Main Panel Neutral/Ground Connection

[jhzl]

Hi all.

I was searching for information on how to wire a “current” project and was “enlightened” on something I had wondered about for a long time: the neutral/ground connection in the main panel. I knew that it worked but could not understand how. Still don’t, really, but I think I have the idea: current follows the path of least resistance. Normally, current flows back to the electric company through the neutral, and not through the panel’s connection to earth, because there is less resistance in the neutral. But because I am a knit-picker, I wonder if this means that there is absolutely no current flow from panel to earth, under normal conditions?

 

[Gooch]

Hi all.

I was searching for information on how to wire a “current” project and was “enlightened” on something I had wondered about for a long time: the neutral/ground connection in the main panel. I knew that it worked but could not understand how. Still don’t, really, but I think I have the idea: current follows the path of least resistance. Normally, current flows back to the electric company through the neutral, and not through the panel’s connection to earth, because there is less resistance in the neutral. But because I am a knit-picker, I wonder if this means that there is absolutely no current flow from panel to earth, under normal conditions?

Current doesn't flow to earth really. the bond is to set the potential on the neutral to the same as the earth. under normal operation, current flows back to the utility xfmr on the neutral. when a ground fault occurs current flows back to the panel on the EGC(equipment grounding conductor) through the main panel bond, and back tot he xfrmr on the service neutral.

The main purpose of the grounding electrodes(rods, water pipe, Ufer) is to 'force' the system neutral to the same potential as the earth.

now if you want to be super technical there is probably always a small amount of current flowing in the grounding electrode conducter/grounding electrode due to inductance on the EGC.

 

[theoldwizard1]

The ground wire in an "electric appliance" is attached to the frame of the appliance. This is done based on the logic that if there was a short from the "hot" wire, it would be most like to the appliance's frame. If the frame was "grounded", a high current path would trip the break/fuse BEFORE a human could be the path of least resistance by touching the frame and ground !

 

[1/2 Cup]

Current doesn't flow to earth really. the bond is to set the potential on the neutral to the same as the earth. under normal operation, current flows back to the utility xfmr on the neutral. when a ground fault occurs current flows back to the panel on the EGC(equipment grounding conductor) through the main panel bond, and back tot he xfrmr on the service neutral.

The main purpose of the grounding electrodes(rods, water pipe, Ufer) is to 'force' the system neutral to the same potential as the earth.

now if you want to be super technical there is probably always a small amount of current flowing in the grounding electrode conducter/grounding electrode due to inductance on the EGC.

On the money, the even do this down under!

 

[BFBOB]

The other factor in the safety aspect: The thinking reader will see the wisdom of grounding the frame, but ask why not just use the neutral since they're the same - connected together inside the panel? The reason is that the separate ground wire does not carry any current under normal conditions. This means it is not subject to deterioration due to electrolytic action or heat cycling, as is the case with the hot and neutral. Thus, it is always in perfect condition to conduct that accidental short and trip the breaker.

 

[malibu101]

The other factor in the safety aspect: The thinking reader will see the wisdom of grounding the frame, but ask why not just use the neutral since they're the same - connected together inside the panel? The reason is that the separate ground wire does not carry any current under normal conditions. This means it is not subject to deterioration due to electrolytic action or heat cycling, as is the case with the hot and neutral. Thus, it is always in perfect condition to conduct that accidental short and trip the breaker.

In bold- Yes.
And since the grounding conductor is not carrying any current it is a much easier path to follow if a fault current goes to ground to open the breaker.

Maybe minutley, but current will flow easier through a conductor that is not carrying any current.

 

[wyliesdiesels]

The purpose of earth grounding/grounding electrodes is to ground out LIGHTNING.

 

[MrMark]

Current does not flow only on the path of least resistance. I see this over and over again and it is absolutely false. Current flows on all available paths inversely proportional to the resistance of the path. Look up the formula for a current divider and you will understand how current splits according to resistance.

The earth is a very large resistance compared to the service neutral. Thus, there should be almost zero current on the ground rods unless you have a metal water main and everyone has copper pipes, like around where I live.

The ground rod is mainly for lightning as posted above by Wylie. It also clamps the Neutral to earth at the service drop for constant voltage levels.

 

[MrMark]

In bold- Yes.
And since the grounding conductor is not carrying any current it is a much easier path to follow if a fault current goes to ground to open the breaker.

Maybe minutley, but current will flow easier through a conductor that is not carrying any current.

I'm not sure this has a lot to do with the grounding system. Physically, resistance goes up with temp rise according to the temperature coefficient and a wire carrying current will thus have a slightly higher resistance. Accordingly, the current divider formula will show slightly less current flow at higher temps for a given resistance wire, but not enough to have it be a design factor or something to worry about.

 

[Kevin C]

My take on this....

Even with higher resistance current will flow through an alternative path. The percentage is dependent on the ratio of the two resistances.

The other side of the equation is how much current is flowing on the neutral. If you put all your loads on one leg of a 240 V system, then 100% of the return current is on the neutral.

If your missing your ground rod wire, this is what you would get with a 100 ft. 2 AWG service drop:

»A 100 ft run of 2 AWG aluminum would have about .025 Ω of resistance (your service drop).

»If you had an imbalanced load of 100 AMP's, all 100 AMP's is returning on the neutral (all loads connected to one leg in the box, I know it's unlikely but for the sake of providing a worse case example).

Voltage drop back to the pole (100' of 2 AWG aluminum):

V=IR ~ 100A*.025Ω= 2.5 Volts. That's pretty much worst case since your loads should be a lot better balanced between legs (probably 10 to 30 amps is the most the neutral will see).

With a large current on one leg and no ground, the potential from the housing of the panel to ground would be 2.5 volts

If the ground wire is connected how much current flows on it?

The total resistance of the neutral system is the neutral wire and the ground wire in parallel.

Assume a UFER ground with 5 Ω resistance.

1/((1/.025)+(1/5))= .0248756 Ω. Not very different than the just the neutral back to the POCO (.025 Ω).

The neutral and the box is now sitting at 2.48 volts instead of 2.5 (no ground wire example).

Current to UFER

V/R=I, 2.48V/5Ω=.496A

About 99.5 percent of the current goes down the neutral. .496 AMP's out the 100 amps is flowing through the ground system.

Scale that back to a more typical neutral current of 20 amps and about .1 amps is flowing through the ground wire to your UFER.

Ground rods can have up to 25 Ω of resistance, so the current flowing through them would be considerably less than a good UFER or ground rod placed in a very conductive soil.