electrical panel disconnects NEC 2005

NEC requires an electrical disconnect at the service entrance to an outbuilding when fed from a main located elsewhere on the property. In addition to the two hots, the subpanel in the separate structure need to have the neutral and the ground also connected to the main panel (in most areas of the US this means whether or not you have a separate grounding point for the subpanel).

Now here's my question. In the newest NEC book from 2005 there is some screwy language about being able to disconnect all of the wiring at the mechanical disconnect.

What's up with this portion? Do I need a better translator? I checked my reading glasses.

Would you like to cite the specific article you have in mind, or keep us guessing? :-)

Ed

"neutral and ground" Just what are you referring to? A "ground" is defined in the NEC as a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some conducting body that serves in place of the earth.. Your terms are so vague that your question cannot be answered.

Not really. Feeding a sub-panel from a main panel, you must isolate the 'ground' bus (the one for all the "equipment grounding conductors") from the neutral bus in the sub-panel. So for a residential setup, that means four wires need to be run from the main panel to the sub-panel, two hots, a neutral wire from the main panel's neutral bus the the sub-panel's neutral bus, and a 'ground wire' from the main panel's ground bus to the sub-panel's ground bus to connect all the equipment grounding conductors in the sub-panel to the main-panel.

In older versions of the NEC it was acceptable to have a disconnect that 'disconnected' the two hots and the neutral (i.e. a three-pole switch) and not 'disconnect' the ground connection between the two panel's ground busses. So even when the disconnect was open, all the EGC in the sub-panel circuits were still connected to the ground bus in the main panel and its earth ground.

The OP seems to have read some language in the 2005 NEC that suggests to him that *all* conductors between the main panel and the sub-panel be 'disconnected' by any disconnect device. Opening the conductor between the two 'ground' busses would mean that all the 'equipment-grounding conductors' fed from the sub-panel would be open-circuit. Seems a tad over-kill, but since there is no other power in the outbuilding, it's not particularly dangerous. (but now you would need a four-pole disconnect switch).

To rephrase his question, "Does the 2005 NEC require that the equipment grounding conductors in a sub-panel in an out-building be separated from the main panel's equipment grounding conductors with the aforementioned required disconnect?" If so, why?

daestrom

There are still circumstances where you can run a 3 wire feeder and reground the neutral in a sub panel in a separate building (no other metalic paths between buildings). That language is scheduled to go away in 2008.

But what about the OP's question? When a four wire feeder is dictated, does the disconnect now have to isolate the ground as well as two hots and neutral? Seems like a 'bad idea', especially if there is some other metalic path. Keeping the EGC / bonding in tact as much as possible would be preferable wouldn't it?

daestrom

Yup I agree

This is not new language, it first showed up in the 1999 but the original propoasl was from the 1995 ROP and I don't have that handy to see the thinking.. Bear in mind 2 things. It is not binding on a residential shed or garage and the means to disconnect the groundED conductor can be the neutral bus bar in the disconnect enclosure This does not affect the groundING conductor at all.

225.38(C) Disconnection of Grounded Conductor. Where the building or structure disconnecting means does not disconnect the grounded conductor from the grounded conductors in the building or structure wiring, other means shall be provided for this purpose at the location of disconnecting means. A terminal or bus to which all grounded conductors can be attached by means of pressure connectors shall be permitted for this purpose.

So, if I understand this, the neutral (groundED conductor) doesn't have to go through a disconnect switch contact. The 'disconnect' switch can simply switch the two 'hots'? The neutral (groundED conductor) can be 'provided' with 'other means' by simply having the "goes in" and "goes out" terminate in a bus with screw connections? A 'straight-thru' neutral (groundED conductor) would *not* be allowed as there is no way to disconnect it with "means of pressure connectors".

Is the same true for the 'ground wire' (i.e. the groundING conductor)? It doesn't have to be a pole on the disconnect switch, but can merely have a "terminal or bus to which [groundING] conductors can be attached by means of pressure connectors...."?? Just no 'straight-thru' conductors? This would make sense.

If so, then a two pole disconnect switch with two other 'terminal or buses with pressure connectors' would be acceptable? The two poles for the UNgroundED conductors, one 'terminal or bus' for the groundED conductor, and another (isolated from the first) for the groundING conductor.

Thanks, some people learn something new every day...

daestrom

I popped this up on some inspector boards and the answer is they want a place to break the neutral to test for ground faults. This language is also in the "service" article 230. It is not meant as a user disconnect.

If the system uses conduit, "ET", or "flex", there may be no [groundING] conductor in the form of a wire. The conduit or flex *is* the grounding device.

The grounding extends through the entire system. The point of disconnecting the neutral it to be able to check for unintended grounding of this conductor.