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# Thread: Old house, 100A fusebox, should we replace w. breakers?

1. Originally Posted by jwelectric
Something else to bear in mind is that a non-time delay cartridge fuse will blow within 6 cycles or 1/60 of a second but an inverse time breaker can carry 6 times its rated current for up to two minutes. The fuse will blow when loaded to 100 amps in less than a second but a 100 amp breaker can let through 600 amps for as long as two minutes but in most cases when loaded to 600 amps will trip in a few seconds, (between 2 and 30 seconds)
How did You come up with these figures ?

2. Im reading it now carefully, a lot of bad stuff can happen in 120 seconds, I sure would rather have a good old fashioned fuse blow almost instantaneously than hang around with a circuit breaker for minutes. A lot can happen, like stuff getting fried.
My brother's house got hit by lightning and his neighbour's got hit worse. Took out everything in the house, but just the dishwasher in my brother's house. Both houses have circuit breakers.

3. Originally Posted by capecod12
Im reading it now carefully, a lot of bad stuff can happen in 120 seconds, I sure would rather have a good old fashioned fuse blow almost instantaneously than hang around with a circuit breaker for minutes. A lot can happen, like stuff getting fried.
My brother's house got hit by lightning and his neighbour's got hit worse. Took out everything in the house, but just the dishwasher in my brother's house. Both houses have circuit breakers.
I would not take that figure to the bank just yet.

JW may just need new batteries in his calculator.

4. Originally Posted by DonL
How did You come up with these figures ?
It is a calculus math problem but there are many sites on the internet where this information can be found. I have a book by Cooper Bussmann at school that I use in the class for fuses. I also use the Square “D” trip curve booklet for breakers.

Fuses and breakers are used to protect electric motors along with running overload protection that is either internal or separate of the motor. Motors in AC and heat pumps will have running overload that is internal the motor and auto reset while motors like garbage disposals will have manual reset overloads.

Look at your AC or heat pump and find the minimum circuit ampacity. This is used to size the circuit conductors.
Now find the maximum fuse or HACR overcurrent device. This is what we use to size the fuse or breaker with.

The overcurrent protective device will always be larger than the circuit conductor ampacity. The reason is because for the compressor to start it must overcome locked rotor ampacity.

Looking at Table 430.248 of the NEC we can see that the running ampacity for a single phase 5 horse power motor at 240 volts is 28 amps. Table 430.52 says that the inverse time breaker (breakers sold at the supply house, HD, Lowe’s ect) is to be sized at 250% or a 70 amp breaker. Why? Because the breaker must hold until the locked rotor ampacity is over come and the motor reaches full speed.

Look at Table 430.251 and we will find that this motor will have a locked rotor ampacity that is six times that of the running ampacity or amperage of 168 amps. The breaker must hold this inrush current until the motor reaches full speed. Under load this can take several seconds.

Using this same rule of thumb we can apply it to the breakers in our home. It will not work out to the exact number as the calculus math but will be real close.
This same method is used in drivers education. Take the speed of your car and divide by 2 then add it back to the speed to see how many feet per second you are traveling. Example; At 60 miles per hour divided by two is 30, added back to 60 equals 90 feet per second but calculus figures to 88 feet per second.
A non-time delay fuse on the other hand is a lot quicker to open.

In most cases during a fault condition most breakers will open between 120 and 180 cycles or two to three seconds.

While teaching at DCC we did a test using a GE 20 amp breaker and a very large rheostat. We loaded the breaker to 30 amps for over two minutes before it opened. We loaded the same breaker to 120 amps and it took just under 10 seconds but over 9 seconds to open. (The watch had 10.2 seconds but we subtracted the reaction time to operate the stop watch)

We did the same test using Square “D” home line and Siemens breaker with similar results.

5. Maybe the figures arent exact, but I just asked my husband about the difference between CBs and fuses, and he said that it is true. Fuses are pretty much instantaneous unless they are DC and have huge voltages going thru them, then they can fail. But that doesn't pertain to us here with house AC.
There is a delay with CBs, guess it depends on the current? I dont understand everything my husband was saying, it gets a bit complex.
I did find this, and it quite explains a lot about what we are wondering, is it OK to put a link to some other site here?
Hope so, as I did find this interesting,
http://www.at-ease.ca/articles/fuses...rcuit-breakers
Im sure learning a lot today.

