running wire

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I am running 12 guage (made for underground) 20 feet to a garage.I will be linking into the 15 amp light circuit.It currently has 6 lights on it -energy efficient lights.How many additional lights can i add and how many plugs -after the gfci can be installed? I can also just transfer the current 2 that are already on the circuit but would rather add the new ones.
Any info is appreciated

[HayZee518]

if you are running 20 amps on the 12 ga line you can't legally run a 15 amp [14 ga] line off the 20 without installing a circuit breaker or fuse to protect the 14 ga line. the formula for POWER is P=EI. Power in watts equals voltage times amps. The GFCI will protect the circuit from ground faults not overloads. The rule of thumb for number of outlets on a circuit is 180 va per outlet, or an amp and a half per outlet. divide the line amps by 1.5 to get number of outlets. Look on the energy efficient fixtures for the number of watts it consumes and do your calculations.

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info

I will be staying with a 15 amp gfi even though the line is 12 its on..I dont believe you can put a 20 amp plug in item into a regular 15 amp plug.Did you mean combined amp as in if several are being used together?

Are most all older built homes(70s) built with light circuits that are 14?

[HayZee518]

a 20 amp 125 volt plug has a different blade configuration than a standard parallel blade plug. devices are measured in watts, i.e. a dryer is so many watts, a lamp is so many watts [60, 100 etc] add up the total watts and divide this figure by 120 volts to get amps. P/E=I

In the past I have wired homes with 14 gauge for lighting and 12 gauge for power circuits. It's the practice now because of practicality to run all 12 gauge in the house for anything fusing it for 15 or 20 amp.