Electrical Question II

ORIGINAL POSTER LAST QUESTION:

Thank you for an excellent and easy-to-understand explanation of white versus green/bare. My question is, from the main to the sub panel, can you just run the 220 volt black/red/white (3 wires = 2 hot, one neutral) and then install a new ground rod for the sub panel to replace the green/bare? REPLY:

The answer is NOOOOO OOOOO OOOOO OOOOO OOOOO O. If you drove a rod into mother earth at that sub panel VERSUS running the 4th bare/green equipment grounding conductor from the main panel YOU DO NOT HAVE A DIRECT LOW IMPEDANCE CURRENT PATH BACK TO THE MAIN PANEL TO TRIP A CIRCUIT BREAKER AND CLEAR A POTENTIALLY HAZARDOUS FAULT. Mother earth, depending on the moisture and mineral content and the grounding electrode quality, is NOT a good low impedance current path, therefore if say a steel cased grounded electric drill (that uses the 3 wire grounding plug) got shorted to a hot live phase wire and since that ground rod you drove is a high impedance path back to the panel, if you touched the case and were standing on moist earth or another good current path back to the panel, YOU COULD DIEEEEE E.

You see the electricity is seeking a current path back to the service transformer (thats where the voltage difference is) and earth isnt a good path. However, if you run that 4th equipment grounding conductor from the main to sub panel (as we told you), it IS A GOOD LOW IMPEDANCE CURRENT RETURN PATH back to the transformer (to trip a circuit breaker to save your life) via the Neutral and Ground busses being tied at the main panel which is served by the transformer.

The reason the Neutral, at BOTH the pole or pad mount transformer, PLUS at the service entrance is tied to a grounding electrode (driven rods or water or gas pipes) to Mother Earth is to bring everything back down to a common zero reference. That driven ground rod you wanted to use simply DOES NOT suffice as a good return current path back to the panel to trip a circuit breaker to clear a dangerous fault.

RUN 4 WIRES TO THE SUB PANEL FOR 120/240 VOLT SINGLE PHASE 3 WIRE SERIVICE 2 Hots,,,,, ,1 White Neutral (a grouned conductor),,,,, 1 bare/Green Equipment Grounding Conductor.

RUN 3 WIRES TO A 120 VOLT LOAD 1 Hot, 1 Neutral, 1 Equipment Grounding Conductor.

Hope this helps

John T retired electrical engineer

A 120 volt fault to a 10 ohm ground connection would result in about 12 amps of current, not enough to trip a breaker. The ground rod itself, and anything referenced to it, would be at 120 volts above the potential of the surrounding earth. Note that the current necessary to inflict a fatal shock is orders of magnitude smaller than the fault current.

Basically, in the US, a new fault protection reference point is established at the output of each transformer feeding an end-user load, such as a house. For fault protection, there must be a low resistance path back to this fault protection reference point, which is typically in the main distribution panel. The ground rod is primarily for lightning and transient suppression, and contributes nothing to tripping the breaker in the event of a fault.

The ground rod does help maintain the fault protection system at something near earth potential for small current faults, which limits, but does not completely eliminate, shock hazards.

It's been a while, but I think additional ground rods are permitted provided they are referenced back to the fault reference point with a conductor large enough to carry fault currents. Additional rods would be useful for applications such as lightning protection, but do little for fault protection.

Keith

John T

I believe your comments are entirely correct for a sub panel within a building or within an attached building where the main panel resides.

However, I believe that if the sub panel is in another building that is not attached to the building where the main panel resides then it is ok to just run 3 wires. 2 hot and a neutral and put in ground rods at the sub panel and tie the ground and neutral busses together in this sub panel.
There must not be any other metallic connections between buildings such as water lines, phone lines etc.

Regards
Gary

Good point about other connections like water or gas pipes !!!!! !!!!

Love this tricity chat lol

John T

John T

I'm having a hard time getting my brain wrapped around one thing.

Again, you are correct, & I understand how it should be done..... :)

What is the difference between a sub-panel with ground rod in a different building being fed with the 3 wires, and a sub-panel with ground rod in the same building being fed with 3 wires?

One is ok, one is not. I don't see where the transformer is involved in either, the loop goes through the breaker in the subpanel & that is what we want to trip?

I know I'm not getting it - trying to figure out the part I'm missing.....

--->Paul

QUESTION: What is the difference between a sub-panel with ground rod in a different building being fed with the 3 wires, and a sub-panel with ground rod in the same building being fed with 3 wires? One is ok, one is not.

When you go out to the barn thats like starting alllll ll over again with what the code refers to as a "seperately derived service" and its like the original house where you ran 3 wires from the pole mounted transformer, bonded the Neutral to mother earth, and bonded the equipment ground buss to the Neutral buss to carry ONLY fault current.

To start with per the NEC the Neutral Buss and the Equipment Ground Buss are not again re bonded at the sub panels (kept seperate n isolated why 4 wires required), cuz if the right (wrong) combination of line breaks occurred you could end up with an energized drill metal case and dieeeee e. So if you would ONLY run the 3 wires with no equipment ground and drive a ground rod to tie the Neutral back to Mother Earth WHERES ANY EQUIPMENT GROUND that provides a path back to the panel for fault current to trip the breaker????? ?? (remember, youre NOT supposed to use the Neutral to carry fault current, thats the job of the equipment ground) Welllll lll the answer is (like if at a remote building) re establish a whole newwwww equipment ground buss tied to the Neutral (per your question)
buttttt tttt then you could be providing more then just the one return current path in that building whennnnn nn its ONLY the Neutral which is supposed to do that and the equipment ground is supposed to carry ONLY fault current. In the one common building all the non current carrying metallic raceways and water n gas piping etc is supposedly alllll ll tied together which is why you dont want to create a whole new equipment ground again tied to neutral again cuz I can envision pipes or raceways ect and/or equipment grounds possibly carrying Neutral return currents
BUTTTTT the equipment ground is supposed to carry ONLY fault current.

