small subpanel grounding

[Ashokan1]

A job I'm on has a small 240v 20A - 2 space/4 circuit - subpanel located outside for some minor use on a farm. One 20A circuit will feed a small mobile trailer on the property. There was no existing fourth wire run from the main panel in the house so the subpanel needs to be grounded. Can I drive in grounds at the panel if there is no other grounded lines (tel, CATV...) from the house nearby? Do I need two 8' ground rods for such a small panel? Thanks.

[Roger]

Is this sub-panel on the same structure as the main house panel and is the main disconnect located in the house panel ...ie...main breaker panel.... or is the main disconnect out on a farm pole and a 4 wire feeder going from the disconnect to the house panel?

I assume you mean a 20 amp 120/240 volt circuit to the sub panel.

[Ashokan1]

This subpanel is located on a post on the side of a field where a small RV is parked. It is about 100' from the house where the main service panel is located.

[suemarkp]

Yes, all detached panels require a ground electrode. And code would require two for this panel, as much of a waste as it seems. The only out, which isn't a reliable one, is to say that the one ground rod you drive at the outside panel is supplemented by another electrode on the property. In order to use that rule, you'd need to have a conductor sized per table 250.66 going from the subpanel ground electrode to the main panel ground electrode. Your neutral is this conductor, bus it is #8 copper or larger (which is the smallest value in table 250.66)?

[Roger]

Take the panel off and convert to a weather proof switch box. Install a double pole snap switch. You didn't clarify but convert to 120/240 multiwire at the house panel if this isn't already what you have. A 20 amp multiwire branch circuit to that pole would not require any ground rods at all. The switch would be your required disconnect for denergizing both ungrounded conductors. You would then have the option of splitting the multiwire into two 20 amp 120 volt branch circuits from the switch box. The switch must be rated 15 amps if one branch circuit is supplied from the pole or 30 amps if two are supplied.

[junkcollector]

Take the panel off and convert to a weather proof switch box. Install a double pole snap switch. You didn't clarify but convert to 120/240 multiwire at the house panel if this isn't already what you have. A 20 amp multiwire branch circuit to that pole would not require any ground rods at all. The switch would be your required disconnect for denergizing both ungrounded conductors. You would then have the option of splitting the multiwire into two 20 amp 120 volt branch circuits from the switch box. The switch must be rated 15 amps if one branch circuit is supplied from the pole or 30 amps if two are supplied.

It's a good idea, the problem is that he says that there are only 3 wires going out to the pole (hot, hot, and nuetral) You would still need a 4th ground wire to the pole, ground rods or not. (feeder or mulitwire)

There was no existing fourth wire run from the main panel in the house

Ashokan, which version of the electrical code are you under?

[Roger]

Thanks Junk, I didn't read the 4th wire not being there. He says only one branch circuit to a trailer so he could convert to 120 volts by re-identifying a hot for egc. I am assuming here he is smaller than #6 with what he has in the ground now. Technically he would likely need his inspectors approval to re-identify smaller than #6 but it is an option if only one 20 amp branch circuit is needed. My personal opinion is that a ground rod or two likely isn't going to add much protection if you don't know the resistance. Under the NEC if the 1st ground rod had 100 ohms resistance then all I have to do is add a second and I'm good to go and I may still have 100 ohms of resistance. So I wonder what the point is...


Maybe we could wish for metal conduit....:smile2:

[junkcollector]

He says only one branch circuit to a trailer so he could convert to 120 volts by re-identifying a hot for egc. I am assuming here he is smaller than #6 with what he has in the ground now. Technically he would likely need his inspectors approval to re-identify smaller than #6 but it is an option if only one 20 amp branch circuit is needed. My personal opinion is that a ground rod or two likely isn't going to add much protection if you don't know the resistance. Under the NEC if the 1st ground rod had 100 ohms resistance then all I have to do is add a second and I'm good to go and I may still have 100 ohms of resistance. So I wonder what the point is...

Well said, Roger, I couldn't agree more.

[Ashokan1]

In NY we are under the 2005 NEC and the 2007 NYS building code for residential.

The homeowners idea was to have circuit breakers at the outside location for convenience. The subpanel is fed with three old #10 direct buriel single strand wires. I have 2x 20A circuit breakers in the subpanel and it is protected in the house by a 20A 2-pole.

It's an old farm I'm just trying to straighten it up a bit to make it more safe. We spent today putting open-air splices into j-boxes w/covers in the basement. It's a real old mess. Unfortunately, that's the condition of a lot of the old farmhouses in NY state. Old wiring with bad workmanship over the years.

[junkcollector]

In NY we are under the 2005 NEC...

2005 NEC does allow 3 conductor feeder, but you need a grounding electode system. So you have two options either do that or downgrade it to a single 120 volt circuit.. In my opinion you could keep the 20 amp breaker in the small subpanel. It can be argued that having a 2 redundant breakers of the same size like that is still a branch circuit. (Others, including the inspector, may disagree)

Is there another circuit in this panel too? You mention the one, and it is a two space loadcenter. I was just wondering if 240 is even needed out at the pole.

