Will it trip the breaker?

[Gary11734]

My first thought was yes, it will trip the breaker. Quick review of Ohms law in my head to confirm it will draw roughly twice the current.

What muddies the question is the somewhat arbitrary choice of 1000w for the toaster. And thus the resulting borderline calculation on the trip curve of a 15A breaker. It might take long enough (5 minutes?) for a particular 15A breaker to trip that the answer to the question might as well be 'no', because by that time something has gone up in flames.

Why not make it a 1500W toaster oven? Then the answer is a pretty clear 'yes' and you can separate the seasoned electricians from the not so seasoned.

Why is the problem such an issue? You did the math, and you say, it will trip, PERIOD.

Some will say, the current will drop in half because the voltage goes up. That is the guys we are looking for to understand why they may be thinking that.

I cannot say this any clearer on why we have this problem in the first place. It is to remind you about resistance and when its a fixed value and when you can change it, like in a dual voltage motor.

 

[don_resqcapt19]

I will try one more time.

Consider this a FIXED resistor. You guys are inserting too much information in this example.

The math has been done. The breaker would trip.

...

While there is no question that the current would exceed the rating of the breaker, there is no reason to expect that small amount of overload to cause the breaker to open. It may open, but it may not.

 

[K8MHZ]

So, the teacher had a bad question? Well, it won't be the first or last!

The apprentices didn't have an issue with it. We solved the problem and learned something about resistance when it's a fixed value.

And those apprentii are going to go into the real world and see 18 amps on a perfectly functioning 15 amp OCPD hold forever and are going to wonder why. Or they may get into trip curves and design tolernaces down the road and have to un-learn what was taught in their 1st year. You may be long gone by the time your 1st year apprentii have an issue with what they learned.

 

[don_resqcapt19]

Looking at the trip curve for one brand of breaker, it shows that their 15 amp breaker will hold 18.75 amps forever in a 25°C (77°F) ambient. The breaker curves are at 40°C and shift to the right when in a lower ambient. In many installations, the actual breaker temperature will often be much less than the 40°C that the curves are based on.

 

[Gary11734]

While there is no question that the current would exceed the rating of the breaker, there is no reason to expect that small amount of overload to cause the breaker to open. It may open, but it may not.

Ok, you have the simple answer that most would get. Some would see this question and think, no the current would halve as the voltage goes up. It is that simple.

Now, as far as my definition of "Seasoned Electrician" in this example, let me change that to, "a guy that worked in the field for a period of time with some knowledge"

Thanks for all the comments.

Gary

P.S. My first-year apprentice teacher was Mr. Hardy. Blame that guy!

 

[K8MHZ]

P.S. My first-year apprentice teacher was Mr. Hardy. Blame that guy!

My first year apprenticeship teacher was an Extra KB8GP that worked maintenance in a paper mill before he came into the classroom. He had an HF rig on the back counter. He would have picked the question apart before it ever got to us.

He would have said something like '18 amps is going to trip a 15 amp breaker? Yeah, right.'

 

[480sparky]

This conversation reminds me of a similar question to ask of an electrician. Will a dual voltage motor use more power at the low or the high voltage? Sadly, many "electricians" respond along the lines of "of course the lower voltage, just look, the current is double. It will cost twice as much to run"

I can't tell you the number of times I've heard "Put 277-volt lighting in.... it costs less to run than 120-volt."

 

[K8MHZ]

I can't tell you the number of times I've heard "Put 277-volt lighting in.... it costs less to run than 120-volt."

It is true. 100% true.

It costs less to run the wire and it costs less to run the conduit.

 

[Gary11734]

It is not as simple as that. Any breaker may be able to sustain a constant current above its rating without ever tripping. The higher the current, the faster the trip. The closer the overload gets to the breaker's rating, the longer it takes to trip. How much of an overload a breaker can handle without tripping depends on the breaker's physical characteristics.

You are looking at this through an engineers eyes. Everyone here had a valid comment, like yours, too.

Thanks for your participation in the thread. I learn a lot in this one thread...

 

[Gary11734]

My first year apprenticeship teacher was an Extra KB8GP that worked maintenance in a paper mill before he came into the classroom. He had an HF rig on the back counter. He would have picked the question apart before it ever got to us.

