I have a 200 Watt Malibu Landscape transformer, with 100 feet of 14 gauge cable, 5 10 watt landscape lamps, and 2 50 watt spot lights connected. It has operated fine for several months, but now the circuit breaker on the transformer keeps opening after 5 minutes or so of use. After being reset, it will stay on for a few minutes and then open again. I've checked all connections and rewrapped the weather tape I used at the connections. Any suggestions as to what I can do or change? The transformer works fine when the cable is removed, so it has to be a load problem or short down the cable somewhere.

May be short down the cable somewhere. Or check that lamps and its connections,loose contacts, rust,a short cause by flies etc. try to replace the old lamps with new one.

Based on your observations, (quote below), this appears to be an overload condition not a short...

"keeps opening after 5 minutes or so of use. After being reset, it will stay on for a few minutes and then open again."

Circuit breakers will trip based on the amount of overload, the more the overload the faster they trip...
To troubleshoot this you could use an "Amprobe" or current meter, first to determine if there is an overload and then connecting devices one by one to determine which lamp or spot light is the offending item...

I have a Malibu 200 watt transformer that was doing the same thing. When you reset the circuit breaker, the lights would stay on for about 5 minutes and trip the breaker.

I checked the bulbs, wire, and connections thinking that the hard winter might have cause a break. Everything was good.

I disconnected the power, took the transformer off the wall, and disconnected the wire from its leads.

There are 6 small Phillips head screws (3 on each side) that hold the plastic front cover to the metal frame. After you remove them, the front cover will separate from the frame. Be careful of the small wires connected to the front switches.

The circuit breaker has two spade connectors attached at the back. Remove them. A plastic nut holds on the outside holds the circuit breaker in place. Take that off, and the circuit breaker will slide out easily.

I found an exact match for the circuit breaker at my local Ace Hardware store where it is called a "Thermal Circuit Breaker." It only cost $8.

Put the new circuit breaker back in place with the new plastic nut. Reattach the spade connectors. (Note the circuit breaker has each connection labeled as line and load. The load is the shorter wire that goes to the connection for the lights. The line is the longer wire that comes from the transformer coil.)

Reattach the front cover with the 6 screws. Reconnect the lighting wire to the terminals. Remount the transformer, and plug in the unit to the outlet.

Everything works great again.

Good morning Don.

Helpful hint, please refer to the date prior to responding. This post was entered in 2012.

However, that said, thanks for the diagnostic process and repair information.