Temporary generator feeding a house

[jmjr55]

We were talking at work the other day about a temporary hook up of a small generator (5kw 2 hots, neutral and a ground) to a person?s home panel. Clearly one needs to disconnect the ungrounded hots all agreed, but we did not agree on disconnecting the utility neutral. Dose anyone have a comment on a generator that is tied into a house panel without disconnecting the utility neutral? That would leave the utility neutral and the generator neutral tied together. One of my colleagues felt this would create a back feed, I felt it should be fine to have them tied together. All current will return to its source, this case being the generator. Thanks for your feed back.

 

[charlie b]

There is no path for current to flow along the utility's neutral conductor. For that reason, keeping the utility neutral connected would make no difference to the operation of the generator, to the operation of the utility source, or to the safety of the installation. I don't happen to know if the code would allow it, and I haven't the time to look it up just now.

 

[tom baker]

Current will return to its source which is the generator. There will be no backfeed.
If you are connecting a generator without a transfer switch, the rules for that are in 702.6
The neutral provides the connection to the buildings GES, see definition of Sep Derived System in art 100. For a connection where the transfer switch does not switch the neutral, a system bonding jumper is installed at the building and none at the generator.
If you disconnect the neutral or transfer it, then you must have a system bonding jumper in the generator to connect the generator windings to the frame - this is clear a fault (IE get the fault current from the generator case into the windings so the OCPD will clear).
Also read in Art 250 sections 30, and 34.
Are you confused? Most are. We'll see some new rules and information on generators in the 08 NEC which will help.
The #1 rule for any generator is to determine whats going on with the neutral.
That determines if the generator needs a SBJ and grounding electrodes.

 

[haskindm]

Unless it is specified to be a "separately derived system" the system neutrals and generator neutrals of even permanently installed generators are tied together. Your installation would be no different. How do you intend to disconnect the ungrounded conductors? If you just disconnect the conductors from the panel they will be hot when utility power is restored and they need to be reconnected. A transfer switch of some sort is needed to do this properly and safely, even with a portable generator.