number of wires in 3/4 inch conduit

Which type of conduit do you have the answers vary with the actual inside diameter of the conduit that you are using.

Each raceway need only have one Equipment Grounding Conductor (EGC) so if you're not using the raceway itself as the EGC then you run one EGC that is sized for the highest ampacity over current protective device that supplies conductors in that raceway. There is no benefit to running an additional EGC unless you need an isolated EGC for that circuit.

You're Welcome

-- Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use." Thomas Alva Edison

Most types of conduit can legally acommodate 6 no. 12 conductors; you'd have to look-up your specific type in a code or other reference book. Two other points: 1. If you have more than 3 current-carrying wires in a conduit, you have to derate the ampacity of the wires. See the code for how much; it depends on exactly how many current-carrying wires there are. Once you do this, you may find that the original 12 ga. wires are now too small. There are some ins and outs of this rule involving shared neutrals and travelers that may allow you to avoid derating. 2. All the wires in a conduit are supposed to be pulled at one time, so the correct way is to pull-out the existing wires, add the new wires to the bundle, and pull the entire group into the conduit. This provision gets ignored a lot, but it's the legally correct way.

cdoc wrote:

Assuming EMT and THHNN/THWN the number is 16. Different types of conduit and wire will change this a little. If you are talking about cables in conduit (two 12/2 romex) you may be full.

OK as long as it is big enough for the largest breaker involved

Derating was mentioned. As long as these are all modern THHN/THWN conductors (90c rated) derating doesn't really become a factor until you get 9 current carrying conductors if there are no other problems, like high ambient heat. You don't count grounds for derating

My calculators at

will provide the answers you are looking for. I have a raceway fill calculator and wire size and ampacity calculators that follow the 2005 NEC Table 310.16 and accompanying sections. I have been working on these all winter and am anxious to see that they are used.