What's wrong here?

Ok, so I posted this on my FB and it received 78 comments and they are still rolling in so I thought I’d share this here as well. I learned this on the MB so I am pretty sure most of you guys know what’s wrong in the photo.

Yeah give it a pull and insert drywall screws was my initial thought.

MWBC on the same phase, bottom right black and red. Among several other things…no disconnect or neutral bonded at the sub panel, double taps, if main panel the neutral/grounds under same bus terminal, and is there a jumper wire on the left between two breakers?

Several things

Came back on my desktop now home but that is a fuzzy picture.
See white on breaker ,possible double tap,etc,however if your going to quiz us take a sharper shot or get a better camera.

I agree with Bob it’s hard to accurately pick the installation apart because the photo is fuzzy. Having said that my guess is that there is an objection to the two black conductors run from the top left CB behind the other CB’s. Likely a split bus panel with that CB feeding the lower half.

Yes Cam. The MWBC breakers are the same leg. Troy was the first to get it on my FB. It’s hard to find sometimes but I always look for it.

It’s a split bus though so it does have a service disconnect.

My understanding is that in this scenario, the neutral can be overloaded.

I don’t understand how putting the breakers on opposite legs stop that from happening though.

Rob? Jeff?

When the MWBC is properly installed on separate (opposite) legs of 120V, the current-potential imposed on the shared neutral will only be the imbalance between the two circuits, or the maximum of a single circuit.

When both circuits are on the same leg of 120V, the current-potential on the neutral equals combined current of both circuits.

If both circuits are wired for 20 amps (for instance), with 12 gauge wiring, and each circuit was drawing 15 amps at any given time, the combined load on the shared neutral is 30 amps, which exceeds the 12 gauge rating for the neutral.

Thanks Jeff. I understand that much actually. I don’t understand how separating them on different legs prevents it from happening. Do you know what I mean? I don’t understand the electrical theory behind it.

Wait, wouldn’t it be 0?

Think alternating current. http://www.allaboutcircuits.com/vol_2/chpt_1/2.html

The sine wave of opposite legs of 120V are exact opposites, meaning at one point (although seemingly instantaneously) the potential for each leg at its maximum positive or negative.

Alternating current. The flow of electrons is never occurring in the same direction at the same time on the two legs of the service.

This is why were also able to get 200 A from each leg of a 200 amp service effectively being able to run 400 A worth of equipment.

Someone please correct me if I just butchered all the instruction my physics teacher in high school gave me. :slight_smile:

Yes, assuming the loads are equal. I misstated that in my previous post. The neutral will carry any imbalance. With load only on one circuit, the maximum potential is no more that the individual circuit will allow.

Ok I think I got it!

At the transformer, L1 and L2 come from the opposite ends of the the coil. Because of this, they are always opposite polarity. (Their Sine waves are opposite each other.) When you have a MWBC, with breakers on opposite legs, that share a neutral, the current in the neutral are opposite polarity (opposite sine waves). If the loads are equal, the current cancels.

If they are the same leg the polarity is the same in both circuits. When the current gets to the neutral, it doubles, possibly overloading the neutral!


Thank you Jeff and Cam! Learning has occurred.

Looks like you got it :smiley:

Took me a while. It was kind of like when you are thinking of a word, but it won’t come. The understanding was at the tip of my brain and it finally hit me with the help of you, Cam and google :slight_smile:

I don’t think its fully explained on the internet anywhere. Time to make a blog post.

I’ve run into this now three times where MWBCs have been moved from a main panel to a generator panel by the backup generator installers. When they move them to the generator panel they inevitably reinstall those circuits on the same leg/phase.