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10-12-2008, 05:45 PM   #1
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Hey guys, I was wondering if someone could answer something that I've never understood about electricity. I've tried asking some of the professors at my school but they can't seem to answer anything at all that I ask them about the wiring in my house. And these are electrical engineering professors! I guess all they are familiar with is the theory side of things, not the practical side.

But anyways, why is it that a light will dim for instance, when a power tool or something else is plugged into the same circuit. Obviously there is not enough power available to give both devices the full power that they need, so why doesn't the breaker trip? And there is nothing wrong with my breakers...they are all brand new. They will trip if a second or third power tool is plugged in, but why don't they just trip as soon as there isn't enough power to go around?

Any help with understanding this would be greatly appreciated!

10-12-2008, 06:15 PM   #2
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This is called voltage drop, and is due to the resistance of the circuit conductors. The conductors' resistance is usually negligible, but when enough current flows, some of the energy is wasted in the conductor itself, as heat. And because volts are a measure of the energy carried by a charge, the amount of volts available falls when the current is large. The current may not be so large that it trips the breaker, just large enough that the circuit resistance becomes apparent.

In other words, the conductors themselves become part of the load, and we see it in the form of a voltage drop. The formula is Vd = I*R. That is, the voltage drop equals the current (I) times the circuit resistance (R).

10-12-2008, 06:29 PM   #3
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Quote:
 Originally Posted by InPhase277 This is called voltage drop, and is due to the resistance of the circuit conductors. The conductors' resistance is usually negligible, but when enough current flows, some of the energy is wasted in the conductor itself, as heat. And because volts are a measure of the energy carried by a charge, the amount of volts available falls when the current is large. The current may not be so large that it trips the breaker, just large enough that the circuit resistance becomes apparent. In other words, the conductors themselves become part of the load, and we see it in the form of a voltage drop. The formula is Vd = I*R. That is, the voltage drop equals the current (I) times the circuit resistance (R).
Perfect. That's exactly what I was looking for. Thank you.

10-12-2008, 06:31 PM   #4

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Quote:
 Originally Posted by dc4nomore And these are electrical engineering professors! I guess all they are familiar with is the theory side of things, not the practical side.
Exactly.
An EE degree is about as useful in construction electric as an accounting degree is.
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 10-12-2008, 06:47 PM #5 Member   Join Date: Sep 2008 Location: Ohio Posts: 177 Rewards Points: 150 Haha, well said.
10-12-2008, 11:50 PM   #6
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All too true!

I've been a commercial/industrial electrician for about 18 years now, and I can honestly say that electrical engineers are generally my worst enemies. About 80% of them anyway. The other 20% are pretty good at what they do, and are a welcome addition to the team.

Rob

P.S. I know I'm stepping on some sacred toes here, but no matter what title it has, a spade is still a spade.

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