How do I connect a sub-panel to a generator transfer switch?

Question

Our house currently has a transfer switch to our main panel. Our barn has a subpanel that houses our electric fence and more importantly, well pump. As of right now, the transfer switch does not provide power to the barn. The barn has a two pole breaker in our main.

I attached pictures of our panel labels (rooms are correct but red outlining for generator is not--we had some work done recently and the electrician didn't move some of the transfer switch wires) and the actual wiring. The barn breakers used to be where the current surge protect is located. The previous owner did have it hooked into the transfer switch.

My question, is there a simple solution to hook up the barn to the transfer switch? We don't need or want the oven, dryer, or garage on the generator (currently hooked into the transfer switch). They just won't be needed during our power outages. Moreover, we don't need the heat pump as we will just use the furnace.

Additional info incorrectly posted as an "Answer":

There is a main switch on the bottom not pictured. It is an older panel in an old house. We have a 10 circuit transfer switch. However, we have some of the wiring going to breakers we don't need on the generator.

Ignore the blue and yellow dots in the wiring picture (they were for my reference).

Answer 1

With the barn being a 100 amp feed your existing system is two undersized to be able to add the barn to your system.

Could it be done? Not by code!

using a mechanical interlock you could feed 240 to the barn but since it s a 100 amp feeder your generator would need to be able to feed that load (by code) if you load shed the barn down to just the pump and fence charger, most mid sized generators could push that with the house circuits but your existing transfer switch is not large enough to transfer a 100 amp load ( even if only 20 amps is needed on the barn circuit.

I would suggest a panel upgrade even if the same ampacity code now allows more breakers and you could add GFCI and arc fault protection IF you wanted (in some states required with a panel change) so by installing a 40 breaker panel or larger you would have room to install a mechanical interlock, the 100 amp breaker could be turned off as the other large circuits are with a mechanical interlock, then you walk out to the barn, turn everything but the pump and fence charger off , go back to the house and turn the barn breaker on, as long as you don’t overload the generator this will work and as a mechanical interlocked system will pass a code inspection, I know because I have wired them up. After the mechanical upgrade you could then sell the 10 circuit transfer switch on line, one customer got more than his interlock kit cost (~75$) .

The advantage to a mechanical interlock is you can change what circuits are energized through out the day/night turn 1 circuit off and another similarly loaded circuit on. you have all the circuits in your main panel to work with, not just the 10 on the separate switch assembly. Your current issue.

Answer 2

You can do it now, at a compromise.

One of the positions on your transfer switch should be for a 240V circuit. (This will need to be common trip). That is the circuit you use.

Note that since the wiring in the transfer switch is only good for 20 or 30 amps, and the power to the subpanel goes through that wiring at all times, you must downgrade the 100A breaker to that lower number. That will limit the power you can use at the barn.

I note that many people who install 100A subpanels just pick the number out of the air, and don't necessarily have load demands which require 100A.

Or, get a generator interlock as Ed Beal advises.

It really is the more versatile way to hook up a generator. Salesmen do a great disservice when they convince you to add a transfer switch to the job, but it is a $550 sale they get commission on. It is a perverse incentive.

However, while I am always happy to see people get plenty of panel spaces, the cost of a whole replacement panel isn't always on the table for people.

In that case, it may be worth talking to a competent Square D dealer, and searching the aftermarket, for a "generator interlock" that would fit your panel. Since you say you have a main breaker off photo, the interlock will only require 2 breaker spaces typically.

You can free those up by consolidating two of your 240V breakers onto a quadplex. Alternately you could fit a subpanel next to this panel, move two 240V circuits to the subpanel, freeing up the 4 spaces needed for interlock and subpanel feed breaker.

Note that dryer, range and subpanel have neutral, so they need "common trip" breakers. That means typically they need to be on the inside breaker on a quadplex.