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Old 02-07-2011, 04:45 PM   #16
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Wiring a sub panel to another sub panel


Now that we know that you only have a 40 A subpanel, your best bet would be to move a couple of circuits from the main into the sub and feed your garage from the main. Or you could upgrade the subpanel feeder and breaker for something like 100 A. I wouldn't feed a 40 amp feeder from a 40 amp feeder unless there was not much load on the first sub panel. Feed your workshop with more than 20 A, trust me.

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Old 02-07-2011, 05:21 PM   #17
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If you are using a 40A double pole breaker, then you are dealing with 40A service from that breaker, not 80A. The breaker will trip when *either* hot leg draws in excess of 40A, not when the sum of the legs draws more than 80A. As noted earlier, a double-pole breaker is essentially two 120V breakers joined with a common trip handle; the breaker does not care if you are actually applying a 120V or 240V load.

You could theoretically use a 40A double-pole breaker to feed a sub panel with ten 20A breakers. The real question is going to be what simultaneous load are you actually going to put on the sub panel at any given moment.
The handle of a double pole breaker has nothing to do with the common trip. The common trip is internal. Ever see a single handle double pole Sq D QO breaker?
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Old 02-07-2011, 06:25 PM   #18
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Stubbie, in reference to the ground rods, I know what they are for.I just added the flow of ground wires (when needed) for other things. During an electric storm the metal weather head and the meter box and all the ground wires in our houses serve as a lightening rod network to ground lightening to that ground rod outside our house. Ergo -- The full available path of ground wires is worth mentioning.

If one gets a loose (or burned) hot wire in an electric drill, the hot wire will find some other way to get to ground/neutral if it touches metal inside the drill (or anything else that conducts electrons) . The drill itself is grounded (via the ground wires) back to the neutral/ground bus in the main panel . Why do you think that is?
I hope your not going to tell me because it is connected to earth ...

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That is why I said the ground wires (we always called them equipment grounds) should go back to the service neutral/ground cable via the neutral/ground bus - instead of through the body of the person using the electric drill. Ground wires are great for giving the hots an alternative path away from the user of electric equipment because average circuit breakers don't open for business until the wire gets to the rated temperature,. (GFCIs are the exception of course
Your confusing a ground fault with a circuit overload. Thermal magnetic trip breakers of the inverse time type trip on over current or thermal overload. What your explaining is not a thermal tripping of the circuit breaker but a overcurrent event that instantly opens the breaker.
The equipment grounds have nothing to do with overheating wires as far as facilitating the opening of a protection device. They carry the fault current during a ground fault to a phase conductor by completing the circuit over low impedance allowing massive amps to flow on that completed circuit with the transfomer ... those amps passing thru the circuit breaker forces it to open on magnetic trip.
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Old 02-07-2011, 06:50 PM   #19
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The handle of a double pole breaker has nothing to do with the common trip. The common trip is internal. Ever see a single handle double pole Sq D QO breaker?
Only rarely in a residential environment

Besides, the point of my post is that a double-pole 40A breaker (whether with tied dual handles or a single handle) does not equal 80A of capacity, which is a valid point regardless of what brand of breaker is used.
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Old 02-07-2011, 07:09 PM   #20
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Wiring a sub panel to another sub panel


Each pole at 120 volts x 40 amps = 80 amps.
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Old 02-07-2011, 08:28 PM   #21
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Stubble, What you said is correct but what you said created or implied a "straw man" argument.


**** This is what I was trying to refer to:

in a breaker, the heating effect on a bimetallic strip causes it to bend and trip a spring-loaded switch.

Since the heating is fairly slow, another mechanism is employed to handle large surges from a short circuit. A small electromagnet consisting of wire loops around a piece of iron will pull the bimetallic strip down instantly in case of a large current surge.

As a practical matter in household electric circuits, it (the ground wire) is connected to the electrical neutral at the service panel to guarantee a low enough resistance path to trip the circuit breaker in case of an electrical fault (see illustration below). Attached to the case of an appliance, it holds the voltage of the case at ground potential (usually taken as the zero of voltage). This protects against electric shock. The ground wire and a fuse or breaker are the standard safety devices used with standard electric circuits.

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Old 02-07-2011, 09:00 PM   #22
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Dear Brric, Thanks for understanding the math.

The problem Clashy may have is probably my fault. I should have never mentioned the term 2 pole.

Maybe I should have simply said two 40 amp circuit breakers which are on different legs of a panel and the two breakers are connected. duh ? 40 amps from one leg of the main panel, and 40 amps from the other leg of the main panel. 40 plus 40 equals 80.

laugh out loud -- would someone like to tell me how the load of that 2 breaker set-up (under a different circumstance) would require no neutral wire? - That question was on the electrician test in Texas about 25 years ago. (we might see the purpose of a neutral wire in an electrical system. And, why most houses are wired with alternating current instead of direct current - even though AC is more dangerous than DC.

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Old 02-07-2011, 09:16 PM   #23
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A 240 volt circuit would require no neutral. AC won out over DC because you cannot transform DC making long distance transmission nearly impossible.
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Old 02-07-2011, 09:34 PM   #24
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Dear Brric, Bingo on the 240.

You know your history of the big debate between Edison and Westinghouse. Edison finally agreed with Westinghouse -- for the reasons you stated. -- Reference: The History Channel
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Old 02-08-2011, 12:46 AM   #25
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Wiring a sub panel to another sub panel


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Dear Brric, Thanks for understanding the math.

The problem Clashy may have is probably my fault. I should have never mentioned the term 2 pole.

Maybe I should have simply said two 40 amp circuit breakers which are on different legs of a panel and the two breakers are connected. duh ? 40 amps from one leg of the main panel, and 40 amps from the other leg of the main panel. 40 plus 40 equals 80.

laugh out loud -- would someone like to tell me how the load of that 2 breaker set-up (under a different circumstance) would require no neutral wire? - That question was on the electrician test in Texas about 25 years ago. (we might see if Stubbie really understands the purpose of a neutral wire in an electrical system. And, why most houses are wired with alternating current instead of direct current - even though AC is more dangerous than DC.
Are you talking about a neutral or a grounded leg ??? There are considerable differences.

Now as for your intent to imply your holding some secret knowledge that no one else can match is rather condescending don't you think?? And yes your terminology is unorthodox and I frankly have never dealt with someone who poses as knowledgeable and explains a 40 amp feeder as making a sub-panel an 80 amp sub-panel because each leg can pull 40 amps at 120 volts....

Now if your intent was to ask about your sub -panel project (which I'm beginning to doubt) why would someone with your knowledge be asking us a question like your original post. And then the questions thereafter ?? Seems odd to me.

As for your challenge about explaining a neutral wire that will depend on whether you can tell me if your wanting to talk about a neutral or a grounded leg as I mentioned earlier. Or is this just more of your terminology problems ....

As for determining why a 240 volt branch circuit requires no grounded leg (notice I did not say neutral as you did) ...surely your joking ....

AC more dangerous than DC ... that explanation I would love to hear... they both can kill you .. just don't try the DC jolts you loose from the contact point theory ... I'll refuse to continue on this thread if you try that one....
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Old 02-08-2011, 09:50 AM   #26
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Thanks to everyone who answered and commented on this thread. I will be ordering a new electrical book - I need a refresher course after being away for 20 years.

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