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#50650 04/09/05 06:29 PM
Joined: Apr 2005
Posts: 6
R
Junior Member
run a new wire and do it right

#50651 04/09/05 07:37 PM
Joined: Mar 2001
Posts: 2,056
R
Member
The other issue is that this is NM cable.
I don't think it was ever acceptable to use 2-wire NM on a range or dryer circuit. Pre-'96, 2-wire installations were required to be SE type cable (never was exactly sure why).
Therefore, if SE cable is encountered on a range replacement, it could be re-used.
Correct me if I'm wrong.

#50652 04/09/05 09:37 PM
Joined: Jul 2004
Posts: 9,766
Likes: 13
G
Member
I have seen 2 wire Romex on dryers but that was when cars had fins. Usually when I saw Romex to a dryer it was 10-3wg. They hooked the white to the silver lug in the receptacle and used the ground on the box/backstrap. I was told it was because a (WWII) dryer/range was grounding to the neutral and the neutral had to be white. SE was the exception if the AHJ would take it.

My 1963 era house has 8-3wg Romex to the range. It is still the original 3p crow foot. I bought a cooktop that is pure 240v.


Greg Fretwell
#50653 04/10/05 01:23 AM
Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
CTwireman,
I have been doing elec work for many years but do not know it all, I'm always open to learn more. Knowing the code or how to do something and knowing the theory behind it are 2 separate things.
Can you explain why touching the main panel neutral/ground bar is different than touching the junction box neutral and/or ground would be different? Other than some resistence in the wire.


The Golden Rule - "The man with the gold makes the rule"
#50654 04/10/05 01:31 AM
Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
jdadamo,
Your Quote:
(Lets assume (theoretically) that you do indeed tie together the neutral and ground at the cooktop, connecting them both to the EGC. Now just for fun, lets sever that EGC. Customer turns on stove and causes some kind of 120 v load. Neutral and ground have been bonded at connection point. Customer touches stove. What happens? They complete the circuit. Dead customer. Your license goes bye-bye.)

Why would that complete the circuit? It is already completed back at the panel. And unless the neutral gets disconnected at the panel from the cook top, electricity will of course takes the easier path, not through the customer.


The Golden Rule - "The man with the gold makes the rule"
#50655 04/10/05 01:35 AM
Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
I know the correct way is to run the 10-3, and in some cases probably good reasons for it. That is what I will do, but it does raise some good theory questions.


The Golden Rule - "The man with the gold makes the rule"
#50656 04/10/05 08:54 AM
Joined: Nov 2004
Posts: 37
J
Member
aldav53, I clearly said in my post "now for fun, let's sever the EGC." So, in that case, no, it is not connected back to the panel. It's sitting there broken. It's a hypothetical situation that I was using to explain to you why it is NOT equivilant to take that approach, because you asked if it was.

#50657 04/10/05 10:42 AM
Joined: Sep 2003
Posts: 650
W
Member
aldav53,

I'd strongly suggest getting a copy of Soares 'Grounding'. This is the _classic_ textbook on why we ground and why we ground in the way that we do. 'Grounding' is not easy, and the historical section will make it clear that grounding is not without controversy. In fact, at one point some of the early electrical safety organizations _prohibited_ grounding of electrical systems. (Check out the historical appendix of the book.)

Alas, the original author (Soares) is dead, and the book keeps being revised to match the current NEC, which means that it now reads as though it were written by committee (the same concepts repeated several times in ways that IMHO don't reinforce useful knowledge, having several examples in a row that look at different aspects of something, but using _different_ base examples when IMHO it would make more sense to look at the _same_ system form different viewpoints, etc.) But the text still covers the essential material, and is worth reading for this reason alone.

You don't need to consider jdadamo's failure condition to understand why the shared neutral for the cooktop is not desirable. The real problem is pointed to in your statement:
Quote

If the neutral and the ground are tied together at the main panel feeding the cook-top. Tieing the neutral and ground (from the cook-top) together at the junction box (using exhisting ground) right below the cook-top would be the same results.

You also said
Quote

And unless the neutral gets disconnected at the panel from the cook top, electricity will of course takes the easier path, not through the customer.

These are two common (and dangerous) misconceptions.

The first thing to understand is that electricity does _not_ simply 'take the easiest path to ground', or even the 'easiest path' in general. Electricity takes _all_ paths back to the _source_ of the current, in proportion to the ease of the path. If you arrange a system where the electricity has two paths back to the source, one through a thick copper wire, and one through a person, then _most_ of the electricity will follow the wire, and a small bit will go through the person. Make the difference in resistance large enough, and the current flowing through the person will not even be noticed.

