Feeding both sides of panel with no 240v breakers bridging across.
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Yes, that's a very cool video. (I especially liked the current dots running back and forth to indicate AC flow). One thing I didn't like too much was descriptions of the neutral wire. They say the neutral wire works in a 120v circuit as "returning the used current to the transformer." That's a useful simplified explanation but they don't show any "used current" returning to the transformer for a 240V circuit. That is correct for pure 240V loads because all current is on the hots. But in a circuit with a mixed 120/240 load protected by a 2pole breaker, like the stove iowagold described (or a breaker that feeds a subpanel), some of that "used current" will go back on the neutral. ("Used current" makes it sound like neutral current has lost its oomph or is not as dangerous as "hot" current; that's not a good way to think of it. ). The visuals did help me understand why I was confused about breakers for 240 circuits; I thought there were two different kinds; one for mixed 120/240V circuits and another for pure 240. I did a simple test a of a spare 240 2 pole breaker. It appears to function exactly as two single pole breakers glued together measuring the current on the two hots independently with two different breakers. That works for mixed 120/240 or pure 240 circuits. Here's what I didn't originally understand:
1. I thought that a pure 240V load would use a breaker that would somehow measure the combined current in a 240v load at only one point and also somehow prevent mismatched current on the two hot legs. Now I don't think that is possible, at least not without using more complicated breakers with circuitry like the ones at the end of the video used to detect ground faults and arc faults)
2. Someone else here explained that normal breakers are only required to prevent the house wiring itself from overheating from excessive current. But when an appliance is connected to the house wiring , then the appliance itself has to protect its own wiring with its own fuses if it can't handle the full rated current of the circuit.
3. Even though neutral wires only carry "used" current (whatever that is) care must be taken not to cause excessive current on the neutral. Neutral wires are not fused, but they can still carry all of the same current as the hot wires and can overheat just as easily. This is only safe in a 120V circuit when the current in a specific neutral wire is always "returning" the current from only one hot circuit that has its hot wire fused at the same capacity as the neutral wire.
4. The neutral wire in a 120/240V circuit can share one neutral wire for its two hots because of a special property of the 120/240 voltage being fed to your home from the utility. As explained in the video, the two hots are 180 degrees out of phase. Here's an example: a 30 amp 240/120V circuit wired with a 3 wire 10 gauge copper cable. At an instant in time; if one hot wire is drawing 25 amps and the other is drawing 20, then the total current on the neutral wire they share is 25-20 = 5. Not 25+20= 45. The 180 degree offset allows this to work OK. The two separately fused hot wires can share a neutral and never risk exceeding its capacity even though the neutral wire is not fused. The maximum current on the neutral, 30 amps, can only occur when only one hot is actually drawing power at 30 amps. The physical attachment of two 120 breakers to make a single 240 breaker is essential to making this safe. It assures that both hots are turned off even if the excess current only appears on one hot leg. It is not electrically necessary for the two 120V breakers to be right next to each other or physically attached; they could be anywhere in the box on two different bussbars, but that would potentially dangerous for a variety for reasons. The interleaving of the two hot bussbars in the panel as shown in the video assures that two breakers installed next to each other are on two different bussbars.
Sorry for the long description; it may sound like a nitpick of a good video. But, in my opinion, wiring safety issues are usually a result of neutral and ground mistakes due to misunderstanding of their function. Everyone is careful with hot wires because they know hot wires are, um, hot!
The video doesn't say much about ground as it relates to the utility power, either. And misunderstanding of ground can lead to safety problems too. For example, I don't think people would understand from this video that a broken/disconnected neutral wire from the utility pole to your house doesn't necessarily disrupt the 120v circuits in your house, or the 240. It may not be noticeable at all. Your house would still get source power from the hot wires; if the "used electricity can't return to the transformer through your white wire, it can usually just go into the ground from your house ground (since neutral and ground are bonded) go next door through the dirt and get back to the transformer through your neighbor's bonded ground and neutral wires! And no breakers will trip. Nobody ever knows until a neutral or ground somewhere gets hot and starts to smoke!
This brings to mind an incident that occurred at my brother's house. The boom from some machinery contacted the overhead distribution power lines, which resulted in the bottom "neutral" burning through and the two ends falling to the ground. The top "hot" wire was still intact.
The lights in the house blinked out momentarily, and then came back on normally. My sister-in-law didn't know that anything was the matter until the utility crew knocked on the door to let her know that they were there to fix the wires. It would appear that the "used" current was flowing from one end of the broken wire over to the other end just fine. It would have been interesting to see if the voltage was any different while the wire was broken, but no one checked that.
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That's a great story Nebrasky. I wonder what would have happened if someone picked up that neutral thinking it was safe. I have to admit that I was under the impression that "white wires are safe" until I found out different the hard way; I disconnected one in a box that was in my way. A disconnected neutral wire is no longer neutral!
I would assume that the full current being used by my brother's house was flowing back through the soil between the two sections of wire. So if you picked one end up off the ground, then you become the next easiest path back.
However, I don't know exactly how much return current there would be. Is there return current back from the transformer when it's feeding a balanced 240v load? Or is it just the unbalanced load flowing back on the main line? I don't know much about what's going on on the other side of the transformer.
If it's only the unbalanced load flowing back, then we're only talking 10-15 amps of 120v. Boost that back up to whatever the distribution lines run at, and we're talking around a pretty small current. Might not be enough to kill someone, depending on the path it took through the body.
I don't know. But I've no intention of finding out either.
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Me either! I have reached, or gone beyond, my level of understanding on this. (But the little voice in my head is still saying "You're not going to let a broken wire sit there like that, are you? Just wear rubber gloves, stand on dry wood, and wrap some tape around the ends; and you'll be fine." I have learned to resist these thoughts, but its still a struggle!
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Central Maine Power Public Service Announcement: No line is safe to touch, evah!
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ayuh!
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