kill you. You can calculate what that will be by holding both leads of an ohm meter, one in each hand. I=E/R. 0.1=E/R_(on the meter.) Algebra... E = 0.1*R. Or, your resistance divided by 10. That's your personal kill voltage if you ground one hand and touch hot with the other.
For added fun, do this again with sweaty hands. Plenty of other ways you can kill yourself with just 110 volts too, like grabbing something hot, and then not being able to let go.
In reply to SVreX:
Yea, it wasn't fun and I don't recommend it. 
I got 277 volts through my left hand and out my chest as it was on the drop ceiling grid. Add that I was sweaty, and up very high on the ladder, and I was lucky I didn't get hurt. I hate 277/480!!
Looks like ill be calling an electrician soon. Thanks guys. I may still do it myself. I had an electrics shop class in jr high. So its only been 30 years. What could go wrong?
I'm going to disagree with everyone.
It really isn't that hard. 3 or 4 wires depending on if you need a neutral (an air compressor usually doesn't). There are a number of books out to walk you through it step by step. Buy a good meter and if in doubt, test it. DO NOT WORK IT HOT. SHUT OFF ALL POWER AT THE MAIN BREAKER BEFORE YOU PULL THE PANEL COVER.
Yes it can kill you, yes it can burn your house down, but screwing up on a brake job can kill you, your family, and the people in the other car, and total your car, just as fast.
Do the research, and do it right. It's not voodoo magic, it follows laws and rules like everything else in the world. The guy that wired your house, was probably a helper, with a HS education. It's seriously easy. I've been wiring on houses with my dad since I was 12. He's not an electrician. I've been doing brake jobs on family cars for about as long. He's not a mechanic. I still do brake jobs on the family cars. I'm certain, I do a better job than the average grease monkey. I'm not a mechanic.
Do the research and do it right. Anything you do is probably less likely to burn your house down than the wiring that's already there. At least you care about the finished product.
True. I've been slowly replacing all of my electrical switches as they wear out and start arcing since the builder used the cheapest available. It's been easy. The new switches average about four times the cost of the original ones I'd bet. I'd also bet they'll never need replacing again. I won't do anything to anything unless I think I can make it better. I also don't have a death wish so I take my time and my precautions seriously. Good times.
In reply to Toyman01:
I remember reading somewhere that electricians can tell if you do your own work, as it's usually either picture perfect or terrifying.
Here's the deal...
It's not hard. But every mistake that has ever happened with electricity happened because somebody just didn't know.
There is so much info out there now, it's pretty tough to choose which to follow. Heck, this thread alone has posted many things that are just incorrect.
Problems happen because of the little stuff. A staple driven too tightly, a screw tightened too loosely, a wire stripped too far leaving bare hot wire exposed as you push the device back into the box against the ground. A loose or non-existent ground. I have worked in hundreds of houses, and it is REALLY common to find loose wire nuts, overtightened romex clamps, evidence of arcing, overworked circuits, jam packed breaker panels and boxes, etc. etc. These are things that are not easy to pick up from a book, but they DO burn down houses.
I think virtually anyone on this board could do this stuff, but that 95% of them would make a mistake because they really didn't know what to look for.
There has been a lot of discussion in this thread about sizing the breaker/ wire/ and panel. Yet the OP hasn't even said if there is space in the panel.
If the panel is full, it's NOT a DIY project. If the OP has any level of discomfort or unease, its NOT a DIY project. If the basic calculations described in this thread are confusing, its NOT a DIY project.
Can anyone run wire? Sure. Is it smart for me to recommend it over the internet to a virtual stranger who has not really demonstrated an aptitude or knowledge on the subject? Absolutely not.
My suggestion to ask for help was not because it is hard. My suggestion to ask for help was because you sound like you need a little help.
In reply to SVreX:
Every failed brake job happened because someone didn't know and home wiring is just as easy to learn as brake work. It's a skill and a easy skill at that.
I've been working in the construction industry for almost 30 years and have seen most of it. 90% of the truly crappy work is done by professionals. I don't have much faith in them as a general rule and I am one. A homeowner, willing to learn and do it right, will always do a better job.
The average person on this board is smart and fairly anal when it comes to doing things right. I don't have a problem suggesting that they try something. The gentleman in question is in his mid 40s and has two fairly nice looking, well kept cars. If he's half as anal as the rest of us, house wiring will be a walk in the park. The most important part, he's asking questions because he is smart enough to know that he doesn't know. That alone puts him above most of the population.
Buy the book, read it, then make the decision on calling a pro or not.
I'd get this one.
http://www.amazon.com/Black-Decker-Complete-Guide-Wiring-ebook/dp/B00K4SEXLK/ref=sr_1_4?s=books&ie=UTF8&qid=1420898343&sr=1-4
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