Why is the neutral isolated in a sub box.

fishbulb · 2003-10-16T12:09:47+00:00

If I have a 200 amp main in my house and later on decide to put a 100amp subbox in my garage.....why do I have to isolate the neutral fromthe ground in the garage box?I am asking this because in the 200 amp main box the grounds andnuetrals are all mixed on the same ground bar.How does seperating them in the garage do anything?

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Don Kelly 22 years ago

---------- Average stroke is about 30-35KA, the probability of a of a stroke of 100KA is about 0.5%. Typically any lightning protection has a let through stroke current level of about 15KA (based on "strike distance " design techniques. However, putting this quibble of mine to the side it is true that it is to provide a current path. As to insulator breakdown, for the sort of insulation levels encountered anywhere near a garage, a typical rod will not be pareticularly effective at preventing this. It does, however, offer a better path to ground than anything else around.

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No problem with the ground return conductor. As for the few hundred ohm ground path- that is questionable - most of the ground path resistance is within a few meters of the ground electrode and the difference between 50 and 100 or 200 ft is not all that great. (many rural 6900KV lines were built without a ground conductor - being grounded by rods only at the transformers along the way. Voltage drops in the ground were small (and safe). I have also seen the results where the ground conductor was open. The resistance distribution that you quote for lightning is also true for DC or low frequency AC AC (with some variations in scale) as indicated by the work of such as Rudenberg and Dwight (about 75 years ago and still in use). A matter of current density as distance from the rod increases (somewhat analogous to the E field as one moves away from a point charge).

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Agreed

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--------- Thanks -This is the point that I knew and had forgotten about. .

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

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John Gilmer 22 years ago

Well, sport, it WILL pull the 120 volts down to "local ground" and that's all you can ask for.

It would be the equivalent of setting up two separate grounding schemes more than 25' apart and connecting power company neutral to one system and power compary 120 volts to the other. You would draw about 10 amps (more or less) from the "circuit" but you shouldn't get zapped.

Of couse, things like metal fence wire and damp sections of the lawn can cause dangerous voltage gradients but within a shed there should not be a problem.

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Nukie Poo 22 years ago

Why do you have to be so snide? Besides, you're wrong: Go take a ground rod, drive it into the earth; take an extension cord and cut off the female end; strip the black wire and connect it to the driven rod. Next, with the cord plugged into an non-GFI outlet, kneel down and take that arrogant mouth of yours and touch it to that rod - or better yet, sit on it. Then, tell us if the empirical results agree with your theory - in your next life, "sport".

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John Gilmer 22 years ago

I would not even think of doing that (but you just did!)

You are just too stpuid to realize that what you fear in only running three wires to the shack happens quite often in real life when the power line to the house partly fails.

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Nukie Poo 22 years ago

I never said I fear anything in running three wires to the "shack". You are confusing me with the original poster. I was simply describing various fault-scenarios and their possible consequences. And, BTW, I quite well know what happens in real life with electrical services - I've been in the electrical contracting business for over 25 years and hold a Master Electrician License (Suffolk County No. 4899-E). I also hold a electrical engineering degree. You, however, look like yet another engineer/armchair electrician with obviously not much hands-on experience in the field. Not to mention that you can't even properly follow a Usenet thread. Go swallow a watch battery and call me in the morning.

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Gfretwell 22 years ago

I suggest you go count the wires coming to the "shack" your family sleeps in. I have never seen a home in the US wired with anything but 3 wire if it was

240/120 single phase.

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John Gilmer 22 years ago

So?

What's your point? This who discussion was based on "worse case" were a wire to open or become disconnected.

In the last few years, we have lost the neutral once and one of the two hots twice.

I simply described the "worse of the worse" case whereby the only wire coming in is the hot and that somehow it ended up being shorted to your "local ground."

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