Generator – interlock required on main panel?

This appears to be an automatic start up generator. There is a 70 amp breaker noted in the picture with a red and black wire feeding from the generator subpanel into the main panel.

Would an interlock be required on this main panel?

Hope you set up the location of the panels… :thinking:

Can you verify that was an automatic transfer switch?

Looks like the 70 amp CB is the normal feed for the transfer switch.

An auto transfer switch is wired to feed its [emergency] load from a 70amp breaker. When power is lost and the generator kicks in, the switch will transfers the load from this breaker to the generator and vice versa when power is restored. As long as it works correctly, it would not be possible (by design) to backfeed into the utility. Therefore, I don’t see why a manual lockout would be needed. It would defeat the purpose of the auto switch. You would manually have to restore power.


The 70 amp breaker is denergized when on generator power. No interlock required because the main panel is dead when on generator power.

There are probably many types out there but it doesn’t look like mine:

Larry, that looks like a manual switch since the switch needs to be changed.

The label on the switch in Larry’s first photo says automatic transfer switch.

Thanks Rob

That is just to run it when power is not out, if one desires, or shut it off if need be e.g. fire?, is how it was explained to me, Jim.

It comes on every Wednesday at 1:55 and runs for 8 minutes.

They pulled the meter, pulled the SEC from the service disconnect in the basement and ran it to the panel that I posted. From there, I forget…but I have pictures with out dead fronts somewhere and I know the the old service disconnect is now a remote distribution panel and the panel that I pictured is the service disconnect.

Yeah, this one:

No labeling other then my electrician friend confirming it was a short bit ago.

Interlock switch is for back-feed of a portable generator.

I’m no expect on this, but this is from what I was told 20 years ago when I had an inexpensive generator panel (with built in transfer switch) installed. The separate generator panel usually has built into it what I think most people would call an interlock device that controls power and ensures power is supplied from only 1 source. You don’t then need to do anything to the main panel to safely switch to generator power if using that transfer switch.

That generator panel is the primary control and connection for certain circuits, like the garage, kitchen, HVAC, well, etc. That panel has 2 incoming power sources: one from the power company (via that 70 amp breaker in the main panel in the above example,) and one from the generator. These 2 incoming power sources both go through a transfer switch in the generator panel that only allows power to pass through from only one of those sources at a time. Some might call that an interlock device. That transfer switch on the generator panel can be manual, or automatic, but it selects and allows power from only 1 source to those circuits connected to that panel. It should prevent a situation of power ever being passed on from both sources.

If you have a commercial power failure, either the switch flips automatically (and your generator starts automatically and once started you will have power) or you manually flip the switch to generator power (after manually starting and turning on the generator). Once flipped to generator power you only have power to those circuits connected through that generator panel. Power from the generator is blocked to the circuits connected to the main panel, just as commercial power from the main panel is now blocked going to those circuits in the generator panel. When the commercial power comes back on, it will still power the circuits controlled by the main panel, but it will not be able to power the circuits connected to generator panel until you put the switch back in commercial power mode. If you have a manual switch, you will have to go switch the transfer switch back to commercial power, which now blocks any power coming from the generator. An automatic switch will sense that commercial power, then automatically switch to all commercial power, and will automatically turn the generator off after it cools down. After making that switch manually, you can then shut off the generator. So an interlock switch in the main panel is not really required, they are “usually” built into the generator panel. I don’t think it would be called a transfer switch if it didn’t block power from more than 1 source. Sorry I can’t answer how to verify that. But if you just have 2 regular electrical panels, one marked generator, you had better make sure there is a transfer switch between those 2 panels.