I've run CAT6 throughout my house, mostly in the basement where the TV area and office are. I have a cable that goes upstairs to my bedroom along the chimney from the basement to feed a wireless access point. There are 3 spots where the cable comes about 4-6 inches away from electrical wires. The power and ethernet wires are parallel for about 18" and about 4" away from each other. Will this make an impact on my data signal?
CAT6, even "unshielded", is very resistant to electrical interference, just as it can carry very high-speed data while emitting little or no interference. Plus, your power cables have pairs of conductors carrying current in opposite directions, so any interference they emit is going to rapidly diminish with distance.
(For fun, here is an explanation of how CatX cables reduce crosstalk and increase EMI resistance.)
In real terms, 4" is plenty of spacing, even if you ran the two lines parallel for much longer distances.
NFPA 70, National Electric Code (NEC) Section 800.133(A)(2) requires communications wires and cables shall be separated by at least 50mm (2 INCHES) from conductors of any electrical light, power, Class 1 non-power-limited fire alarm or medium-power network-powered broadband communications circuits. Exceptions are if separate raceways or conduit are used for separating the communications cables/wires from the power conductors.
This is for parallel runs. They can cross perpendicular at less than 2". An electrical inspector can fail a permit inspection and keep you from obtaining a certificate of occupancy (CO).
Differential Manchester Encoding generally prevents induced voltages on your Ethernet cable from power lines from creating an issue. Because he voltage induced on both lines of a twisted pair is the same the differential is zero.