Existing 3 wire service feed to sub panel: bonding grounded neutral and ground bars?

Question

I know variations of this question have been asked (the closest I could find is here), but I'm still confused.

@Tester101 explained if the existing feed to the subpanel is 3 wire with a grounded neutral and two hots and is not electrically connected to your main panel, your sub should have a bonded ground/neutral.

Is this an acceptable way to ground the sub panel? My sub panel is not at all connected to the main except by the 3 service conductors and has the neutral and ground bus separate. The ground bus is grounded to the earth (2 rods), gas, and water. Now I've read that this is very dangerous as the earth is not a low enough resistance for grounding (makes sense).

I read the code that Tester101 linked to in the post above but didn't see where it said to bond the ground to neutral in the sub

I guess what I'm most interested in is a good explanation about why it would be okay to bond the neutral and ground within the sub panel in this type of situation when every other time it's a big no no.

I live in Oregon for the record.

Answer 1

The three wire allowance is an exception to 250.32(B)(1), which allows existing installations that were compliant with previous code versions to exist. If you want to bring the installation up to current standards, you can install a properly sized grounding conductor between the panels and separate the neutral and grounding bars in the second panel.

The reason you need to bond the neutral bar in the case of a three wire feeder, is to provide an effective ground-fault current path. Remember, electricity is trying to return to the source, not to the ground (earth). In the event of a ground-fault, you want the fault current to be able to have a low resistance path back to the source. This low resistance path should allow the fault current to be great enough, so that a circuit breaker (or other protective device) can activate and open the circuit.

If you read the text of the code, you'll find that it says "the grounded conductor run with the supply to the building or structure shall be permitted to serve as the ground-fault return path". By bonding the grounded (neutral) and grounding bars in the panel, you're using the grounded (neutral) conductor as a ground-fault return path.

Hope this makes sense, and answers all your questions.

Answer 2

You can have the best of both worlds.

You are allowed to retrofit a separate equipment ground wire. Just run a bare copper wire from the sub-panel grounding system to the main panel ground. Use any practical route you please, there's no requirement that it run with the power cable. (Because it's a ground, and does not carry current except in fault conditions, and then, not for very long.)

Also don't ground to gas lines.

Answer 3

Your main panel is probably connected to your kitchen, which probably uses gas and water. In that case, there's probably a ground connection between the main panel ground system and your gas and water pipes.

The same pipes go to your other structure, the one with the sub panel you enquire about. Those pipes might be connected to the ground system there, creating a ground connection, through the gas/water pipe, between the detached structure and the main panel.

If that's the case, then you're out of the code, and you need a 4 wire feeder. Either upgrade the current feeder or add a separate ground wire. And unbound neutral from ground at the sub panel.

Why? Consider the following: you have a 3 wire feed, bonded N-G at sub panel, and a water pipe connecting 2 structures. The neutral wire somehow gets severed. Current would flow from the hot to the load to the sub panel neutral bar, then jump to the sub panel ground bar, then to the water pipe, then to the main house, to the main panel ground bar, to the neutral bar, to POCO.

You wouldn't notice the defective wiring, you would overload the grounding system, and energize all metal boxes and pipes.

Now, why add a real copper wire and not rely on the water pipe to act as a copper grounding wire? Corrosion, dual purpose, future repairs and adding a union, ... The next person working on the water line will not know if serves an electric purpose.

Also remember to add a grounding electrode near the sub panel. That's for lightning and such. Completely independent of the grounding system.

Answer 4

The only time the neutral and grounding conductors are connected together is at the stand alone main disconnect or the main panel. Unless in mostly commercial installations where a transformer is installed ( separately derived electrical system) which feeds a main disconnect, gets grounded properly, like any other main disconnect or panel. Any panel installed in residential, commercial, and industrial that is not the main panel it is considered a sub panel and sub panels feed other sub panels lots of them, in all sub panels the neutral conductor ( current carrying conductor, and the grounding conductor ( non current carrying conductor) are isolated from each other no exceptions. This means a separate grounding conductor starts at the main disconnect or main panel and terminates to the grounding bar at the sub panel.