We have 2 carriage lights on our patio and 3 flood lights on our eves. The carriage lights are controlled by two 3-way switches and the yard lights are controlled by another set of two 3-way light switches (one in the bedroom, one in the living room). We are expanding our patio and are adding two more carriage lights which we want controlled on the same switches as the existing carriage lights and we are moving the flood lights to the eves of the expanded patio cover. The new lights and the moved lights will all be within 10 feet of the pool which is a code violation unless we change the circuit breaker to a GFCI breaker.

They are all on the same circuit breaker. The circuit breaker they are on has a BLACK wire going to it from a 12 gauge 3-wire. The RED wire from the same 3-wire goes to the circuit breaker next to it and controls our security system and some outdoor outlets. I'm not sure if that is called tandem breakers. They are not tied together, they can be tripped separately.

My question is, can we replace just the one circuit breaker that the lights are on with a GFCI breaker or do we also need to replace the other breaker that the RED wire goes to that is next to it? If we have to replace both breakers, do we need to replace both of them with GFCI and then what do we connect the two neutral white wires from the two GFCI breakers to since the 3-wire only has the one white neutral wire coming off it? Or can we just replace the one breaker with a GFCI breaker and leave the other breaker, which has the red wire to it, as-is?

It is a single pole, 15 AMP Square D circuit breaker. And the current wiring and breakers in the panel are all original to the house when it was built 21 years ago and therefore passed inspection back then. Doesn't meain it is right though or up to current code...

 

You seem to have a multi wire circuit with a breaker configuration that does not meat current code. Under current code if two single pole breakers are used they must be handle tied. Your installation may be grand fathered. You can not change one breaker to GFCI. You can not use two single pole GFCI breakers. You would have to use a 2-pole (240 volt) GFCI breaker. Simplest solution would be to leave the breakers as is and use a dead face GFCI before the lights.