Would someone please explain the reason behind the rule to isolate the neutral from the ground in a subpanel? Thanks in advance.

 

This is done so that there are no neutral currents straying through the bonding system.

 

Please amplify. What would be this danger since the neutral and ground are common at the service panel.

 

The neutral and equipment grounding conductor are kept seperate so that you dont create a parrallel path for neutral current travel in the event your neutral would open. If you were to terminate the neutrals and grounds on the same bar or bonded bars, if the neutral would open, then the egc would begin carrying neutral current back to the main panel. The egc should never have current on it. Only in the event of a ground fault and then only breifly before the circuit breaker trips.

 

This can be a big subject, and more then one good reason.

Add this reason to the list.
Example:
Say your pulling 50 amps thru the black hot wire.
feeding your tools, the 50 amps returns thru the white Neutral wire to the sub panel.

If you have two wires (the same size) in parallel. say the neutral and ground tied together in the sub panel.
The current will be shared between two parallel wires the neutral and ground wire, running between the subpanel and main panel

With that, the neutral will have 25 amps running thru it and the ground will have 25 amps running thru it.
from the subpanel to main panel.

The ground will become part of the power line.

 

Roger: When you say "... if the neutral were to open....", are you referring to the neutral between the service panel and the sub panel? If so, then I see your point about parallel paths. However, it is hard to see really what safety compromise would result, since (in the case of a 100 amp sub panel for example) a #2 wire at the sub neutral goes to the service neutral, and a #2 wire goes from the sub ground to the service ground, where the neutral and ground are bonded together. Why does it matter which wire conducts the "return" current?

I'm truly not being argumentative, but I'm just trying to understand the reasoning behind this code.
(I presume that most of the code is based on safety issues.)

Thanks again.

 

There are lots of different scenarios, but let me give you just one:

• Suppose you run separate neutral and a grounding wires to the subpanel.
• Further suppose that you bond the neutral and grounding wires in the subpanel.
• Now suppose that one day, the neutral wire of the feeder breaks somehow. You won't notice this event at all because the grounding wire in the feeder will begin to carry all the neutral current. Years go by.
• Now years later, the grounding wire in the feeder also breaks. At this point, the hot current has no return path at all, and all of your appliances stop working.
• Suppose your refrigerator is powered off the subpanel. Like everything else powered from this subpanel, it's not working. However, something unusual is happening. Because no current is flowing, there is no voltage drop through the refrigerator. So there is 120 volts on the neutral wire coming from the refrigerator. If the grounding and neutrals were isolated, nothing bad would happen. But since we bonded them in the subpanel, that same 120 volts is transferred from the neutral to the grounding wire in the subpanel, so there is 120 volts on the grounding wire too. That 120 volts is fed back from the subpanel on the grounding wire to the refrigerator, and through the grounding wire in the refrigerator cord to the chassis of the refrigerator.
• Next person to want a snack is electrocuted.

 

Another scenario - I guess - is that because the wired "ground" is in fact just one possible path to ground, a person can become a partial path under some circumstances, even with the ground wires intact.

 

If the grounding conductor is carrying current, because of an inappropriate neutral to ground bond in a subpanel, the the grounding conductor is energized. If the grounding conductor is energized to the subpanel, then it is energized throughout the house because all grounds are connected together at some point in your distribution. If it is energized throughout the house, then your refrigerator door, etc will be energized.

 

There is one more reason. A voltage drop occurs between the main panel and the sub panel. This means that the hot is less than at the sub panel than at the main panel. This is not an issue if you size the wires properly to minimize the drop.

However, and more importantly the neutral voltage is higher at the sub panel than at the main panel. This means that if you bond the neutral and ground at the sub panel, then there will be voltage on the ground. Minimal to be sure, but voltage nonetheless.

 

Great explainations all! Thanks! Knowing the reasons behind the rules makes the picture complete.

 

They were great answers.