We have a 5000 watt Coleman generator. When the house was built, the electrician placed a 100 amp doublepole breaker at the base of our panel and from that ran wires to a subpanel that powers the circuits which can be fed by the generator. There is a three position transfer switch (positions are: electrical utility(normal)/off/generator). All the neutral wires are screwed to a bar in this transfer box(it appears that only the hot wires are switched). The ground wires are all screwed to a separate grounding lug in the transfer box. So much for the background info. Now here's the basis of my 4 questions: In the garage, the electrician installed a twist-lock plug that has separate neutral and ground(4 prongs). The generator outlet has only 3 connectors. It is 220v and looks somewhat like a duplex outlet. 1: I was told to just connect the neutral and ground wires in my cable that goes from the generator to the house together at the ground pin on the plug that inserts into the generator. Is this correct? 2: The generator also has an internal wire that bonds the neutral to the case/frame of the generator. Should that be disconnected so that the neutral is no longer bonded to the case. 3: Do I need to hook this the external grounding lug of the generator to a ground rod if the neutral is no longer bonded to the case? 4: If the neutral is no longer bonded to the case and I want to run a tool appliance directly by plugging into the 110v oulet on the generator, do I have to do anything special? Thanks very much, Dave
Portable generators that have the winding center-tap (neutral) bonded to the frame by the manufacturer are primarily intended for use where not connected to a building wiring system.
Generators that are designed to be permanently connected to a building wiring system through a transfer switch are usually manufactured with the neutral isolated from the frame (floating), as shown in Diagram 1.
Note - These diagrams do not represent actual installations, and are intended to illustrate basic principles only.
Because the grounded (neutral) conductor is not switched in the transfer switch, no other grounding electrode connection is required at the generator.
If a generator that has the winding center-tap (neutral) bonded to the frame is used with a transfer switch, as in Diagram 2, the switch should break the grounded (neutral) conductor, which will require the generator neutral to be grounded to an acceptable grounding electrode.
[This message has been edited by electric-ed (edited 11-21-2002).]
What could conceivably happen if the generator does not have the floating ground (neutral is bonded to frame) and the neutral is not switched at the transfer switch?
When on utility power the neutral is in parallel with the equipment bonding conductors (green). The unbalanced load current, which should be flowing in the neutral only, will divide, and some will flow in the equipment bonding conductors.
sorry for not understanding, but could this potentially feed back into the line and shock someone working on the downed power lines? Wouldn't it preferentially go to the ground circuit/ground rod for the house?
Re: generator wiring woes#17170 11/22/0209:52 AM11/22/0209:52 AM
could this potentially feed back into the line and shock someone working on the downed power lines?
No, the transfer switch will prevent this. In Diagram 3 the generator is not connected and would not be operating. The problem is that the EGCs (green) are not sized for, and therefore not permitted to carry, the unbalanced load current.
Wouldn't it preferentially go to the ground circuit/ground rod for the house?
No, this current should flow back to it's source, the utility transformer winding, by way of the grounded (neutral) conductor.
[This message has been edited by electric-ed (edited 11-22-2002).]
Re: generator wiring woes#17171 11/22/0210:19 AM11/22/0210:19 AM
How would I rectify this situation with my current setup? The grounding conductor from the twistlock oulet in the garage to the transfer switch is 6 gauge. My electrician told me that technically everything should work fine, but the ground connection in the generator (bonding wire or generator outlet) may be undersized. The wiring from the generator back to the panel is oversized for the load (possibly 20 amps max).