Why can't I trip the breaker?

[gs8212]

I'm adding general lighting, and receptacles for workbenches and a compressor, to an existing 2-car homeowner attached garage (20x20). I had another thread asking how to size the circuit for the compressor rated at 115 volts / 15 amps, and there was some concern I'd be tripping a 15 amp circuit so there seemed to be an unofficial consensus to just put in a dedicated 20 amp for the compressor and forget about it (thanks to everyone who replied to that thread).

As I was designing the new circuits on paper, I couldn't shake the thought that I'd already been running the compressor off an existing 15 amp without problems. Also, I finished my basement and did all the wiring (taking the homeowner electrical test required by the county, getting all the permits and inspections, etc.) so I am minimally knowledgeable about electricity. The finished space in the basement is 860 sf and has 4 15-amp circuits for lighting and receptacles. Putting all this together, it just seemed to me that the circuits throughout the house are over-engineered.

All this thinking - let the jokes begin - caused me to wonder what it would really take to trip a 15 amp circuit (was this a dangerous exercise?). I confirmed an existing 15 amp circuit in the garage powers:

• 2 - 1/2 hp garage door openers at 6 amps each (12 amps) and
• 2 open receptacles

I plugged into the 2 open receptacles of the circuit the following:

• circular saw rated at 14 amps
• my compressor rated at 115 volts / 15 amps

I turned on the compressor which was empty at the time, hit both GDOs, and then hit the circular saw, and everything ran fine. I couldn't get the breaker to trip.

Why couldn't I trip the breaker? Does length of the wiring run, temperature - about 14 degrees, or other things affect all this? Just curious.

 

[hayhauler71]

I would say the amp rateings you posted are at spike not running amps

 

[Ironcrow]

Look up the time-current curve for the breaker. A breaker will run its rated value for forever. And typically something around 10 times its rated value will trip it instantly but about 5 times its rated value will run for, say, 5 seconds. This accommodates in-rush current from starting up electric motors and such. So, in your experiment, trying to hit the in-rush currents on all your loads at once is not going to get you there.

 

[gs8212]

Very cool. Learn something new every time I visit this sight.

 

[happy2rv]

I suspect the answer lies in a combination of both of the previous answers. The ratings on the appliances you listed I believe are maximum sustained current draws. So, the garage door openers at the start of their cycle might have been drawing peak current, but the compressor "compressing" against an empty tank, and the circular saw free spinning (assuming you weren't actually cutting something) wouldn't be drawing anywhere near their maximums.

If you really want to know how much is being drawn, get a clamp ammeter, something like this and measure the current draw. Also, a more realistic test would be using loads with a more level current draw. Electric heaters for instance are going to have a fairly constant draw as long as they are on.

 

[gs8212]

Thanks happy2rv.

 

[Kevin C]

That and your skill saws current draw is at its rated HP, just running it without a load might only be a couple of amps. Similarly with the compressor, its peak power draw is when its close to full.

 

[gs8212]

I think i just figured it out. Having the 6 amps of draw from the GDO basically on its own circuit isn't a bad idea as it would **** if my wife tries to pull in on a freezing cold snowy day and trips the breaker because i wanted to save a few bucks and had a garage heater on the GDO circuit.

 

[jake00]

I had a similiar setup but had Xmas lights plugged in and on... I was using my miter saw, when all of a sudden, the compressor kicks on , and everything is dead...

 

[nehog]

... I confirmed an existing 15 amp circuit in the garage powers:

• 2 - 1/2 hp garage door openers at 6 amps each (12 amps) and
• 2 open receptacles

I plugged into the 2 open receptacles of the circuit the following:

• circular saw rated at 14 amps
• my compressor rated at 115 volts / 15 amps

I turned on the compressor which was empty at the time, hit both GDOs, and then hit the circular saw, and everything ran fine. I couldn't get the breaker to trip.

Why couldn't I trip the breaker? Does length of the wiring run, temperature - about 14 degrees, or other things affect all this? Just curious.

Compressor, lightly loaded, likely drew only about a quarter of the full load amperage. Saw, same, not cutting draws only idle current, about an amp or two probably. GD openers likely were not drawing any near six amps each--that would be a maximum current at full load.

Want to trip breakers: plug in resistance heaters. They provide a steady load. Motors are unpredictable, unfortunately.

 

[G_P]

Want to trip breakers: plug in resistance heaters. They provide a steady load. Motors are unpredictable, unfortunately.

