Understanding the need for a floating neutral in a sub-panel.

After an hour of reading online, the consensus seems to be that any stray current finding its way onto the service enclosure will not flow strongly because the service is connected to a grounding electrode buried in the earth which offers lots of resistance to current flow.
A sub-panel with no grounding electrode and a grounding conductor that connects to the service panel grounding bus bar offers a parallel path (parallel with the neutral conductor) for any current getting onto the sub-panel enclosure, increasing the chances of strong current flow and electrical shock to anyone touching the sub-panel.

But, the sub-panel is connected to the same grounding electrode as the service panel. The only difference is that the grounding conductor connecting the sub-panel with the grounding electrode is usually a lot longer and of smaller gauge that the service GEC, and it’s routed through the service panel grounding bus bar.

To use the water hose example that gets used so often, if you connect two hoses, one 10 feet long and one 1000 feet long to two hose bibs, turn them both on, and then plug the hose discharge ends, that water is not going to flow, it doesn’t matter how long the hose is.

What’s the purpose of the floating neutral in a sub-panel?

To keep the current returning on the neutral from flowing through the exposed metal enclosure and pipes.

How is the sub-panel enclosure any more exposed than the service panel? Both enclosures are grounded to the same grounding electrode.

This might help:


There’s no grounding system in that diagram, and the neutral feed is disconnected in the sub-panel. I guess if the sub were not bonded, it would be more difficult for current to get onto the sub-panel, so yeah, that helps. Now if both panels were properly grounded…

Two reasons the grounded conductors (neutrals) and the equipment grounds must always be isolated after the main service panel are:

a) To provide a low-resistance path for current travel to the transformer in the event of a ground fault. This path should not be energized to ensure that it is fully available for the fault current in the event of a malfunction.

b) To *prevent the ground wires from conducting returning neutral current *during normal operation. Neutral current in the equipment ground wires can energize the casings and enclosures of appliances and equipment. (To prevent enclosure from conducting current).

If the grounded conductors (neutrals) and the grounding conductors are bonded at a Sub-panel you have current traveling on the enclosure and if the grounded (neutral) conductor comes loose….the enclosure and the grounding conductor (equipment grounding conductor) will carry the current.

They are properly grounded by the metallic raceway connecting them. The use of the EMT as an EGC would be code compliant.

I swear, the way this stuff goes over my head sometimes I feel like Forrest Gump.
If a ground fault- like an ungrounded conductor coming into contact with the sub-panel enclosure- were to energize that enclosure, the breaker protecting that circuit would trip or the fuse blow. Why would a grounded (neutral) conductor not in contact with the enclosure help return current from the energized sub-panel enclosure to the transformer?

So the current would flow with a bonding strap connected, but without the bonding strap, the fault would trip the breaker, problem solved.

The neutral is the groundED conductor. The green or bare is the groundING.


I answered a similar question in another thread and this was that post

It helps if one understands why we have grounding electrodes for in the first place.
In the 2011 NEC we have eight grounding electrodes and are instructed to bond any and all that are present together to form the grounding electrode system. They are 1) 10 foot of metal water pipe, 2) Steel of a building that is effectively grounded (connected to earth) 3) rebar in the bottom of the footer, 4) ground ring that circles the building, 5) rod or pipe, 6) chemical electrode, 7) plate electrode, 8) oil drum buried in the back yard.

If the electrode system is a metal water pipe, single rod, pipe, or plate electrode then it is required to be bonded to another electrode as outlined above.

The electrode system serves four purposes and these four purposes only. They are installed to help dissipate lightning, line surges, unintentional contact with higher voltage lines, and to stabilize during normal operation.

In most of Florida ground rods are not used due to the water table. Driving an 8 foot ground rod can cause water to ooze out to the earth. Orlando is only 8 feet above sea level so think about knocking a hole in the bottom of Florida should one drive a rod eight feet down and sinking Florida to the bottom of the sea.

I live in the center of NC and am 689 feet above sea level so driving an eight foot rod into the earth will not knock a hole in the bottom of the most beautiful place God ever created and it will not sink to the bottom of the sea.

To say one electrode is any better than any other electrode is nothing but a wives tale. Remember why we are installing an electrode for, it has nothing to do with how our system works but is there only to dissipate unwanted current to protect our systems from high voltages generated from an outside source.

Should we drive a rod and connect it to a 15 amp breaker the breaker will not trip. 120 volts divided by 25 ohms will equate to 4.8 amps but should the primary of the transformer touch the service drop then it would be 7200 volts divided by 25 ohms that would equate to 288 amps and clear the fuse on the primary side of the transformer. Lightning is a very high voltage that is seeking a connection to earth therefore an electrode is needed. Lightning 30 miles down the road can cause a surge on our homes and again an electrode is needed. Look at your switch cover, see those two 6/32 by ½ inch screws? We connect to earth to keep those screws stable during normal operation.

