Hello everyone, I am giving a customer a quote on a standby system for a building with two services, one three phase and one single phase, both 120/240. My question is about the ATS and grounding. The single phase service supplies the building and the three phase service supplies condensers outside. The single phase service is connected to a grounding electrode system, the 3 phase is not. If I am correct the code requires that both systems be connected to the same GES at the building. I will be utilizing two transfer switches, but I am thinking that if I connect the 3 phase system to the GES currently at the building then I can use standard switches, but if they are left as they are, I may have to utilize ATS switches that switch the neutral as to not have two ground return paths to the generator. At first I thought I would have to install two generators, but an industrial dealer I have been working with says I can do it with one industrial upsized generator with two breakers installed, one for each service. I was worried about the imbalance in the alternator since the condensers won't run year round. I know that if I switch a neutral then I, in a sense, have a separately derived system and must ground and bond at the generator. I am not sure when an application requires switching a neutral, and what my specific situation requires here. Just need some understanding. Thanks in advance.