Many generators that have a bonded neutral to frame can be modified so the neutral is floating, or separate from the frame and generator ground. If the neutral connection (bond) to frame can be removed, if the manufacturer supports this, it’s usually the best solution as it would result in a “floating neutral” generator and would be ideal for connecting to most transfer switches.
If the generator manufacturer does not support modifying the generator to remove the bond between neutral and ground/frame, the system can be setup to meet all codes by using a neutral switching transfer switch. A “non-separately derived system” is where the generator neutral is floating, and its neutral is always interconnected with the neutral for the utility – this is the most common way backup systems are setup – including automatic backup systems.
If you need to isolate the neutral then you need to use a GFI type transfer switch such as the Reliance Controls "X" series switches.....see link for details.
GFI Transfer Switches
Note: Typically these switches are not required unless the 120/240v outlet on the generator is GFI protected which is found on Professional generators such as the Generac "XP" models and not the "XG" model.
The fact that a generator has a bonded-neutral does not rule out compatibility with 2-pole transfer switches, which make up the majority of all transfer switches. The only time that a portable generator will not function with a 2-pole transfer switch is if the generator has a bonded-neutral AND there is a GFCI breaker or GFCI mechanism protecting the receptacle being used to connect the generator to the transfer switch. Most 120/240v generator receptacles are not GFCI protected.
National Code does not require switching the neutral in general, but arguably the neutral should be switched when using a bonded generator based on how the system is setup and defined as a whole. Any time a bonded-neutral generator is connected through a transfer switch to a house wiring system, what results are two neutral-to-ground bond points: one at the main panel and one at the generator. This can be construed as having two bond points in the electrical system, even though the portable generator is not part of the system, and therefore one of the bond points should be removed (or a transfer device implemented that isolates the neutral).
Having two bond points in this scenario creates 'dual paths' on the ground and neutral between the generator and the house panel, aka "ground loop", meaning the ground wire from the generator to the house panel will share some of the return power with the neutral wire. This leads to a very small amount of voltage on the ground wire between the generator and house panel. It can be argued by an installer or authority that this power on the ground is objectionable, and a special neutral switching setup might be mandated. This is also why if there is a GFCI breaker protecting the generator receptacle, it will sense this power being lost on the neutral (to the ground) and will trip every time the generator is connected, even under No load.
* If the generator does not have a GFCI protecting the in-use receptacle, there will not be a trip and the generator will power circuits successfully. Now it’s just a question of whether having this setup is acceptable to the home owner, installer and authority if applicable.
Using bonded generators without switching the neutral has been a common application for many years, regardless of how the code is interpreted, but about 10 years ago generators started adding GFCI's (per OSHA job site requirements) and wiring them into the larger receptacles, which will limit their applications when used at homes. The 2014 code will require new generators to have built in GFCI’s, but only on the 15/20 amp 120 volt outlets, which in most cases are not being used to power a transfer switch. If we look at all of the portable generators that are bonded neutral, and then separate the models WITH a GFCI breaker protecting the 120/240 outlet, the percentage of these generators is extremely small. These are the only generators that will functionally require a 3-pole transfer switch that breaks the neutral.
Breaking the neutral with a “3-pole” (neutral switching) transfer device eliminates the ground loop (dual paths) between the generator and house panel created by having two bond points, and will eliminate the problem of the GFCI breaker tripping creating a “separately derived system” where the generator neutral is never common with the utility neutral.
I hope this information was helpful.
