subpanel wiring question


Old 12-12-00, 04:48 PM
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I've read the posts regarding wiring a sub panel off a main breaker box. Have enjoyed filtering the information and always relate what I'm reading to all the electrical work that I have done in my house in the last 30 years. I've also just read the article on page 114 of "Fine Homebuilding" relating to the 10 most common wiring mistakes that are made, some intentional, some not.

Now for a couple of questions:
Assume a subpanel is physically adjacent to, i.e. right next to the main panel, wired with correct size wire, 4 conductor including ground from main panel to subpanel. Except the neutral and ground are wired to the same lug in the sub panel. Where exactly would a break in the neutral have to be for the ground wire to become activated(energized, is that the word??), where it would not otherwise be energized if the neutral had it's own lug and wired "correctly"? The "Fine Homebuilding" talked about sparking etc on ductwork and metal siding. Assuming all recepticles and other units to be wired correctly, where physically would the problem have to be for danger to happen?

Next question: What is the problem with fastening two black wires to the same breaker in the panel? I mean why does the NEC prohibit it? I'm not talking about a sloppy job like in the picture in the magazine. I'm talking about a nice neat job. It's easier than trying to "stretch" the slack out of another cable circuit to put in a junction box in order to tap off another outlet or two.

Once again I'm enjoying all the questions and responses. TIA
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Old 12-12-00, 06:28 PM
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I will try to answer your questions, begining with the 2nd one 1st. In order to be able to terminate two wires under any screw, the device the screw is attached to has to be listed for such. Alot of breakers are now listed as suitable for two wires, but it is up to the manufacturer to say so. If, somewhere in it's labeling, either inside the breaker panel, or on the box the breaker came in, there is a remark that you can put two wires under one screw, then go ahead. But if it doesn't give permission for more than one, then one is all you can do.

Now for your 1st question, concerning a fault in the sub-panel. The danger is not really a factor in this case, however the code is violated. The code says you tie the neutral and the grounding conductors together at only one place, which is the service. the reason for not doing it anywhere else is that if there is a short circuit or a ground fault anywhere downstream from the overcurrent device feeding it, then, if the neutrals and grounds are tied together, the current will try to get back to it's source, and with the two conductors tied together, the current will be split between the two. You add into that the conductor length, and loss, by the time the current gets back to the overcurrent device (the breaker) there may not be enought current to trip the breaker. With one wire, most all the current will flow, getting back to the overcurrent device quicker, and causing the breaker to trip faster.

Remember, this current will have to travel from the fault, back on the euipment grounding conductor, thru the grounding conductor between the two panels, to the neutral of the service, out to the power supply (usually the transformer out in the alley or street), thru the transformer, back on the hot conductor, back to the service, thru the breaker, and then back out to the fault. If this circuit is hindered in any way, the current flow may not be large enough to trip the breaker.

This in a nutshell, is why you do not tie the neutrals and the equipment grounds together anywhere but at the service.

Hope this answers your question.

Rick Miell
Old 12-12-00, 07:30 PM
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A Bit More Info...


#2 first--There's a reason you shouldn't put two conductors under a terminal screw of a breaker listed (by the UL) for one conductor. As current flows in the conductor, it will expand. As it cools when current os off, it will contract. Two conductors means possibly twice as much expansion and contraction. A one-wire only breaker terminal has not been proven to maintain a secure connection in those circumstances. So you could wind up with a loose wires...a recipe for arcing and subsequently disaster.

#1--The return current will flow back on both the neutral and the ground, as the previous commentor said. The ground is likely to include the metal couduit and case of the subpanel. I don't think it's so farfetched that someone (who's grounded) will eventually touch that metal. Granted that the closer the subpanel is to the panel, the less hazard there is, but given that the cost for materials (an isolated busbar for the neutrals in the subpanel) is miminal, why not do it right?

Old 12-13-00, 07:47 PM
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I agree with the two previous replies pertaining to only one wire in a lug that is approved by UL only for use with one wire. They are good explainations.

I have a few problems with the explanation of whether danger exists paralleling the neutral and grounding conductor on the load side of the main service disconnect or main service panel.

The previous explainations did not approach the real danger involved in running the neutral and grounding conductor to the same grounding or neutral bars on the load side of a service rated panel. This includes subpanels etc.