He would have said something like '18 amps is going to trip a 15 amp breaker? Yeah, right.'

Yes, Mr. Hardy was an electrician by day and did Apprentice class duty by night. He did more of the head cocking watching us 18-year-olds trying to become electricians...

I learned a lot from that guy. No wonder I still remember his name from 46 years ago...

 

[Strathead]

Yes, Mr. Hardy was an electrician by day and did Apprentice class duty by night. He did more of the head cocking watching us 18-year-olds trying to become electricians...

I learned a lot from that guy. No wonder I still remember his name from 46 years ago...

It doesn't change the fact that the question is bad and the instructor should have realized that either before asking it, or as soon as he started answering. I have just started teaching and I had to concede on two questions on my new 50 question test as bad questions that needed rewording or eliminating. Ii suspect if I use the same test next year (corrected) we will probably find at least one more. But that is my obligation to correct. Not to attempt to justify as you have done throughout this entire thread. I am sorry to be blunt, but at some point you have to concede that you asked a bad question here. :slaphead:

 

[Gary11734]

It doesn't change the fact that the question is bad and the instructor should have realized that either before asking it, or as soon as he started answering. I have just started teaching and I had to concede on two questions on my new 50 question test as bad questions that needed rewording or eliminating. Ii suspect if I use the same test next year (corrected) we will probably find at least one more. But that is my obligation to correct. Not to attempt to justify as you have done throughout this entire thread. I am sorry to be blunt, but at some point you have to concede that you asked a bad question here. :slaphead:

Thank God we all have opinions.

I think it was a fantastic question.

You are not blunt that you have an opinion.

Yes, Mr. Hardy was a fantastic teacher... The good old days are long gone...

 

[drcampbell]

... What electrician would think a 120 volt light bulb with 240 volts across it would use less electricity?

All of them, I hope.
Assuming it's a plain ol' ordinary light bulb, not something with a wide-voltage-range electronic front end, it will have a brief but brilliant career, and then it will use no electricity at all.

 

[jaggedben]

Why is the problem such an issue? You did the math, and you say, it will trip, PERIOD.

Actually I didn't say that at all.

Some will say, the current will drop in half because the voltage goes up. That is the guys we are looking for to understand why they may be thinking that.

Let's say your question had to be answered yes or no with no comments. You have a set of people who answer 'no' because they don't understand ohms law; those are the least knowledgeable. You have a set of 'math guys' (and gals) who say 'yes' because they know ohms law but aren't aware of trip curves; these folks are middle level. Then you have a group who say 'no' because they know about trip curves; this is actually the most knowledgeable group, but they have the same answer as the least knowledgeable group.

Your stated goal is to distinguish the first group from the rest, and the question is doing a poor job of that. Adjust the math so that trip curves aren't in play and you have a better question for your stated goal.

 

[mivey]

It doesn't change the fact that the question is bad and the instructor should have realized that either before asking it, or as soon as he started answering. I have just started teaching and I had to concede on two questions on my new 50 question test as bad questions that needed rewording or eliminating. Ii suspect if I use the same test next year (corrected) we will probably find at least one more. But that is my obligation to correct. Not to attempt to justify as you have done throughout this entire thread. I am sorry to be blunt, but at some point you have to concede that you asked a bad question here. :slaphead:

Pretty much the way I see it also. Poorly worded question.

 

[480sparky]

It is true. 100% true.

It costs less to run the wire and it costs less to run the conduit.

But it still costs the same to operate them.

 

[roger]

Pretty much the way I see it also. Poorly worded question.

I have to go with that too. If we are really looking for understanding the math (or if the theory is understood) a better example could be used or asked?

Roger

 

[brian john]

It won't do anything if you do not push the toast down handle.

 

[don_resqcapt19]

It won't do anything if you do not push the toast down handle.

That is not always true...my mom and dad got one as a wedding gift in 1946 that automatically lowers when you put the bread in...the only one I have ever seen that did that...and my mom still uses it.

 

[K8MHZ]

But it still costs the same to operate them.

Of course that is the case, but you did say run.

I was just funning with you, sorry that wasn't obvious.