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.
Best Answer
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.