I Understand that “Two phase wiring” means:
One live wire to neutral is 120V, and the same the other way round. But between the two live wires is 240V. Measured from the neutral point the AC waveforms on the two live wires are 180 degrees out of phase with each other, hence 2 phase.
But i want to know if there is any prohibition to having two different phases wired to the same receptacle. In many older kitchens they will have two breakers wired to the same receptacle with the connection tab between the hot side of the receptacle broken off so they are isolated from each other. BUT when the wires are run back to the panel in these same older houses. The panel often has all of the breakers lined up in a row. Newer houses or bigger panels will have a left side and a right side, isolating the two buss bars. But in the older/smaller panels they would have to alternate the two breakers (with a unrelated breaker between them) or the two circuits [at the receptacle side] WOULD be on opposing bus bars… and the hot side of the plug would have a 240v potential. If it gets a little loose in the box and bumps the side if the box… its a bigger spark/arcflash. I have asked this same question several times to electricians and never gotten a satisfactory answer. And this is not the sort of thing that a plug in tester can detect. I’d have to meter between the two hots.
Thanks for your time. And is this something that i would have to include on my inspection?
A single duplex receptacle is permitted to have the tab(s) removed and be fed by two circuits. You would need either a 2 pole circuit breaker or two single poles with a handle tie. You could have a MWBC or two circuits with their own neutrals and the 2 pole or handle tie requirement is the same.
Just another note, it is likely that you’ll never see two phase wiring during an inspection. A 2 phase system did exist but it has been obsolete for many decades. I’m not sure what you mean with the question about bus bars.
The 240v /120v wiring you are referring to is SINGLE phase. There is only ONE secondary winding on the transformer it is 240 v. There is a center tap on this secondary winding… Either transformer secondary end tapp to the center tapp is 120v.
Zinsco was a popular panel in the 1960’s All the breakers are mounted in a single row. What you don’t see is the TWO busses that alternate down this single row of breakers. No two adjacent full breakers mount to the same bus bar.
In a more modern panel with two columns of breakers the breakers in each column still alternate from one bus bar to the other. Two adjacent full breakers in either column do not mount to the same bus bar.
When the breakers are all lined up in a row. the bus bars alternate A-B-A-B-A-B, with the contact points to the breakers. But i think that you answered my question with having the two breakers joined as a single pole, so when one trips the other trips. With them both need their own neutral. Otherwise i would assume that whenever both breakers shared a single device (duplex receptacle) they would both have to be on the same bus, A or B but never A&B because of the potential 240v. I have seen those Multi plug devices that cause me to worry.
No, the duplex receptacle can be either on a MWBC which is a shared neutral between two circuits on opposite legs or on two circuits each with it’s own neutral. Handle tied single poles or multi-pole circuit breaker required.
Regarding the devices in your photos it would be interesting to see how they’re wired when they plug into both receptacles on a duplex.
If they have a single plug, it’s all from that one. They will have a plastic peg that goes in the ground hole of the lower outlet.
If they have two metal plugs, it’s top and bottom. I have one of these in my house on an outlet that has a switched top. The top 3 receptacles on the outlet will switch, the bottom three don’t. I don’t use it for 6 plugs, but because I have 3 anti-social transformer blocks that won’t fit anything else and need the space.
I had this one in my house. Clearly two sets of plugs. I tested both sets and I as I assumed the top plug is connected to the top three horizontal receptacles, the bottom plug the bottom three. Basically it’s two single three receptacle adapters in one unit.
Wrong. If they were both on the same bus (say 2, 20A circuits) then the sum of both circuits (40A) would return on the single neutral (both circuits in use simultaneously, maximum draw).
I believe that he’s stated that in his scenario he has separate neutrals for both circuits so it doesn’t matter if the ungrounded conductors are on the same leg or not.
So if both hot conductors were on the same bus, AA then they would require their own neutral, (thank you for explaining that by the way). But if they were on a separate bus AB then they would not require their own Neutral! If i under stood that correctly that is most informative. And i am grateful for the feedback.
My original question regarding having both hot conductors on separate bus because of the 240v potential inside of the box… Was answered by Robert Kenneys helpful Wiring diagram. I suppose if they teach you how to do it in the code book it must be kosher. Thank you all for helping me get to the bottom of this. I’ve bee wanting to know for a while now.
I was referring to the image used by Robert Kenney. Im not sure where he found it. But the image has a tag similar to the tags I’ve seen in the CEC. So i assumed he pulled it from his code book. What book is used to teach from then, if not the NEC? In Canada the CEC is the only book required for Electrical Apprentices to own. They use it exclusively to work through their modules. They must learn how to use it and learn the code directly from it. If Americans don’t use code books as teaching guides, what do they use? Obviously there are other training materials. But to not use a code book sounds mighty foreign to me. And the teaching aids cant stop talking about the the code book. So why not familiarize students with the NEC while in the classroom? Perhaps teach them how to use it before requiring them to utilize it… Electrical Training Products
There are hundreds of books that can be used for teaching the electrical trades, but the NEC isn’t one of them. Trying to learn the electrical trades by studying the NEC is like studying guardrails to learn how to drive.
