Any electricians in the group?

This probably isn't the best group. You need one that deals with household electrical issues. I don't know about newsgroups but I'm pretty sure I've stumbled across internet forums that deal with things like this. I am not an electrician but know a bit about it so I'll try and give you some help.
The breaker may be faulty. Best way is to substitute a known good one. You may have another 20A in the panel you can swap around. Please do not do this yourself if you don't know how to do it safely. You can get killed.
I suspect the real reason is that your barn is a fair distance from your panel and the startup surge for the compressor (which for a brief period of time if the motor starts is several times its rated full-load current) is causing a significant voltage drop along the wire. This means the motor can't quickly develop enough power and tries to draw even more current causing the breaker to trip.
The key to more power in your barn is related to the wire size. For normal length runs your 20A breaker will likely be supplying #12 wire. If you have #10 wire then you could install a 30A breaker safely; this is unlikely if the original installer just wanted some lights and light-duty machinery in the barn.
If you want to run a welder, compressor etc then you need larger wire with the appropriate sized breaker for it. The best solution would be to install a sub-panel in the barn supplied from your main panel. I would say that a panel providing 60A at 240V would be a minimum and that may be marginal if you were welding at high-current and the compressor started. Installing a panel with the suitable cable to handle 60A or more won't be real cheap but without it you will lose the ability to work in the barn. It would be best to have a local electrician come in and evaluate your specific needs.
Billh

I'm not an electrician but here is the usual suggestions: (Insert usual comments about "if you don't know what you are doing, keep your fingers out and get someone who does know what they are doing!")
2 questions right off: how far is the barn (and what kind of obstacles, where is the main breaker panel, etc) and what size equipment do you plan/want to run? ie a Lincoln Tombstone welder wants a 50 amp circuit, a small wire feed is happy with a good 20 amp circuit.
Blowing the breaker: a couple of things come to mind. First is that your long run of wire to the barn may be dropping the voltage so much that the compressor is starting hard. The other thought is a 'tired' breaker. The cheaper brands of breakers are not really meant to snap very many times. Swap the barn breaker with another breaker and see what happens.
Fixing it right means running a heavy 3 wire plus ground cable to the barn and installing a full sub panel out there. 240 volt 30 amp is the minimum you want to consider, 60 amp is much better. Biggest expense is the cable, a couple hundred feet of copper 6-3wg is not cheap! I don't have my NEC handy so I can't suggest the correct size wire.
Depending on your power company policies, how far from existing meter to the barn, and how much power you need, I have seen situations where it is cheaper to run a second power meter just for the barn. There are some downsides but a phone call to the power co is free.
When you dig the trench consider using conduit plus consider burying an alternate conduit for either a spare power cable or for a phone, CATV, security, whatever. Flexibility is nice, conduit is cheap, treches are hard work. Beware of horses, they are real hard on the conduit near the barn! (Bury it deep and armor it heavily!)
Technically you do not need to use the last two slots on your main pannel and can run the cable directly from the main disconnect to the sub pannel in the barn but it certainly is nice to have a breaker in the main box to control the barn. Again, flexibility is nice.
Drop the same question over on rec.crafts.metalworking Bruce Bergman should pick up on it, give you a much more complete answer.
Rick Barter wrote:

Er...I think I'll be calling and electrician. :)
rvb

Rick, The breaker maybe the right size for the cable but wrong for the distance. If the barn is over 50 cable feet then you need to go one size bigger for each 50 feet or reduce the size of the breaker on size for each 50 feet. Michelle
Rick Barter wrote:

An excellent synopsis and I agree with you. Though I would make that a 100 amp service out at the garage. Depending on the run distance and a bit of judicius scrounging..the wire may be easy to find used from a demo job somewhere in the proper sizes.
This is one place that it pays to have a real electrician involved.
Gunner
Gunner "Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

Most everything folks have said has been right on, you certainly need a proper upgraded sub panel in the barn if you want to run more than lights there.
One thing that wasn't covered fully is the "tired" circuit breaker. Most circuit breakers are only rated to be accurate for like 3-4 trips, after that the trip point can drift. Manually switching generally doesn't count against those trips, although there are breakers specifically rated for switching duty. In any event, if hard compressor starting has tripped that breaker more than a few times the breaker should be replaced since it's trip point could now be 15a or 25a or something else other than it's 20a rating.
Pete C.