This will do it. Space heaters will draw a large and constant load. Same with incandescent lighting. Plug in a few of those 500w halogen work lights and that will put a decent load on the circuit.

 

[goldie lox]

just get yourself an amp probe, and check your hot wires at the panel. this will tell you what you're drawing.

 

[Syberia]

At least you're doing better than me. My garage was wired seemingly as an afterthought, and shares a (thankfully, 20 amp) circuit with all 3 of our bedrooms. If I try to run any high-draw tool for any length of time, the breaker trips, and not only do I have to go reset it, but I get to experience the joy of resetting the clocks in the bedrooms as well.

 

[teamextreme]

The inverse time/current breaker curves are the key. You can run 21 amps through a 20 amp breaker and it won't trip. At least not right away. It will take quite awhile for it to eventually trip (could take as long as several minutes). Run 40 amps through it and it will trip within a few seconds.

 

[gs8212]

My garage was wired seemingly as an afterthought, and shares a (thankfully, 20 amp) circuit with all 3 of our bedrooms.

I am lucky wih that. Am going to tie 11 4-ft led lights to the existing 15 amp circuit as they are only about 4 amps total for the 11. I can't get to the attic for the existing circuit so I just bought the metal clad 14-2 to tap into it from the ceiling side and run power to the lights and then down to the switches. Probably 3 switches.

This thread and one other are edumacating me on wiring and designing my existing little garage. I'd start a build thread but it is humiliating compared to what I see you guys do. [emoji4]

Thanks for all the help.

 

[MoonRise]

Want to trip breakers: plug in resistance heaters. They provide a steady load. Motors are unpredictable, unfortunately.

Pffft, neophytes.

If you really want to trip the breaker instantly, all you need is a brand-new, fresh out of the box NW wire clamp. Metal, of course.

Sort of like this kind:

http://www.homedepot.com/p/Halex-3-...Cable-Clamp-Connectors-5-Pack-20511/100133208

And it also has to have a 'special' feature added to that particular metal clamp of a relatively small 'burr' on the inside of the floating clamp shoe, probably less than 1/8" long. Install the clamp body into the knock-out hole of the service panel box where you will be running a new 20 amp circuit.

Then you have to take a nice fresh new length of 12 AWG NM-B and pass it through said metal cable clamp and attach the floating clamp shoe and gently snug it up.

But you have to unknowingly manage to line up that relatively small 'burr' (think metal splinter) that is inside the floating clamp shoe and perfectly line it up with the hot wire that is inside that nice new piece of 12-2 NM-B cable.

And that metal splinter that you didn't even know was there then has to perfectly pierce the outer jacket insulation of the NM-B cable, and then also perfectly pierce the individual insulation jacket on the hot conductor that is inside the 12 AWG NM-B cable.

Then, after all your rough-in wiring is done and after you finish things up and put all the outlets and switches and cover plates and everything in place for the branch circuit you just installed, flip that 20 amp breaker and it will trip instantly. With a very loud bang and a flash of light as well.

And the breaker will continue to instantly trip, along with the loud bang and flash of light, for the next two or three times as you trouble-shoot the entire brand new branch circuit including disconnecting pretty much the entire branch circuit from the circuit except for that 'feeder' coming out of the service panel. You remember that brand new piece of 12 AWG NM-B? The one with the brand new metal cable clamp securing the cable coming out of the knock-out of the service panel?

That breaker will continue to trip until that formerly unknown burr/splinter inside the floating shoe of the cable clamp finally vaporizes.

And that, my friends, is how you trip a breaker.

Either that, or use some more mundane things like some high-wattage incandescent lights or high wattage electric heaters.

 

[toplessHO]

also depends on what type breaker it is and how far away from panel.
Short version I was in an attic very hard to get to panel had a hot circuit
that I needed to reroute
I flashed the wire against the metal hurricane strap and it stuck.
pulled it off and climbed down found the proper breaker and turned it off and tagged.
Not something I would recommend but proof that breakers dont always work as intended.
another story ...installed a new 200 amp outside panel that fed existing panel inside.
HO was trying to put up a ceiling fan and shorted it out,
called me because he had lost power on the entire house. 200amp main had tripped as well as the subfeed breaker in the newly installed SqD panel.
FPE inside didnt even trip the 15 amp that was feeding the shorted branch circuit.
Care to guess the brand of the one I arc welded in attic with?