I currently have a Forced-Air boiler as my 2nd zone in my attic. I believe there is a 12-2 that feeds the boiler, an outlet and 1 switch for a light bulb. I wanted to add extra lighting with 2 shop lights I had purchased since its rather dark with just the one bulb. Would I have to continue using a 12-2 wire or can I use 14-2 since the lights will not be drawing much power?

You have to stick with the same gage wire as what that breaker is on that branch circuit.

Also watch what watts are being pulled on the added circuit. Here is a BASIC standard to figure out the added amps and Watts on there already.

Its volts times amps = watts A 20 amp has 2,400 watts in the line. But by the electrical code your only allowed in usage 20% less than 2,400 watts which is 1,920 watts on that line

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You'd have to stay with 12/2 to keep the 20A circuit size.

I must say, this would be continuing a violation as central heating must be on a dedicated circuit.

Its volts times amps = watts A 20 amp has 2,400 watts in the line. But by the electrical code your only allowed in usage 20% less than 2,400 watts which is 1,920 watts on that line
I know this has been repeated over and over but it is still not true.

Except for specific instances, such as a single plugged in appliance or a continuous load, the statement that a circuit must be limited to 80% is a myth.

Thank you for the information. I had the entire 2nd floor and attic re-done. I am not sure how then it pass inspection, unless if the light bulb was added afterwards. So then in order to have it up to code I would simply have remove the bulb from that circuit and run a homerun from the panel in the basement to the attic?. The run will be approximately 40feet.. I plan to use a 15-amp circuit breaker. Because of the distance should I use 12/2 or is 14/2 sufficient?

Depending on what will ultimately be on the circuit 14/2 should be fine