Is this a recently-developed problem in an installation that previously worked OK? Or is this a botched job that never worked properly?

"This timer itself will not support two separate circuits."

That is exactly what you have described though.

Two circuits, one timer.

Something is not wired correctly, and you could even have 240 V on the circuit that keeps tripping.

Since we bought the house in 2007 these lights have worked fine on both the garage and on the house. I talked to Intermatic about the T101 and this model will not support two separate circuits. I just presumed that when the timer went on, it sent power to one relay/contact which activated and turned on the house flood lights, and I presumed the circuit the timer was on was the circuit the garage flood lights are on. This makes up the two circuits controlling the flood lights. The garage itself has two other circuit breakers for its ordinary lights and for the 20 amp plugs. Thus this all has worked for a long time until just recently.

"I just presumed that when the timer went on, it sent power to one relay/contact which activated and turned on the house flood lights, and I presumed the circuit the timer was on was the circuit the garage flood lights are on. This makes up the two circuits controlling the flood lights."

I cannot recall seeing timer that is wired as you have described.

Most have a single hot input line for the timer and the output that is controlled.

When the timer closes the circuit it pulls power from the single hot input and supplies it to the output.

The neutral connections are made around the timer.
One for the timer so it can operate, the other to match up with the switched output from the timer for the controlled circuit.

My first response was based on the fact that we do not have enough information. That is still the case. The only way a single pole timer switch can properly control loads on multiple circuits is for the timer to operate a multi-pole relay or multiple relays. Is there a relay in the hookup somewhere between the timer and the loads? Anyone made any changes inside the electrical panel from which these circuits originate? And who determined that these loads are on separate circuits? How did they determine this?

It sounds like someone used a relay to operate the house lights. This relay may have developed a defect that couples the coil to the contacts.

This would be an unusual defect - but the way this system is wired is quite unusual. Baroque, even.

Wild guess, but it suggests an installation done by someone with a moderate amount of electronics experience, and very little home wiring background.

Perhaps I'm missing something, but I don't see any reason not to just remove the house lights from their separate circuit and connect them in parallel with the garage lights.

davidr: If we connect the house lights with the garage lights, and the short circuit is in the wiring between the house lights, then would I not trip the other circuit breaker? I think this arrangement was done from day one by the original electrician, and the whole purpose was to use one Intermatic timer to control all the outdoor flood lights, and for some reason two circuits were used. The Intermatic timers like the T101 are so cheap, they could have used two timers so the garage flood lights were on one and the house flood lights were on another. By now it is probably time to stop speculating and at least get an electrician out for one hour to tell me what was originally done - I am curious, and then either get a test procedure to fix the problem and restore the house flood lights or make a new installation.

"a test procedure to fix the problem "

You will not find a "test procedure" anywhere for a job like this.

You have to look at what is there, understand it, then figure out what is wrong.

This is not like debugging electronics.

Makes me wonder it this timer isn't a 2-pole unit, like for a water heater. They look the same on the surface but there is an extra pair of connections.

"Makes me wonder it this timer isn't a 2-pole unit, like for a water heater."

There is no reason to bother with a 2-pole timer for a WH.

Opening either leg will turn of a 240 V circuit.

At least Rheem finally realized this also.

I have a Rheem condenser that uses only a single pole relay, and it has far better contacts than the typical ones seen on high current 240 V contactors present in many brands of condensers.