Because electricity will take _all_ available paths, the grounding done in premises wiring systems is known as 'single point grounding'. All metal that _should_ be at ground potential is 'bonded' using equipment ground conductors and grounding electrode conductors. This makes sure that during normal operation all of this metal is at the same voltage, so that there isn't a shock risk. Then this entire 'grounded' system is connected to the neutral of the electrical supply at one, and only one, location. Because there is only a single point of connection, there is no closed path that the electricity can follow, and thus current does not normally flow in the ground system. That latter point is the crux of the issue.

The neutral is the 'grounded conductor'. It is insulated and _expected_ to carry current. The grounding system, on the other hand, includes essentially all other metal, and may or may not be insulated, but is _not_ expected to be carrying current. When you make _two_ connections between the neutral system and the grounding system, you change the story: now there are closed circuits in the grounding system, and current _will_ flow in the grounding system.

Now, if the metal frame of the range were bonded to the neutral conductor, rather than to an equipment grounding conductor with an isolated neutral, but the range itself were completely insulated from everything else, than I would agree, this would be exactly the same as the 'proper' connection back at the panel. But if the frame of the range has _any_ metallic contact with any other grounded metal, then the net result would be multipoint grounding and the introduction of current into the grounding system. If this current were flowing, for example, in the water pipes, then even without any sort of fault condition or failed conductor, you could have a shock hazard.

As iwire noted, previous versions of the NEC permitted the use of the _insulated_ neutral conductor to bond the frame of things like ranges, presumably because the total imbalance current of this load would be pretty small, and thus the amount of current potentially placed on the grounding system would be _very_ small. But it is interesting to note that this sort of shared neutral/ground was only permitted if the feed went back to the _main_ panel. If the feed went back to a subpanel, then the feed to the range had to be a four wire feed.

I'd also like to call attention to a situation where multi-point grounding is commonly seen, even though it is undesirable. In urban settings, it is quite common to see several houses, possibly an entire block, fed by a single transformer. Each house has their own ground to neutral bond in their main panel, and the city water supply pipe is used as one of the grounding electrodes. The city water pipe is a metallic system that connects several homes, and is clearly a 'parallel path' for neutral currents. In some cases, the neutral in the electrical system can fail, but the entire electrical system appears to function normally, since the neutral current is flowing through the pipes to an adjacent house, and then through the ground bond in that other house. Plumbers have been killed working on the plumbing pips because they've interrupted the neutral current, and experienced plumbers place jumper cables across sections of pipe that they are cutting apart. The next time you are in a home that has metal water pipes coming in from the street and a shared transformer, put a clamp meter around the pipe or the grounding electrode conductor to see what sort of problems multipoint grounding can cause.

-Jon

[This message has been edited by winnie (edited 04-10-2005).]

#50658 04/10/05 04:55 PM
Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
winnie,
Very good explanation. The only part that I might question some is if a person touches the cooktop, the resistence is going to be so much higher than the wire path, they wouldn't get shocked. But of course we don't want to take any chances at all, and should wire it with no chance of a path. The point about making a connection at one point only, makes great sense.
Again, very good..
Thanks,


The Golden Rule - "The man with the gold makes the rule"
#50659 04/10/05 05:57 PM
Joined: May 2003
Posts: 2,876
E
e57 Offline
Member
What are the chances someone might touch the sink and the cooktop? Or the cooktop, and a grounded appliance? Or someone else touching one of those items? That parrelel path to ground from the current carrying surface of the cooktop posses a hazard.

Let me tell you of a service call horror story: I got a after-hours call from a resturaunt, where the dishwashers had reffused to work, and the manager only called to appease them.... I was about to leave when he told me that. Then tells me why - He said the dishwashers "claimed to be getting shocked from the sink." So I went back and finangled my best spanglish with the guy mopping the floor, who pointed to the sink and a metal counter. Then to the sink and another metal counter. 180 from one to the sink, 120 from the other, 240 to both. The garbage disposal was using the plumbing as a neutral, and the counters were installed with 3" screws that penetrated different circuits on each hot metal wall. I told the manager who didn't beleive me either, and refused to have me fix it. I asked one of the dishwashers how long it had been going on, he said years! (I called the boss who had me pull the breakers, and sharpie the hazard on the panel before I left. We called the city in the morning.)

So imagine this cooktop, and its "ground fault", any time your customer touches an appliance, or the sink, getting more than 5 milli-volts across their heart.


Mark Heller
"Well - I oughta....." -Jackie Gleason
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