[Roger]

For what it is worth I'm not sure how you make it better/safer with old direct bury wire still in the ground. I'm not even sure what that wire could be if it is individual wires sized #10. As far as safety is concerned installing a ground rod at that pole improves nothing as far as human safety. The ges is intended to protect property and not humans.
My opinion now that things are more clear, if your after safety determine how much power you need out at that pole and whether 120/240 is needed or just 120 volts as Junk asked. Abandon the present 3 wire and lay new 4 wire cable or pvc conduit with individual wires with an equipment ground (4 wires)i f 120/240 is needed then you have improved safety IMO. Configure this for a sub-panel with isolated neutral and equipment ground and ground rods. Or configure as a multiwire and switch disconnect.. no ground rods if kept at 20 amps circuit breaker..
Or just run a new 120 volt uf-b or other method out to the pole and don't install the ground rods.

I suspect that cost has something to do with what your options are but only adding ground rods is not going to make that installation that is existing safer.

[Ashokan1]

An elderly woman who lives alone runs this small farm. Not much money at all so cost is definitely a factor. A total redo is out of the question for her.

Maybe I'm not understanding something or explaining this properly. If I ground the subpanel with a grounding rod or two, how is this not making it safer? The subpanel feeds two GFI weathertight outlets on the backing board each with it's own 20A circuit breaker.

One will be used to power a small RV trailer parked next to the subpanel. The other is for occasional use in that area of the farm. Isn't grounding the panel providing a grounding conductor for all grounds and case grounds in the system after the subpanel? The 15A male plug feeding the trailer is a grounded three-prong. Isn't this grounding of a subpanel not only safe but required?

[suemarkp]

As Roger indicated, ground rods are more for equipment safety and not human safety. You may never need the rods, but code does require them unless you happened to run a branch circuit there instead of a feeder. How lightning knows the difference I don't know. You could also perhaps do one rod and hope it is 25 ohms to the earth. Then, you don't have to drive the second one (some material cost savings and perhaps a lot of labor savings).

[Roger]

Isn't grounding the panel providing a grounding conductor for all grounds and case grounds in the system after the subpanel?

No it is not. You are confusing the equipment ground with the grounding electrode system. Two different purposes. Grounding the sub-panel to earth has nothing to do with the equipment grounds of the branch circuits originating in the sub-panel. Fault currents that are generated from a ground fault in a branch circuit travel what is called the "effective ground fault path". They do not travel to earth via the grounding electrode conductors. Fault currents travel the "effective ground fault path" back to the source. The source being the transformer. Fault currents use this path because it is by far the lowest impedance path that returns you to the source. All other paths such as the grounding electrode conductors are high impedance/resistance because in order for the fault current to get back to the source it would have to travel into the dirt then travel through the dirt to the transformer ground rod then grounding conductor to Xo or center tap of the transformer. Remember current seeks it's source not earth....but it might use earth to get there if no other low impedance path is available to it.

Study the attached diagrams and commentary and I think you will see what is going on with the equipment ground fault path vs. grounding to earth for protection of property from events like lightning. The equipment ground is for human safety without it the overcurrent protection device cannot open to clear a fault.

[Ashokan1]

Now it makes a lot more sense, you're right I was confusing the two. Now I see they have separate and distinct purposes.

Thanks Roger and everyone for your patience and help.

[scuba_dave]

Learn something new everyday
Thanks for the pics & explanation

[Roger]

Remember residential single phase voltages are relatively low when compared to other voltages like lightning and the utility primary voltages. 240 and 120 volts will not push large amperages to earth nearly the way lightning will. Lightning has no problem with the resistance/impedance of dirt....:smile2:
For example lets say we have a fault on a 120 volt branch circuit however the the service neutral is open breaking the effective low impedance fault path. The fault current upon reaching the neutral bar at the main panel no longer can utilize the service neutral to get back to the source... being... Xo of the center tapped transformer. It now must look for another path to get back to the source, that being the grounding conductors to the electrodes then through the earth to the transformer ground rod. Now if we use the NEC's bench mark of 25 ohms and ohm's law we would have 120/25 = 4.8 amps not nearly enough amperage flow to trip a breaker on a 15 amp branch circuit.

It is also important to note that current will seek all paths made available to it but it will disproportionately flow on the lowest impedance path to get to it's source. At the main panel (service equipment) bonding takes place between the grounding electrodes, a metal water pipe, branch circuit equipment grounds and the service neutral. With the service neutral far outweighing the others as to low impedance back to the source from that point in the effective fault path. So nearly all current whether fault currents or return currents on the grounded legs (white wires) takes this service neutral path back to the source. Take the service neutral and open it... now all current seeks alternate paths...that being the grounding conductors to the earth. Problem is if only about 0 to 10 amps can flow on those paths due to resistances/impedances of dirt... circuit breakers cannot trip and clear faults. This results in all metal in the bonded paths back to the source coming to line voltage as the breaker remains closed even though a ground fault has occurred.

[Ashokan1]

So your point is that it isn't any safer for us humans as it will not trip the circuit breaker but that low of amperage passing through us to a ground is potentially fatal.

[Roger]

Yes my point exactly