Edit to add this; I just read the previous post, yes, my husband can do that kind of math, but its a bit beyond me. Thanks for the details. Learning fast, I am.

6. JW is correct that fuses will blow quicker than Breakers of the same size.

I do believe fuses are safer. But keeping spares is a smart thing to do.

Most home style breakers contain a heating element and a bimetallic Cutout, Heat makes them open.

The breaker may take a dead short or UP TO 10X rated Value to trip Immediately.
The surge current of normally operating devices (Mostly Motors) is the reason that breakers have delays built in.

Normally they will trip at there rated current in Milliseconds (MS). Could be many, depending on the load.

If it was very many seconds, the Fire could have already started, and that would defeat its purpose.

I disagree with that link as it states.

"A fuse might make the difference if you get hit with lightning."

I Fuse can not protect from a direct hit from lightning. It will jump across it.

A gas discharge tube connected to a good ground maybe, but not a fuse of any type can fool Mother Nature.

Making sure the current flows into the ground, instead of your equipment is the key.

7. We have plenty of spares, tho some are rather old.
Maybe we will stick with what we have.

8. Originally Posted by capecod12
We have plenty of spares, tho some are rather old.
Maybe we will stick with what we have.
Not a bad choice, You may want to Pick up more spares when you go to the big town.

Old is good, If it is working.

They are getting harder to find.

9. We do have a lot of them and we also have stockpiled a lot of 100W lightbulbs in our cellar too.

10. Originally Posted by capecod12
We do have a lot of them and we also have stockpiled a lot of 100W lightbulbs in our cellar too.
Way to go.

Don't forget the bullets...

Keep everything dry and they will be OK.

11. Yeah, we have a few of them too, who doesnt?

12. I lived in a house many years ago and the wires ran parallel on insulators.

It was cool to see real time old school.

That was back before a ground was ever ran to any outlets.

13. Originally Posted by jwelectric
While teaching at DCC we did a test using a GE 20 amp breaker and a very large rheostat. We loaded the breaker to 30 amps for over two minutes before it opened. We loaded the same breaker to 120 amps and it took just under 10 seconds but over 9 seconds to open. (The watch had 10.2 seconds but we subtracted the reaction time to operate the stop watch)

We did the same test using Square “D” home line and Siemens breaker with similar results.

I need to make some corrections. I talked with Tommy today about our experiment. Tommy was one of my students who has done nothing but go forward in the electrical profession. He remembers the experiment and kept his notes to wit I need to make some corrections.

The max load was 100 amps not 120. We also determined that there was a plus or minus 15% in the ammeter that was being used also. The meter was out of an old control panel and wasn’t completely accurate. The GE breaker fared better during the testing than the other two with the Square “D” taking the longest to open.

We also used a NON-20 amp fuse that opened faster than he would work the button on the stop watch with the old pot set at the same resistance for the 30 amp test on the three breakers. It opened faster than the old ammeter would and we never got a reading on the amperage so for the sake of the experiment assumed it to be a 30 amp load as it was the same setting for 30 amps for the breakers although the connection was made a little different.

Please forgive my mistake as my memory is not as good as it once was and this test was done in September of 2002.

14. Originally Posted by jwelectric
I need to make some corrections. I talked with Tommy today about our experiment. Tommy was one of my students who has done nothing but go forward in the electrical profession. He remembers the experiment and kept his notes to wit I need to make some corrections.

The max load was 100 amps not 120. We also determined that there was a plus or minus 15% in the ammeter that was being used also. The meter was out of an old control panel and wasn’t completely accurate. The GE breaker fared better during the testing than the other two with the Square “D” taking the longest to open.

We also used a NON-20 amp fuse that opened faster than he would work the button on the stop watch with the old pot set at the same resistance for the 30 amp test on the three breakers. It opened faster than the old ammeter would and we never got a reading on the amperage so for the sake of the experiment assumed it to be a 30 amp load as it was the same setting for 30 amps for the breakers although the connection was made a little different.

Please forgive my mistake as my memory is not as good as it once was and this test was done in September of 2002.

Got to love them students JW.

They don't have the CRS yet.

I do not rely on my memory because I ran out 1 Megabyte ago.

I bet the students really enjoyed you as a teacher.

Good teachers are hard to come by, and they will remember You Forever and a Day...

You are the best.

15. jewelectric is kinda right. If you can upgrade to a bigger "Breaker" panel, do it.Prices aren't falling.

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