I want to install an additional outlet under the counter in my bathroom. There is only one other outlet in the room and there are multiple wires wire nutted together in the box, but only three connected to the duplex. Can I connect to the vacant poles on the duplex and run the cable to my new outlet or will that be too many wires in the box.

The way you described connecting the nuw pug is right if you are sure to connect the ground wire also.

As far as too many wires I an not sure that you don't already have two many wires in that box. Maybe an expert will give you the right answer.

You might take out the box that the old plug is in and put back in an extended box, I think that would alow extra wires.

Need to know:

1. What size box, height, width, and depth, and shape.
2. How many total wires.
3.What size wires.

The box is 3.5"x3.5"x2" and has two Romex 12ga. Cable in it now.

Need to know:

1. What size box, height, width, and depth, and shape.
2. How many total wires.
3.What size wires.

The box is 3.5" x 2" x 3.5 and has two 12 gauge Romex cables coming into the box. They are all wire nutted together (black to black, white to white and ground to ground) with leads coming from the wire nuts to the existing GFCI outlet.

That's a 24.5 cu. in. box
Each 12AWG wire needs 2.25 cu. in. so there is enough room for 10 in that box.

Your current box fill count is 7
4-12AWG current carrying conductors originating from outside the box.
1- all equipment grounds
2- the duplex receptacle

All that is longhand for yes you can add another piece of 12-2 w/g romex to the box and extend your circuit to a new receptacle.

I wish more people knew enough to ask this type of question.