The grounding electrodes have no role in the normal operation of our electrical systems. We wouldn’t know if there was one or not in everyday use of our electrical systems. The equipment grounding conductors of our electrical systems are bonded to the grounded neutral conductor at our service equipment in order to establish a low impedance path for clearing fault currents in our systems. This fault clearing path will work without a grounding electrode system of any kind. The grounding electrode system plays no role in the fault clearing of our systems.

The grounding electrode system is installed for the reasons outlined below as mentioned above;
250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by 1) lightning, 2) line surges, or 3) unintentional contact with higher-voltage lines and that will 4) stabilize the voltage to earth during normal operation. (numbering was added by me)

The grounding electrode plays no part in clearing a fault. Bonding of the EGC to the service neutral is what clears faults.
No electrode is needed for this to happen.

Obviously a grounded conductor cannot clear fault current from a sub-panel to which it is not electrically connected because it has a floating neutral, so how is the sub-panel protected from fault current?

The grounding system has nothing to do with fault current? So I can connect an ungrounded conductor from a circuit breaker directly to a grounding electrode (routing it through a metal enclosure along the way, and the breaker won’t trip?

The enclosure is grounded, meaning that somewhere in the system it is connected to the service neutral. That will provide a low impedance path for the fault current to flow and open the OCPD ahead of the subpanel feeder. The connection of the enclosure to the service neutral is provided by the EGC run with the feeder. The EGC may be a wire or can be a metallic raceway.

I thought the whole point of the floating neutral is what happens if the EGC becomes disconnected (diagram in post #4. In post #12, Joseph says that grounding plays no part in clearing a fault. If the EGC becomes disconnected and the grounding and grounded conductors are connected (which they are in the main service panel, then the grounding conductor becomes the low-impedance path back to the service panel where current can then flow back onto the Neutral bus bar and continue to the pole.

This means electrons are flowing through the ground and sub-panel.and someone can get shocked.

The sub-panel enclosure is usually grounded to the service neutral by the electrical connection to the metal enclosure of the service panel which is bonded to both the grounding and neutral bus bars.

That’s unless there is an isolated main disconnect in a separate panel, in which case the neutral would be bonded to the enclosure with the disconnect and the main distribution panel would also have a floating neutral. I’m pretty sure I understand how it’s supposed to be configured. I just don’t understand why.

Current returns through the neutral conductors and bus bars whenever a circuit is closed and electrons flow. For some reason, we want to keep that current off the sub-panel, so we isolate the neutral by floating the bus bar in all sub-panels. If the neutral bus bar is bonded at the service panel,

  1. Why is it OK for electrons to flow through the service panel and not through sub-panels?
  2. Why don’t we get shocked when we touch a service panel if electrons are flowing thrrough the panel?
  3. If the GEC becomes disconnected at the sub-panel, what prevents anyone touching it from getting shocked?
  4. How does ground fault fit into any of this?

I think that you need to look at this is terms of difference in potential. The neutral and EGC/metal enclosures are all connected together at some point, typically the main bonding jumper, so the difference in potential between them is theoretically zero volts. If you touch the neutral and the metal enclosure nothing will happen since there is no current flow.

  1. Under normal circumstances they don’t in the sub-panel either.

  2. I mean the conductor that connects the neutral bus bars in the service and sub-panels.

Robert, I appreciate your efforts, but I am totally confused. I don’t understand this at all.
I see no difference between a service panel and sub-panel that are a foot apart, except that in one, it’s OK to bond the neutral and ground and in the other it’s not.

As far as looking at it from the viewpoint of potential? I don’t understand the concept of potential. I’ve read a dozen explanations, and I still don’t even know whether- used in describing potential- ground is a verb or a noun.

The isolated neutral problem seems to have something to do with current accidently getting onto the enclosure and from there onto the grounding system, but both sub and service panels are connected to the same grounding electrode. Why does having a disconnect in it make the service panel any different from the sub-panel as far as what happens when current accidentally gets onto the enclosure?
If current gets onto the enclosure, someone is going to get shocked unless… unless what?

I remember going through this 10 years ago and it doesn’t make any more sense to me now that it did then.

Kenton lets say the second panel is two hundred feet away and you have it fastened to ground at the far end.
Now we can have what is some times called eddy currents ( stray voltage ).
These can effect many things including decay of water pipes fencing Under ground oil lines .
Here is an example where it effected cattle.
In Canada you can only have one ground at the main panel .
For safety reasons to take neutral and Ground wire as well as the feeders to all other panels .