Why does the yellow power cable connect the outlets on separate circuits, causing power to flow through one cable back to the panel?
Asked by Susanna Fowler 
2 months ago
Last Updated: November 4, 2024
I’m in the process of getting a new shop set up, and I made sure to have the electrician install three separate 20 amp circuits for all my power tools. I just noticed something interesting in the picture below – the yellow power cable to the outlet on the left seems to be running through a box before reaching the outlet on the right. I’m not an expert, but if those are supposed to be on separate circuits, why is the power flowing through one cable back to the panel for both boxes? Just curious!
Both of those devices will be connected to the same circuit where it is currently placed.
Isn’t great
The box on the left is receiving power from 12-3 romex. It contains a red and black hot wire, each for a separate circuit. The black wire feeds the left box, while the red continues to the box on the right (or vice versa, depending on wire nut placement). There are 3 circuits shown here.
That’s right, Alex! I’ve wired it like this multiple times.
In my opinion, sharing the neutral is not a good idea. Why not just give the man what he asked for?
Neutral wire shared.
If it’s wired with 12/2/2, it’s compliant. Using 12/3 without ensuring the neutral isn’t overloaded is not recommended.
The man didn’t mention not sharing a neutral.
Ask him to do it right and provide you with what you need instead of sharing a neutral when dedicated circuits are preferred
That’s the impression I get too. However, technically, there might be an issue with one of those circuits sharing a neutral. It’s best to keep them completely separate. Are they tandem or on 2 separate breakers?
There are two circuits.
Josh uisinger I completely agree with 310.15(E).
I also think this isn’t a dedicated line.
Why would it be considered dumb? It doesn’t matter when the circuits are on different phases.
– So you’re saying one neutral can be shared for two circuits? Does that mean a total of 40 amps could be placed on it if both 20 amp circuits are fully loaded at the same time? Is the 12 gauge neutral sized for that load?
Being exact about dedicating neutrals is important.
Understanding basic wiring and theory doesn’t make it dumb.
Yorgy Kemenes Essentially, you are placing the combined load of 2 circuits onto a single neutral wire.
Are you suggesting that two receptacles, each connected to a different hot wire, cannot utilize a shared neutral?
Yorgy Kemenes Well, when both circuits are active, it does indeed decrease the current flow on the neutral. I see you are already aware of this, just wanted to chime in.
It really depends on your location, as using a 12-3 setup without a single 2-pole breaker can be a code violation in many places. Some areas don’t allow sharing a neutral for separate branch circuits. This falls under NEC regulations, particularly in the 200s range, around 200-215. It’s necessary to have ‘simultaneous disconnecting means’ to ensure both hot wires are deactivated together, making the neutral inactive.
I was under the impression that ‘sharing neutrals’ was a thing of the past.
Absolutely, and in a 3 phase panel you can have 3 circuits sharing a single neutral. It’s always 3 full circuits, even if the neutral is shared, as long as they are on different phases. This is very basic electrical knowledge, not sure why it needs an explanation.
How does a FI function with a shared neutral? I had to replace mine because it kept tripping when I used the other circuit.
Requested 2 separate circuits – both at 120 volts. If both circuits have 15 amps each, the neutral would have 30 amps, exceeding its rating
Dealing with this will never cease for us guys 🤣
Sharing is actually caring. 😉
Its ampacity is dedicated.
Son no, AC power uses alternating current, so the neutral does not carry the load of both circuits at the same time. There’s no issue with this installation
He didn’t request dedicated circuits, he asked for separate circuits, which he already has.
Son It’s acceptable for them to have a shared neutral as long as they are on separate poles. (I’d say “phase,” but some here might nitpick about terminology).
Like I mentioned before, the current on each leg offsets the other. If there’s 15 amps on one line and 15 amps on the other, the neutral has zero amps flowing through it.
Yorgy Kemenes If you review your code, you’ll see that using 3 circuit wires with separate neutrals is not allowed.
Son The wires are on different phases and carry only the imbalanced load. The neutral will never exceed 20 amps if wired correctly.
Son The neutral current is determined by the difference between the currents on L1 & L2. For example, if L1 has a 15A load and L2 has a 5A load, the neutral carries only 10A due to the phase difference.
If both are 12/2, then they are sharing the power.
Was a 4 wire feeder installed by him? It’s not common, but it could save on labor time. Without seeing the splices, it’s hard to tell just by looking at the box.
Mentioned it’s not a feeder.
Talking about branch circuit. Let’s keep it simple for those who may not be familiar with technical terms.
It might be a 12-3 cable, which contains a black, red, white, and ground wire all in one cable. This allows the red and black wires to be used for two separate circuits.
The cable is 12/3 with ground, meaning you can share a neutral with 2 hots as long as they are connected to two different phases. It seems like you’re getting exactly what you requested.
So it seems like he ran a 12-3 romex to the first outlet and a 12-2 romex to the second one. The 12-3 has black and red circuits sharing one neutral.
Looks good to me.
Ng
Are you referring to a two-pole AFCI?
Clarified that you don’t need afci in a garage unless it’s livable space. Just use FI.
Make sure it’s on a common trip breaker and not on the same phase in the panel.
He’ll need a 2 pole breaker for that circuit as they share a neutral. The code mandates a dual function arc fault breaker.
It’s hard to say from a picture, but is the wire going into the first box a 12/3?
12-3 works fine, but you can’t use arc faults. I guess it’s fine for a shop, just not a house. Everything else looks good.
Arc faults can still be used on 2-pole circuits, which is especially convenient with E breakers that don’t require a neutral connection.
I’m not sure, but I don’t think I’ve ever seen an arc fault without a neutral needed on a single pole. I suppose a double pole breaker could work since they have to turn off at the same time anyway.
There’s nothing wrong with it… I know this might spark some debate, but the neutrals and grounds both go back to the same place. It’s still two circuits if the incoming wire is 12/3.
Have you all seen what is written on the stud? It says 12/2.
Ilberto Muro, it seems like the date might actually be 12-3(thicker) instead of 12-2, mistakes like this can happen in construction.
So if the first box shows 12/3, that means there are 3 circuits. But if it says 12/2 on the wall, then there are only 2 circuits. Having three circuits will require a 2 pole 20 and a single pole 20a breaker.
Just let him do his thing
The setup he’s got should prevent the lights from turning off or everything shutting down at once if you overload and stop your table saw.
You all should really brush up on your code books. Many of you are mistaken and need to learn. Start at article 250 to educate yourselves.
‘t forget to consult your electrician!
Just a heads up, it needs to be a 2 pole breaker