Isolating ground and neutral in main panel after installing generator transfer switch

[I start fires]

So I have a customer with a residential 200A 120/240v single phase service. Panel has a 200A main breaker. This is the only service disconnect after the meter which is quite typical. Also as is expected, all branch circuit grounds and neutrals originate from a common bus within the panel. The customer wants to install a generator. One option would be to connect the generator by backfeeding through a breaker and installing one of those lockout devices that forces you to shut off the main disconnect before energizing the generator breaker. However if the customer wants an automatic backup generator I believe I would need to install a new 200A disconnect right after the meter connected to an automatic 200A transfer switch and then connect to the existing main panel. By code after the 1st disconnect, the neutral and ground must be permanently separated. I guess this requires me to install a separate ground bus in the main panel (which is basically impossible because of the lack of available space.) Also, there is no way the existing branch circuit neutral and ground wires are long enough to rearrange to complete the isolation. Bottom line, I don't see how isolating the ground and neutral is even remotely feasible in an existing panel with 40 or so breakers. I know that many homes have these auto-standby generators installed but I can't imagine the installer is going to reconfigure any ground or neutral conductors in an existing rat's nest panel. Am I correct? Are all of these transfer switch generator installs resulting in a code violation? Is there some type of bonding exemption for retroactively installing a transfer switch or am I missing something?

 

[Little Bill]

First, if you need a 200A ATS (auto transfer switch) then get one that is "service rated". That way you don't need an additional disconnect because it would be the disconnect.

Second, if you install an ATS or disconnect, you will need to separate the EGCs and neutrals in the panel as it would now be a subpanel. No exemption for retrofitting. You may have to splice on some tails for the EGCS and neutrals but that is acceptable. You would need to make room for adding a ground bar.

 

[LarryFine]

. . . I can't imagine the installer is going to reconfigure any ground or neutral conductors in an existing rat's nest panel.

That's exactly what must be done . . . and that's exactly what we do . . . every time. In addition, the grounding electrode connections (water, rods, etc.) must be moved to the new disconnect. The existing panel is now a sub-panel, and must be wired as such . . . every time.

 

[jortego128]

That's exactly what must be done . . . and that's exactly what we do . . . every time. In addition, the grounding electrode connections (water, rods, etc.) must be moved to the new disconnect. The existing panel is now a sub-panel, and must be wired as such . . . every time.

Larry, I have a similar situation where I am replacing an existing 100A(!) panel with a new 200A panel and also installing a manual service disconnect/transfer switch between the meter and the panel.

I understand you are saying that the grounded service conductor (neutral) must be bonded to the case in the manual transfer switch, and that a separate EGC must be run with the service conductors leaving the transfer switch and feeding the 200A main breaker panel in the dwelling, and that panel must have a separate ground bar installed and the bonding screw must not be installed.

Now, note that the manual transfer switch does NOT break the neutral and thus is not considered a separately derived system. My question is how then should I interpret the following:

Generator – If the grounded (neutral) conductor in a transfer switch is not opened,
then the grounded (neutral) from the generator will be solidly connected to the utility’s
service grounded (neutral) conductor. Under this condition, the generator is not a separately
derived system, and a neutral-to-case bond shall not be made at the generator or at the
generator disconnect [250.20(D) FPN 1].

If a neutral to case bond is prohibited per the above, would it be permitted then to simply install an EGC to the case of the transfer switch to the ground wire coming from the meter (which would protect the case in case of a conductor to case fault), and then simply bond the neutral to the case in the main breaker panel and thus eliminate the need for a separate ground bar?

Ive been wrestling with this for a bit, because at first it seemed obvious to (as you said) simply bond at the transfer switch and run an EGC along with the feeders into the panel, install a ground bar, and do not install the bonding screw in the panel. But reading above, it almost seems the bond could indeed be put in the panel as long as the transfer switch is not bonded (which seems to be prohibited, anyway?)

Thanks in advance for your help!

 

[LarryFine]

Generator – If the grounded (neutral) conductor in a transfer switch is not opened,
then the grounded (neutral) from the generator will be solidly connected to the utility’s
service grounded (neutral) conductor. Under this condition, the generator is not a separately
derived system, and a neutral-to-case bond shall not be made at the generator or at the
generator disconnect [250.20(D) FPN 1].

That is referring to the generator's disconnect, not the house's disconnect/transfer switch. The requirement that everything downstream of the main disconnect maintains neutral/EGC isolation is not changed.

Not switching the neutral means that one grounding electrode system can serve the house regardless of the power source. If the neutral is switched, each source requires its own grounding electrode system.

Added: In fact, you're also supposed to replace 3-wire major appliance circuits with 4-wire circuits, receptacles, etc.

 

[jortego128]

I forgot to add the source for that italicized reference-- I found it in here.

https://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/Neutral-to-GroundConnections~20020521.htm

 

[jortego128]

That is referring to the generator's disconnect, not the house's disconnect/transfer switch. The requirement that everything downstream of the main disconnect maintains neutral/EGC isolation is not changed.

Not switching the neutral means that one grounding electrode system can serve the house regardless of the power source. If the neutral is switched, each source requires its own grounding electrode system.

Added: In fact, you're also supposed to replace 3-wire major appliance circuits with 4-wire circuits, receptacles, etc.

This is the only way it made sense to me, but wasnt sure-- this is a transfer switch for a cord-and-plug connected portable generator, it is NOT switching the neutral. So then, the way to proceed is to bond at the switch, run 4th (ground) wire into panel, terminated to a stand-alone ground bar, leaving the panel neutral to ground unbonded.

Thanks!

 

[I start fires]

I don't see why a neutral to case bond for the generator can't be made at the generator's disconnect providing the transfer switch is the generator's only disconnect. This same "service rated" transfer switch is also the only disconnect from the meter and therefore must be the location of the one and only ground and neutral bond in the entire system. As long as the generator neutral/ground bond and main service neutral/ground bond are made in the same bus bar within the same transfer switch enclosure I can't see how this would be a violation. No objectionable neutral currents would be able to travel through any enclosures or other exterior paths. No need to rout the generator ground conductor through the transfer switch just to connect it to the main panel's isolated ground bar as far as I can see. What If the generator was connected to the transfer switch via an armored cable? That would end up bonding the neutral/ground of the generator at the transfer switch anyway. Does this make sense?