#2 copper main on 150amp breaker?

So your saying if the breaker wasn’t there its possible that the amperage could rise to abot 8000?

Yes, the potential is there.

In most cases, the conductor will vaporize in a flash (explosion) of molten metal before that type of current is reached. If the conductor is large enough to carry that type of load for a second or two, there will also be fireworks at the transformer.

Hopefully, you will never be unfortunate enough to actually experience that occurrence :wink:

Ok thanks guys. It all makes sense now

Yes, it can and will be close to the maximum during a fault with or without the breaker. High current faults are common occurrences. Virtually all electrical equipment is designed to withstand faults of at least 10,000 Amps with no noticeable effects.

If, for example, you were to connect a wire between the two phase legs of a 240V system (such as on a 240V breaker), and threw a switch, thereby creating a bolted fault (short circuit), the fault current would be close to 10,000 Amps but you would not notice anything. The fault would clear without incidence.

Equipment and conductors will only be damaged in a 10,000 Amp fault if the equipment is defective. A common error made by electricians is to install equipment without checking the transformer impedance. Fault currents of up to 200,000 Amps are common. If Class H equipment is used on a system with an available fault current of 200,000 a fault would cause a violent explosion.

Transformers intended to supply residences are designed with an output impedance that is sufficiently high to limit the available fault current to 10,000 Amps or less so there is rarely any reason to be concerned with the equipment class. If you inspect commercial properties, you should always check the transformer impedance and verify that the equipment is suitable for use in the system.

Jeff,

There would be an explosion only if the conductor or transformer were defective or the system was not designed properly. Ground faults and phase-to-phase faults, both bolted and arcing, are very common. The currnet that flows in most faults will be in the thousands of Amperes. There should be no noticeable effect.

What clears a fault is an overcurrent device be it a breaker or fuse. If the conductors coming from the secondary side of a transformer to the first overcurrent device be bolted then there is going to be all hell breaking loose until there is something that opens to clear the bolt.

Without an overcurrent device to open this leaves the conductors that make up the winding of the transformer or the conductors between the transformer and the point of the fault.

The service entrance conductors have no overcurrent device between the main disconnecting means (the main in the panel) and the secondary of the transformer. Should a fault occur on these conductors there will be melting of something be it the SE conductor or the transformer.

The length of time it will take to burn something to clear this fault will be dependent on the resistance of the conductors, the impedance of the transformer and the available current.

The one thing that is sure and certain is that the fault will continue until something opens. I have posted here many times the outcome of my removing a panel cover when a #12 screw caused a fault in one of the service conductors and the resulting replacement of the panel and everything supplying that panel including the transformer on the pole.

Jeff is correct that the conductors will vaporize and the resulting bang is going to be anything but pretty.
This short video is a fault that was bolted to get it started and we can see the result
http://www.youtube.com/watch?v=-iClXrd50Z8

I understand that it’s technically an “arc-flash” as opposed to an “explosion,” but standing next to it I’m not sure I’d be able to distinguish the difference :smiley:

Arc flash is by no means pretty in the mid 1970’s I was the operator of a 750 ton chiller don’t rememer the exact voltage somewhere in the range above 1200 V. Two legs of the 3 phase went phase to phase when no one was present in the equipment room. I came in to start the chiller and found the MCC front panel across the equipment room. This cover had been bolted on with about 7/16 bolts sheared them off like a pretzel

Jeff,

I understood your use of “explosion”. As I said, that will only happen if one of the components is defective or the system is not designed and built properly. Up to 10,000 Amps can flow during a fault in a residential electrical system without damage to the system.

Had the electrical system and chiller been designed to the IEEE242-2001 Standard, that would not have happened. I had a similar experience with a Berg Chiller. The entire interior of a control panel was destroyed. Upon investigation, we learned that Berg had used Class H fuseholders. That was ruled a manufacturing defect.

I have investigated hunderds of electrical system failures that were attibutable to improper electrical system design or construction. Electricians are not trained to design systems for proper selective coordination but electricians and electrical contractors are often put in the position of building a system without enginering specifications or engineering review.

Call it what you want but rapid air expansion is a blast in any language. When solid copper turns a gas it expands 67,000 times its size.

Call it what you like but I promise it is anything but wonderful.

In an electrical flash the temperature can rise to 35,000 degrees F. The surface of the sun is only 9000 degrees.

The blast that is present is capable of producing a ton of pressure expanding outward of the blast.

The light produced by the blast is blinding. There will be all sorts of flying objects expanding with this blast of light. It is worse than being in a war zone.

The technical term is blast but I suppose explosion would be understood and if you are the one involved you will call it an explosion.

Mike,
Juan said “You lost me at “and”” so, I was trying to keep this simple. Not everyone here is interested in getting down to fine details.
Juan originally said “So since there is no resistance the current rises above the breaker rating?”

