I have a box with a double receptacle and a switch where the switch powers an exterior outlet on the other side of the wall. In the photos below, the orange is the switched hot for that outlet. All receptacles I've tested in my house so far are grounded to the box, this one included. I was hoping to replace the switch here with a smart switch, for which I'd need to procure a new neutral connection. My question is, can I split the neutral wire coming into the box and pigtail it to the smart switch?
When installing a smart switch, can I pigtail off of the neutral from the existent outlet in the same box on the same circuit?
Yes, that's the correct neutral.
Hot goes to inside receptacle and then to switch. Which means the receptacle controlled by the switch, and any smart switch itself, must use the same neutral as the inside receptacle. There are currently two neutrals connected to the receptacle, one to the switched lights and the other back to the breaker.
Remove the two white wires. Add a short white to the receptacle and another for the smart switch and connect them together with a wire nut.
Outdoor receptacles should be GFCI protected. For a bunch of reasons, including the addition of a smart switch, that is best done at the breaker panel or another location before the switch. If you currently have an outside GFCI/receptacle you may want to put that inside in place of the inside receptacle and install a plain weather resistant receptacle outside. But if you do that then instead of combining all neutrals you will need to properly separate LINE and LOAD.
The two existing neutrals are electrically connected inside the receptacle. Presumably one of the two continues to another outlet. You can verify this by removing the black insulation tape and checking that the bridge of the receptacle on the neutral (white) side is still in tact.
If this is indeed how it's wired (likely), then yes, wire the two existing neutrals together, add a pigtail neutral to go to the receptacle, and tie in the neutral from the smart switch. For reliability it is best to connect the pigtail to a screw terminal rather than a backstab, but they are electrically the same. Same for the lives.
2If actually wired as described by the OP, the neutral tab must be intact, but good call on double checking. It's always good to know for sure.– FreeManDec 13, 2021 at 13:30
Neutrals must be monogamous to their partner hot(s)/live
This is because currents must be equal, balanced and "cancel each other out" in any cable or conduit. (NEC 300.3). This rule applies to AC power. It is so the electromagnetic fields cancel each other out, and don't cause vibration -> metal fatigue in wires, or eddy current heating in nearby metals, or other mischief.
Separate from that, AFCI and GFCI protection looks for imbalanced currents in a circuit, and will trip if a device takes live from one circuit and neutral from another.
Take neutral that runs with a hot/live
When dealing in cables, it's simple - any cable which has live/hot wires going to your device, it is safe to take neutral from that cable.
Your installation is conduit, but it obviously only has 1 circuit per pipe. So that's no worry here. Also, in conduit, white wires are always real neutral.
Let grounding work in your favor
This is an EMT metal conduit installation. Ground is handsomely carried by the metal pipe and boxes. The switch automagically grounds via its mounting screws, and no ground wire is needed.
Receptacles can do that too, but only if they are a better "Spec-Grade" ($3-4) receptacle that is marked "Self-Grounding". This has a special gizmo on the mounting screw hole to assure good contact.
Spec-grade receptacles have another feature you can use. Their screws take 2 wires each (per instructions), meaning 4 per side. That will help you out with the 3 wires you need to connect to neutral.
Wait. Did you say hot(S)?
Yeah. Most AC power is distributed as split-phase (240V with neutral in the middle at the 120V mark), or as 3-phase with neutral in the middle. As a result, 2 or 3 phase wires on opposite phases can share a neutral, which will only carry differential current, and that will never exceed the highest current on a phase. If split-phase phases are 49A and 50A, neutral current is only 1A. So Multi-wire branch circuits are possible which share a neutral.
It also means the circuit breakers are able to protect the neutral, even though the neutral is not on a breaker of its own. Offer void if misconfigured.
minor point: AFCI doesn't look for imbalances, it looks for high frequencies in the current (300kHz I believe) associated with arcing.– P2000Dec 14, 2021 at 3:18
@P2000 Yes, that is the primary mode of AFCI detection, but almost all AFCIs also have a weak GFCI to detect hot-ground faults and neutral-ground faults, since that is a cheap way to detect those arc faults. Dec 14, 2021 at 3:23