What are you talking about?

Well, I got my license in 1990, but have not been working full time as an electrician for a while now. The wire run out to the barn may be undersized, the connections to the breaker may be bad, the breaker may be tired or not an HACR type breaker which allows for motor starting surges. If you want to use all of the stuff you mention, some of it simultaneously, I would suggest a 60 amp subpanel or better.

Faulty breaker, possibly a loose conductor. Replace the breaker and re-tighten all connections.
Hire a sparky to put a subpanel in the barn with separate circuits, each with enough capacity to handle what you plan to use. Feed this subpanel with conductors of sufficient size for the total load of the subpanel and fuse at the main panel with the proper sized breaker to protect these conductors.
Keep in mind not to exceed the total capacity of your main panel (likely 100 or 200 amps), but this will be a permit job, so your hired electrician will (hopefully) do it right.
--- Rich

Generally they get easier to trip, rather than harder. Ive had this problem before when pushing the breaker hard in hot weather. Have had to change them out because they got too sensitive as they got older
Gunner
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

You might think about running 240 volts out to the barn. I am assuming that you now have three wires running to the barn. One hot, one neutral and a ground. By running one more wire you could have 240 volts at the barn and be to draw twice as much power out there. 240 volts will run a fair sized compressor.
Dan

Whoops - I think that is only Europe and Asia.
US uses two Hot wires and a safety Gnd.
I have by code a 4 wire 220 on the wall - Two hots, 180 degrees out of phase and a neutral and ground. I have a 4 wire to 3 wire adapter cable - but it is a bit short for nice use or is a bit long.
So be sure to know what type of 220 you are using.
Martin

I was figuring that if he ran one more wire, he could have 120 volts in the barn for lights and portable power tools and 240 volts for compressors and welders.
Dan

He would need a 4 wire system - 2 hots for the 220, one neutral that with either hot lines is 120 then the safety GND. At the far side - barn - copper coated steel rod pounded into the ground and then a lead from it - typically bare copper to the junction box and joining to the safety ground. In some areas, this could be a three wire - two hots and a neutral - then mandatory safety ground locally. Red, Black and White is common enough.
Luckily, I have a pole and transformer for the shop. Sadly, the other utilities - water, phone are 2-3 inches sub surface and running down the side of a drive way that is sand and small stone. Guess what gets washed up and where a drain line needs to be put. I hope I have the center of the road for a long enough section to funnel water. Then it is ripe for a bridge. Always paying more when someone cheaps out in the first place.
Martin

"lionslair at consolidated dot net"
> snipped-for-privacy@krl.org wrote: >> I was figuring that if he ran one more wire, he could have 120 volts in >> the barn for lights and portable power tools and 240 volts for >> compressors and welders. >> >> Dan >> > He would need a 4 wire system - 2 hots for the 220, one neutral that with > either hot lines is 120 > then the safety GND. At the far side - barn - copper coated steel rod > pounded into the ground > and then a lead from it - typically bare copper to the junction box and > joining to the safety ground. > In some areas, this could be a three wire - two hots and a neutral - then > mandatory safety ground > locally. Red, Black and White is common enough. > > Luckily, I have a pole and transformer for the shop. Sadly, the other > utilities - water, phone > are 2-3 inches sub surface and running down the side of a drive way that > is sand and small stone. > Guess what gets washed up and where a drain line needs to be put. I hope > I have the center > of the road for a long enough section to funnel water. Then it is ripe > for a bridge. Always > paying more when someone cheaps out in the first place. > > Martin > > -- > Martin Eastburn > @ home at Lion's Lair with our computer lionslair at consolidated dot net > NRA LOH, NRA Life > NRA Second Amendment Task Force Charter Founder > >