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3 vs 4 wire panel to sub panel with load.

I added a drawing. of both wiring configurations. The only advantage I can see is the 4 wire gives you a dedicated ground fault circuit vs sharing the neutral wire. Open neutral between the two panels would make the ground hot in the 3 wire.

Open neutral between the main panel and transformer would make the ground hot in the 4 wire.

I am trying to understand how this works and why the code changed. In this case you have a detached shop 250 feet away with no plumbing or other connections to the house with the main panel.

The current NEC code requires a 4-wire feeder so there is a separate grounding connector between the Main and Sub panels. The neutral and ground are not bonded in the subpanel. In this setup if a hot wire coming in contact with the non-current carrying parts of the electrical system, (outlet covers, panel covers etc), the 4th ground conductor will provide a low resistance patch back to the source tripping a breaker. This 4th grounding connector ties the ground rod at the sub panel to the ground rod at the Main panel making a grounding system.

How were the sub panels wired when 3 wires were allowed? Was the neutral and ground bonded in the Sub panel so the neutral wire served as the grounding connector? Was this code changed for the same reason 220 volt plugs went from 3 to 4 wire?

  • it's more likely that they never had a ground and the installations were grandfathered in. – ratchet freak Apr 25 at 11:12
  • I am just trying to understand how it works. – grantr Apr 25 at 15:54
  • Issues with a 3-wire panel: Bonded ground and neutral in the sub panel Open neutral= Would cause full voltage to energize the panel chassis and everything connected to the ground system. Ground rod would provide too much resistance to trip a breaker. Separated ground and neutral: Ground fault - hot wire touches chassis: Would cause full voltage to energize the panel chassis and everything connected to the ground system. Ground rod would provide too much resistance to trip a breaker. – grantr Apr 25 at 15:55
  • So, in the main panel if the neutral feed wire becomes disconnected, would this open neutral also cause full voltage to energize the panel chassis and everything connected to the ground system. Ground rod would provide too much resistance to trip a breaker – grantr Apr 25 at 15:55
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Back in the old days, grounds didn't exist at all. Mains electrical was wired as an isolated system. It still is, but now we've added ground as a safety shield.

When they started to require grounding, they went to pains to make sure they weren't mandating old work be retrofitted. Old work is "grandfathered" - if it was legal at time of installation, it is still legal today. However grandfathered work does not have the safety features now required; because of that many upgrade it voluntarily.

In some cases: dryers, ranges and subpanels, they allowed groundless connection, but attached ground to the neutral wire "since they're the same back at the panel". That proved to have problems, as you mention in comments. Notably a simple wire break of the neutral wire can result in all things which are supposed to be grounded now being electrified.

Now, in a subpanel, it is normal for the load not to be balanced, and for there to be say, 20A of flow on the neutral wire. What happens when that neutral breaks? Now the current is trying to return via the grounding rods. And at 25 ohms, which is the max resistance allowed for a single ground rod to be acceptable, that doesn't have a chance of safely earthing that much current.

  • So if the main panel were to lose the neutral connection back to the transformer, its grounded parts would be energized as well? – grantr Apr 25 at 17:06
  • If it happens before the neutral -ground bond, it won't electrify grounds, but creates two different problems. – Harper Apr 25 at 17:32
  • So if the neutral wire comes out of its connector in the main panel or at the transformer, the panel housing and all other grounded parts of the system would not become hot? – grantr Apr 25 at 17:49

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