Read inside the box. If you install 14 AWG conductors to the new box, instead of 12 AWG conductors. You'd need at least a 22 cu.in. Box, or a 3" x 2" x 3 1/2" device box.
- 8 for current carrying conductors.
- 1 for grounding conductors.
- 2 for device fill.
11 total, times 2.00 cu.in. for 14 AWG conductors = 22 cu.in.
However, since you're using 12 AWG, the calculation looks like this...
- 6 for 14 AWG conductors x 2.00 cu.in. (12 cu.in).
- 2 for 12 AWG conductors x 2.25 cu.in. (4.5 cu.in).
- 1 for grounding conductors x 2.25 cu. in. (2.25 cu.in.).
- 2 for device fill x 2.25 cu.in. (4.5 cu.in.).
Total = 23.25 cu.in.
For more information on box fill, check out this answer
As for connecting the new wire. There should be no problem attaching it to the receptacle. However, since you're using 12 AWG conductors, you'll have to use the screw terminals.
If you want to do pigtails, you can. I know some guys don't like the backstab connections, so if you want to avoid using them you can do pigtails. Ideal 76B® Red Wire-nut® Wire Connectors are rated for 2 to 5 #14 conductors, but check with the manufacturer of the connectors you're using. Again, if you're going to do this, you'll want to use #14 conductors instead of the #12 you've installed.
I wouldn't even need a pigtail jumper ground because the green screw has continuity with the 10-32 screw and ears of the receptacle anyway, right? Am I missing anything?
Since they used metal raceways you could do it this way only if the receptacles are of the "self-grounding" type. They are more expensive and have a metal clip on one end of the mounting yoke. Or, if your boxes are surface mount and you remove one of the plastic screw keepers. See (A) below.
Chicago has their own Electrical Code but I would be surprised if it is any more lenient than the National Electrical Code. They are usually more strict. The NEC allows self grounding receptacles to be used for grounding in this article, attention to (B):
250.146 Connecting Receptacle Grounding Terminal to Box.
An equipment bonding jumper shall be used to connect the grounding terminal of a grounding-type receptacle to a grounded box unless grounded as in 250.146(A) through (D). The equipment bonding jumper shall be sized in accordance with Table 250.122 based on the rating of the overcurrent device protecting the circuit conductors.
(A) Surface-Mounted Box. Where the box is mounted on the surface, direct metal-to-metal contact between the device yoke and the box or a contact yoke or device that complies with 250.146(B) shall be permitted to ground the receptacle to the box. At least one of the insulating washers shall be removed from receptacles that do not have a contact yoke or device that complies with 250.146(B) to ensure direct metal-to-metal contact. This provision shall not apply to cover-mounted receptacles unless the box and cover combination are listed as providing satisfactory ground continuity between the box and the receptacle. A listed exposed work cover shall be permitted to be the grounding and bonding means when (1) the device is attached to
the cover with at least two fasteners that are permanent (such as a rivet) or have a thread locking or screw or nut locking means and (2) when the cover mounting holes are located on a at non-raised portion of the cover.
(B) Contact Devices or Yokes. Contact devices or yokes designed and listed as self-grounding shall be permitted in conjunction with the supporting screws to establish equipment bonding between the device yoke and flush-type boxes.
(C) Floor Boxes.......
(D) Isolated Ground Receptacles......
Check with your inspector to see if you are under the Chicago Electrical Code and if it allows this.
Best Answer
You can't use Romex. You need AC cable.
Your jurisdiction does not allow the use of Romex cable for electrical wiring. Romex is a brand name of a type of plastic cable called NM-B.
I'm well aware that the Internet is full of instructables and videos that say to use Romex. This is a cheaper wiring method that is not allowed in your town, for reasons.
This is obvious from the builder's use of AC type armored cable and backstabs. The AC type with steel boxes is a deluxe wiring method the builder would not have used if it wasn't compelled by law.
If you have any doubts, go talk to your city's permit issuing office and ask them about wiring requirements there.
But I only know Romex, I don't know anything about AC cable
It's not that big a deal. You use AC cable obviously. The cable clamps appear to be built in to the existing metal box. You'll need to make a bottom entry because all the top holes are full, then loop upward.
So you'll need to get a metal box that has AC cable clamps built in. The local electrical supply house can help. If you bought from the local big-box, you can try there but their advice is terrible, so don't rely on clerks. If you wonder why they're consistently terrible, it's because electricians and electrical supply houses constantly troll the big-box stores looking for competent clerks, and hire them away.