If you connected the normally larger neutral feeder to the same bar as the usually smaller grounding conductor. Keeping in mind that the grounding conductor can be a metal conduit, bare conductor, etc. Also keeping in mind tha the grounding conductor is an equipment grounding conductor connected to all metals associated to that panel, including the metal of your washer, kitchen sink if with metal pumbing, furnace, dishwasher, etc. All metals in the house is required to be bonded to the bare or green equipment grounding conductor. This means that people including your kids are exposed to all these metals, including during the time that they come in out of the rain, step out of the bath tub, etc. Now a wet body makes for an inviting conductor for a path to ground. Electricity also always seeks the easiest path to ground. Ground does not always mean mother earth.

Now consider if you connect both the equipment grounding conductor to the same bar as the neutral or grounded conductor then you have made both conductors the same as one entity. Now consider an imbalance where there is a large load on the neutral or grounded leg. This is common because of the intermittant loads of a dwelling. Your neutral could carry the full amount of the capacity of one of your hot feeders serving the house. Now you would be carrying as much as 200 amps or more on that neutral feeder of you sub panel, that is usually sized the size of your thumb or at least only two sizes smaller than you hot conductors in a dwelling setting. Therefore your neutral is now carrying a large current through it or at least the capabilities of carrying that large current if you have good connections and a return path to carry the unbalanced load. If both the neutral and equipment grounding conductor is connected as both the line side and the load side of that feeder and if the larger neutral feeder failed then that smaller and commonly bare equipment grounding conductor must try to take of as the neutral path, you then would have a bare current carrying conductor. That bare conductor is too small to carry the load of the larger neutral and is seeking an easier path to ground. YOUR IT. JUST TOUCH THE METAL OF YOUR SINK, WASHER, DRYER, BATHROOM TUB FAUCET, ETC, THAT IS CONNECTED TO THAT SMALLER EQUIPMENT GROUNDING CONDUCTOR IS WAITING FOR AN EASIER PATH TO GROUND. IF YOU TOUCH ANY METAL CONNECTED TO THAT ENERGIZED CURRENT CARRYING EQUIPMENT GROUNDING CONDUCTOR YOU JUST MADE THAT EASIER PATH TO GROUND FOR THE CURRENT TO FLOW. MOST LIKELY RIGHT THROUGH YOUR HEART



If a neutral or grounded leg is isolated as it is supposed to be wired, then your metal through out your house is protected by the equipment grounding conductor. Only if there is a short circuit would that equipment grounding conductor become energized. If this happens then a hum appears and the breaker kicks out within its interuppting rating for a short circuit. It happens very rapidly and this rapid response of the breaker protects people from being hurt. If the neutral and the grounding conductors are married on both sides of a feeder on the load side of a main service then the short circuit protection is null and void or unworkable to the point that people can get hurt or killed.

The article you read about the ducts etc. sparking where a neutral and grounding conductor is married on the same bar of a sub panel even next to the main service panel is absolutely true. Just ask an old engineer in a factory almost all of them have either seen or heard of the sparking you read about.

Hope this helps

Old 12-14-00, 10:09 AM
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Location: Fayetteville, NY, USA
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OK, my 2 cents:

All but wg talked about the "terminal screw" on a breaker. If this is what you have then absolutely never place 2 wires on a single screw. But wg mentioned lugs. Many if not most breakers have a lug, or a clamp into which your wire goes and is clamped in there tightly when you tighted the screw. I have a Square D panel and the breakers have two separate lugs on each. Although I am thus permitted to run 2 circuits off 1 breaker, I don't. This is why I went for the 30 space panel when I upgraded. Lots of individual circuits now possible, and one breaker kills only one circuit. The whole situation is just tidier, to me.

One clarification to RickM's post. He said that you only connect neutrals & grounds at "the service", and I felt that not everyone may know what that means. In a nutshell, the enclosure that houses the first disconnect your electrical system sees, typically but not always the main breaker in your load center, and everything else in that enclosure, is considered your "service equipment", which is a specific category of equipment and a technical term in the NEC. So many people with an electrical background will often just call it your "service". This is plain and simply the only place you can bond your grounds and neutrals together, and at your service equipment you're required to. Anywhere downsteam of that box you're absolutely forbidden from joining the N's & G's.

Hope that sheds a little more light on your topics.


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