I bought a new DeWalt 779 Miter Saw, rated at 15A. Worked fine for a few days in the garage, then I added two more outlets on the same 20A circuit that I had been using. After a few cutting sessions, the breaker started tripping when using the new outlets (20A also). (I have two refrigerators and a laydown freezer on at all times on that circuit too.) In order to finish a project, I ran a 100ft extension cord to a outlet on the front porch and the saw worked great. Because of this, I decided to add another outlet in the garage, running 20A Romex from the porch outlet to the inside wall of the garage to use for the Saw. Lo and behold, when I found the breaker to shut off the circuit to the porch, I discovered that that outlet is on the same circuit as I was using before. However, the Saw and a huge floor fan both work perfectly from the new outlet, even with extension cords. Any ideas why it seems all these things can coexist on that circuit at the same time, except when the saw is plugged into one particular outlet? I guess it could be a problem with the one or both of the new outlets, but what would I look for or test before I plan on replacing them?
This is an interesting situation and the problem you have could be for more than one reason. I am going to try and explain it without getting too deep into the science behind it.
First you have simply too many pieces of equipment attached to your circuit. Since I don't know specifically the name plate ratings of you refrigerators or the freezer I have to use generic amperage's. A standard 20-25 cu/ft refrigerator uses 3.5 - 4 amps when it is running and a 10 cu/ft freezer uses about 2.5 amps. Your 15 amp miter saw will run about 1700 watts or 14 amps under full load. So if you add this altogether 3.5+3.5+2.5+14 it comes out to 23.5 amps.
Logically speaking you should be asking why doesn't it trip all the time? Well to start refrigerators and freezers only run about two hours a day, and your Miter saw pulls 14 amps if it is under full load. If you are cutting smaller softer wood then there is a good chance your Miter saw is only running at about half of full load or more like 7 amps. So when a any of these appliances are coming on there is a start up surge and if any other equipment is running it may be enough to trip a breaker.
That explains why you are tripping a breaker, but why doesn't it trip if you put the miter saw on a 100' extension cord?
That would have to do with voltage drop, demand amperage and fault current. Very basically it means that when you add a 100' of extension cord you get a situation where when your miter saw starts and it calls for 14 amps, the length of the circuit creates so much voltage drop it can't deliver 14 amps. Instead it is delivering say 10 amps. This causes the motor and the circuit conductors to heat up causing more resistance causing less amperage to be delivered to the load. So there is not enough energy being passed through the circuit to cause an overcurrent situation which means the breaker wouldn't trip. This could also be true of the old circuit if there were corrosion or loose connections causing more resistance in the line.
Even though it appears that adding an extension cord fixes the problem it really doesn't. Compressors and motors don't like low voltage situations. They heat up and overwork and eventually burn up. That is why major appliance manufacturers specifically state not to use extension cords, otherwise it will void their warranty.
As I stated earlier I don't have specific information about your situation so I can't make a definitive conclusion, but based on generic information and the situation you have given, this would be my best guess as to what is happening.
I would suggest you investigate this particular circuit and check it for loose fittings and corrosive wiring and verify correct wire sizes. I would also suggest you add a few circuits to distribute the load a little better.
Hope this helps and good luck.