Am I makin any sense ????? ????? ????? ?

QUESTION 2: I don't see where the transformer is involved in either, the loop goes through the breaker in the subpanel & that is what we want to trip?

ANSWER yes its the breaker we want to trip and the line current must pass through the breaker to trip it BUTTTTT TTTTT TTTTT TTT ONLY ONE LINE ISNT CARRYING THAT CURRENT, ITS THE HOT PHASE WITH THE NEUTRAL SERVING AS THE RETURN CURRENT PATH BUTTTTT TTTT the voltage difference is at the transformer and its the neutral at/on the transformer where that current is trying to get back to NOTTTT mother earth. The transformers line voltage is with respect to the other line (240 volt) or the Neutral (120 volt) and its line to line or line to neutral the voltage exists and where the current wants to flow via the panelboards breakers and the neutral buss UNLESS a fault exists in which case the equipmenmt ground will carry current back to the transformers Neutral via the Neutral buss in the panel whichhhhh h is bonded to the equipment groudn buss. The whole loop concerns the transformer and its voltages and its lines and neutral, the panel just serves to distribute it and prevent overloads or faults cuz of the line current passing through circuit breakers.

PS Gerald, I like n agree with your comments that the NEC allows but dont really like only the 3 wires to the shed cuzzzzz zzz the neutral ran to the shed may have to carry fault current versus if 4 wires were ran it wouldnt have to.

PS Gary, greattttt ttt post Im gonna re read n study on it, dont go away now lol

Hope this helps but its startin to confuse me lol

Ol John T retired EE

In an out building their thinking is that alternate neutral paths are much less likely to exist. They even state that the out building must not have any other metallic connections between it and the building where the main panel is located. . If it does then 4 wires are required and neutral does not get bonded to ground in the sub panel.

If you did have say a water pipe between the main building and the outbuilding with the pipe bonded at each building to the panels then some neutral current could also flow in the water pipe. With only 3 wires (2 hot and a neutral) you would have the neutral bonded to the ground terminal, ground rods and any water pipes etc. also bonded to that same ground connection. Now the neutral current can flow on the neutral wire and also the water pipe back to the main panel as the neutral and ground wires would be connected in PARALLEL.

You ALWAYS want to bond water pipes or any other metallic lines to the panel�s ground connection. If you don�t you could have a different voltage between the panel ground and the water pipe as an example especially during a fault. This could be disastrous. So this is the reasoning for the 4 wires.

This is kind of flawed reasoning though. If you think about the house that may be next door to you that also has city water connections, gas, cable TV etc. there are metallic connections between the houses other than the power company connections. Each house has its own ground rods and only 3 wires feeding each. So the chances of multiple neutral paths do exist between power ground and those other metallic connections entering your house.
The nec does require all those other things, water lines, cable TV etc. to be bonded to the common power ground. And your neutral and ground connections bonded together in your main panel. But in doing so some neutral current can flow in those pipes connected between houses when both houses are fed from the same transformer due to the voltage drop in the main wires to the houses. It is quite common to feed 4 or 5 houses from the same transformer.

If you were to do that same type of bonding in your out building with a sub panel (if you had water lines etc. running to the out building) it would be illegal. You could have the same type of multiple neutral paths as can exist between houses the only difference is that your out building gets fed from a breaker in your main panel rather than having its own main panel. Ground connections would be the same.
It can get interesting!

But you should wire things the way the nec says. You can never go wrong with 4 wires.

Regards
Gary

I understand follow NEC, my point is not to get around that..... ;) My questions are only to help understand better.

Last year I had the main backbone of the fam rewired. Pro electricians designed & did it, county inspected it. I watched, but did nothing.

So, it was 3 wires to all 11 buildings. One ground rod at each building.

House, old chicken barn, and barn have water. All runs of water involve plastic pipe (or a concrete cistern seperating), but no one ever asked me. As well, the well is 2 feet from one building, and the pipes go to the foundation of the old old chicken barn.

No one asked about or was concerned about bonding any building other than the house, or about 4 wires. Everything is 3 wire.

Curious about that too, but not terribly disturbed about it.

--->Paul

Do you have one main breaker only in the panel at the house and all other buildings fed from a breaker in that panel and no main breaker in any of the panels in the outbuildings?

Or do you have main breakers in each panel in each building? If so are those panels still fed from a breaker in the main panel at the house or are they fed directly from the meter?

Thanks
Gary

Transformer, feeds to a meter, feeds to a 200amp main breaker more or less centerally located. From there 3 runs - make that, 4 runs of wire, one to the house with a 100 amp panel, other 3 runs go to the other 10 outbuildings - mostly 200 amp wire running to terminal boxes & split to buildings. Each building has it's own main panel. Some are only still the old 20 amp fuse box, but the idea is 60 amp each building someday perhaps.

So, the house is it's own line from the meter/ main disconnect. No sub-feeds from any building to any other.

--->Paul

Gerald J.

The preferable practice is to run the fourth wire and then the ground rod at the outlying building is acceptable but not required.

Gerald J. Electrical engineer in Iowa.