Short answer: Yes

Long answer:

The current will rise to the limits established by the combined transformer impedance and additional system impedances. This will happen with or without a breaker. The breaker or fuse will clear the fault but will NOT prevent the currents from rising to levels in the thousands of Amps. Breakers and fuses are designed to withstand AND interrupt currents as high as the avaialable fault current on a system.

Therefore, if a conductor vaporizes or a piece of electrical equipment explodes, it is the result of a defective or damaged component or an inproperly designed or installed electrical system. As I said in previous posts, the system will be subjected to very high current flow during a fault but the fault will clear without incident - no vaporized conductors, no explosions, etc.

There are ways to limit let-through current but they would not generally apply to residential systems. There are special fuse classes that specifically limit the let-through current. I have attached a chart for a Class J fuse as an example for the benefit of those who may be interested.

The video you posted, while impressive, does not address the questions posed by Juan. I was addressing Juan’s questions. Had there been a Class J or a Class L fuse, for example (or, depending on the AFC, perhaps a K1 or K5, etc.), the fault would have cleared without incident. In fact, Bussman has a whole series of videos that demonstrate exactly what I am saying. In one instance, they will use an improper class device and then they will show the same thing with the proper class device. In the first instance, there is an explosion and in the second, there is not. They go a step further and show both in extreme slow motion. In the second instance (the one with the correct class device), the cable starts to form into a circle then relaxes. At normal speed, the movement cannot even be detected.

So, going back to Juan’s orignal question, the current during a fault WILL rise to a level many times greater than the circuit breaker’s overcurrent rating. The breaker does NOT prevent the rise in current. The conductors will not vaporize. The equipment will not be damaged. Unless - there is a defective or damaged component or the system was not properly designed and installed.

Way to go now George! How are you going to explain the graph?:smiley:

He will defer to you. :mrgreen:

Way out of my league!

Now we are making progress.:shock:

Mike,

From what I can see, no one is challenging the definition of “explosion”. This thread is drifting into something other than what was presented by Juan.

Juan questioned whether currents flowing during a fault would exceed the breaker’s rating. Once again, the answer is yes, they will.

Jeff mentioned that the current will be limited by line and equipment impedances. In the interest of clarity and completeness, I added transformer impedance to that because the transformer impedance is the main limiting factor.

I further added that no damage will be done as a result of the high fault current in an electical system that was properly designed and installed and is free of existing damage or defects.

Kevin,

I will not be drawn into strawman arguments. I posted the chart for the benefit of anyone who may be interested and may not have known or believed that current flow during a fault is very high.

When you added the impedance of the tranny to the equation I included the effect of the same ground fault between the tranny and the first overcurrent device.

We are both addressing issues in the grounding and bonding of an electrical system.

I like the chart you posted for the let through for a 600 volt time delay fuse, but, we are discussing a 120 volt circuit using an inverse time circuit breaker. As a general rule of thumb multiplying the rating on the handle by 6 will give the amperage it will take to open it within a two cycle period, .033 seconds (magnetic trip). Most inverse circuit breakers will hold 135% of their rating for two hours before opening (thermal trip).

If we look at the chart you posted we can see if we were using a 600 volt 15 amp time delay fuse that the fuse would open instantaneously at around 1250 amps. You are correct that a 14 AWG copper conductor would not sustain damage in this fault. The peak temperature on the conductor would be even less on a 120 volt circuit protected with a 15 amp inverse time circuit breaker.

I am not trying to start any sort of debate and was only trying to add information to the discussion as I hope all of us are trying to do. Being the tranny was brought into the discussion, and me being as safety minded as I am, I brought in the effects of the fault between the tranny and the service disconnect (the removing of panel covers.)

If I did wrong please forgive me. :frowning:

Mike,

You are not wrong. As you pointed out, the discussion is about residential electrical systems, not commerical systems. The reason I mentioned current limiting protective devices was to make it clear that while there are ways to limit let-through current, residential electrical systems rely on the transformer’s impedance to limit the current. It would be a mistake for anyone to think that the currents during a typical fault do not rise into the thousands of Amperes.

I don’t often jump into discussions on this message board because by the time I see the threads, the original questions have already been answered by you, Jeff, Michael Larson or others who obviously understand the questions and have the correct answers. There are occasions, however, where additional information is warranted. I thought this was one of those occasions.

Another point to consider with residential electrical systems is that they are moving in a direction that is going to require anyone involved with them to have more of an understanding of electricity and electrical systems. Understanding fault currents is just as important as understanding the drawbacks of using high input impedance testers to check electrical systems.

As population densities increase in some urban areas, and the demand for electricity grows in individual households, utility companies are being held back by residential design criteria. The day will come when they will need to change the rules. One logical change would be to use larger transformers with lower output impedances, thereby significantly raising the available fault current (AFC). That is not impossible today but it is not practical to do as things are now. Today, 10,000 Amperes is the industry standard for residential. It isn’t likely to stay that way forever. So, there is some value for home inspectors to understand other types of electrical systems. I remain mindful, however, that this is a home inspector forum. If there is not a logical connection (or what I percieve to be logical), I avoid bringing other stuff in to discussions.