I'm thinking about running circuits for lights and an infrared heater to a patio. The heater is a 6000 W unit with two individually controlled tubes – I'm thinking this will start off in 12/2/2 NM-B and then transition in a J box to THWN 3/4 PVC when it leaves the house for a short underground run to the arbor over the patio. The lighting will be a single duplex outlet in a weatherproof box on the arbor for a string of LED lamps. The run will be 12/2 or 14/2 NM-B to the J box and run with the heating circuit in the 3/4 PVC as THWN.
My understanding is that in the PVC I could (maybe even should) pull a single 12 AWG conductor as a shared ground for all three circuits.
So, my question in what happens at the J box? There will be three EGC conductors there: one from the 12/2/2, one from the 12/2 (or 14/2), and the one shared by the circuits as they run through the conduit. Should they all be tied together in the J box? Or should the ground coming from the conduit be spliced to the 12/2/2 ground, ignoring the ground from the lighting circuit? Or something else entirely?
The, related perhaps, question that nags in the back of my mind is "what about ground loops?" Would tying all three grounds together create a potential ground loop (assuming that the NM-B grounds are connected to the ground bus at the panel as well)?
Best Answer
So you're paralleling two separate runs - a 12/2/2 and a 14/2 - from the panel, to a J-box. Beyond that they run as THHN wires in conduit.
You're concerned because you've been well-trained that cables and conduits should be tree topology not loops or bubbles - and there's a bubble between the 12/2/2 and the 14/2. Good sense.
And that sense is particularly astute when it comes to neutrals. You must be very careful to keep neutrals identified and segregated so each neutral is identified with its partner hot.
Remember that in individual wires (e.g. in conduit), no remarking is allowed for function: you can't re-mark white wires to be hot. Which means... blue tape on a white wire does NOT make it hot; it only makes it "the neutral for the blue circuit".
However, with grounds, the more the merrier. Keep in mind ordinary service current travels on neutral. Safety ground is for nothing but ground fault current, which is expected to be abnormal and transient.
"Ground loop" is a term-of-art relating to audio engineering, and the impacts of having audio equipment powered by an AC source in the audible range. It's also used by airplane pilots to describe the airplane doing an accidental "donut" because they forgot crosswinds can affect the airplane even when rolling on the ground.