Main panel when a house generator is installed

Where a house has one main panel (like most), and a house generator has been installed with a transfer switch, does the original main panel now become a sub panel?

The reason I asked is this.
I had an estimate this week for a house generator installation. In the scope of work was this sentence Convert main service panel to sub-panel and isolate all neutral and ground conductors.


No. Should not… Any pic’s. Want to see Interlock Kit .

Is this a standalone generator or a portable generator? Generators can be set up as either a separately derived system (SDS) or a non-separately derived system (Non-SDS) so the neutral/EGC thing is related to how the system is designed and installed.


This is a standalone generator.
18kw Generac standby-generator, 100 amp automatic transfer switch.

1 Like

Mine did…

What is your current service rating, 100amp?

Yes. 100 amps.

1 Like

The OP specifically asked about a transfer switch, not an interlock. Typically the transfer switch becomes the service and the old service becomes a distribution panel, aka subpanel.


The auto transfer switch will become the main and your current main panel a sub. They feed the service entrance cable right into the switch and then the switch either draws power from the POCO or the generator to feed the sub. When you flip the 100amp OCPD in the switch (to disconect from the utility), the generator should kick in in a minute or so. I do this to test load the generator. Read all the manauls if you are going to do any of the servicing yourself :slight_smile:


Thank you all.


Great topic, Peter, :slightly_smiling_face:


That’s how they did mine per Generac.


Thank you, Jim. Much appreciated.

Hi! Peter.

In my opinion if you have a “whole” house emergency/alternate generator, you only have ONE (1) Main Panel. That would be the same one was there before the generator was installed. The automatic transfer swictch is not a Panel, is a Switch that diverts the power source between the Public Utility and the local generator. So, two input energy sources come to the ATS (Automatic Transfer Switch). The switch detects the absence of energy in the Utility side and triggers a solenoid that moves the source to the local whole generator, right?

If the generator is not providing all the load capacity to energize all the circuits in the dwelling, you are in the presence of a non-whole or partial load generator installation and as a consequence you have TWO (2) panels, one is the original Main and the second one that is a host to those circuits that will be activated in the event of a public power failure, that one is a Secondary or Sub Panel Panel, Right?

Now, about the requirement to isolate the neutral and ground conductors, I really do not get it! Also the language “convert main service panel to sub-panel…” is not clear to me…! That will be a major undertaken. Usually to install a generator, the loads that the generator will handle will be set in a “new” sub-panel. This is very confusing! Please upload pictures and electrical blue prints to better understand the situation at hand.

NOTE: A GENERAD 18KW Generator could be consider a WHOLE house generator. That is enough power to energize the complete house, but those calculations should be done carefully ahead of the installation. That GENERAD generator most likely has the two-phase 220V set up to feed both phases in the house and move all 220V charges, including A/C condensers and probably a Cloths Dryer too. The fact that the house was originally been energized with only 100Amps tell me that there is not much internal electric demand and that 18KW generator should be sufficient to feed all the demand of that dwelling. But this is all talk, need more information to understand your line of questions.


My Generac installers and all the electrical inspectors around here don’t agree with you.

Like Jim, Simon said;


The simplest way to install these systems is to do as Jim and Simon suggested, make the ATS the service disconnect and everything downstream becomes a sub-panel. Code wise it does not need to be done that way but for a dwelling system that would be the most common type of installation.

1 Like

@pmarin you would be better served using the terms service and panel. There is a difference. A panel can have a main and not be a service. Do you understand a service vs a distribution or subpanel?

The transfer switch becomes the service.

Great! I guess we all can agree to desagree, eh? Right?

I am not reaching any conclusions, not with the information provided in this exchange, but I can deduct that we all have different understandings of the words used in this case. A) Panel (Is a metallic box, Right?); B) Main (What? a Main Electrical Distribution Panel where all the Circuit Breakers meet?) No?; C) Transfer Switch? Are we (you all) talking about another metal box containing some automatic disyuntors and some electrical solenoids that actuate to “switch” or better said, “Swap” incoming energy feeders coming from either the Generad generatior and/or the utility public service drop? Is this device automatic or requires human intervention to manually go from either position when the external utility power is absent?

Jumping forward, it doesn’t make much sense to continue this chat unless we are all clear and agree (on the same page!) on the proper electrical engineering terminology and avoid the field jargon trying to impress each other instead of getting to the bottom of the question/concern presented by Peter.

I walk away sharing with you all this link [ Wiring our Generac generator transfer switch to the meter can - YouTube ]. Peter, please invest good 30 minutes watching this video and all your questions will be answered without arguments.

1 Like

“The transfer switch becomes the service” … Not quite…! It will still be just a “switch” and nothing else.

Such device will receive “service” from two sources (Utility Meter & Generator) and will deliver its output accordingly to the main breaker that could be located in a separate box (panel) or directly up-on-top of the main distribution panel inside the dwelling, where all the rest of the circuit breakers are located. This layout only applies if we are hooking up a whole house generator than can assume the whole (redundant) load of all the circuits that are already connected to the main distribution panel, Agreed?

Yes sir, we can!

Just sharing how it is done and expected around here, Pedro.

I think Rob helped by sharing this:

1 Like