Breakers not tripping?

My background is in construction and maintenance and I currently work in an old warehouse with a ton of red flags to learn from. The electrical system / distribution panels are from the late 50s early 60’s and in desperate need of an upgrade. I once had some wires short and observed that a breaker refused to trip and it was only after the jbox and wire melted away did the arcing stop, …obviously a defective breaker!

Today our resident welder asked me to fix his plug with a broken prong for his portable mig welder. Being unsure of weather I should wire it with a 15 amp or 20 amp plug, I decided to grab my clamp meter and line splitter and see how many amps it would draw under use. I found it drew more than 25 amps under use for about 30 seconds, long enough to put down an couple of inches of weld. Anyway my questions are…

If a breaker is rated for 20 amps, am I correct in assuming that because its not tripping immediately after it draws over 20 amps that it is a defective breaker? Or is there some kind of wiggle room there?

I asked the welder if it was common for him to trip breakers and his response was “YES!”, then he proceeded to show me a large hand full of receptacles that worked for him, all 20 amp breakers, all very old. Is this hard evidence that all these breakers are faulty?

A side question unrelated to breakers, but I called the manufacturers of the welder, a Lincoln sp-135plus, and found out the factory installed a 15amp plug with 12guage wire. Is it common for power tools to have wires and plugs rated for less power than they draw because its not a continuous power draw?

Hoping someone out there has some answers :slight_smile:

No, you would be incorrect. In fact, breakers loaded to 100% of capacity will likely never trip.

Here is a chart for reference.

Search breaker trip curves to see why the breaker did not trip. It is not necessarily defective, but it depends on the amount of overage and how quickly it gets to that level. A breaker can hold 125% for two hours and still meet the standards.

Welders have a duty cycle that affects the current draw.

Thank you!

I found:

135% and 200%

A breaker between 0-30amps at 135% should trip in less than an hour. 20amp x 135% = 27amps
A breaker between 0-30amps at 200% should trip in 2 minutes.

Apparently the bigger the breaker gets the longer trip time.

So now Im trying to figure out temp max for breakers before they trip. and while Im at it, max temps for wires. Im finding alot of numbers out there and so far its hard to cess out reliable information VS unrelated/random ones.

Why do you say a bigger breaker gets a longer trip time?

The breaker should keep the temperature rise within limits. What is the purpose of your research?

I want a more complete understanding of components and the mechanics of things and not be a guy who just say, “that’s not code its wrong”. I like to understand the whys and why nots, the degrees of why something is wrong or right. To be able to take into consideration more variable than a few when looking at an issue or question.

I just have a habit of reading things and asking questions and wanting answers to them. Im confident I have a ways to go but Im up for it.

I’ve read that from various sources, but the easiest to read table/pdf is this:

I had never seen the data presented in Chart 1. Thanks.

A breaker can by UL standards hold 125+% indefinitely, and after that it takes time. Breakers have a inverse thermal time current curve. In short it means the less overload the longer it takes to trip. The more over load the faster. A 20 amp breaker pulling 40amps could take 30 seconds to trip, and one pulling 25amps could take an hour.

A breaker doesn’t trip immediately until it gets into the ‘magnetic trip range’ which is an intentional mechanism to open the breaker immediately under a direct short circuit with no resistance. The magnetic pickup usually tends to be around 10 times the handle rating, with some being more some being less. Single pole QO breaker actually tend to be less around 5 to 6 times. Double pole breakers tend to be higher than single poles to allow for heavy motor starting like AC units.

In the case above, is 27 amps from normal operation or are you intentionally trying to trip the breaker?