Hello, just finished the electrical portion of my training so I decided to have a look at my panel wiring as practice. I noticed a 4 conductor wire (red, black, white and ground) that is feeding both the dishwasher and microwave with the 2 ungrounded conductors on separate breakers. I know there is a logical explanation for it but I can’t wrap my head around it. Does this mean if I were to shut off the breaker to service the dishwasher that it could be still live from the ungrounded conductor running from the microwave breaker?
Google Multi wire branch circuit or shared neutral circuit. The basics are correct.
Older editions of the code allowed two breaker on opposite legs to serve a MWBC in a residence if both hots did not land on the same yoke. Newer editions require two handle tied single poles or a double pole breaker to ensure both hot are off at the same time.
Understand the concept because you will see this condition in about 75% of your inspections.
Good to work through that one before you encounter it in the field. Because the grounded (neutral) conductor is shared and is current carrying, both breakers have to be off for either side to be safe to work on, which is why breaker handles need to be tied together. The load on the shared grounded conductor (assuming the MWBC is configured properly) will be the difference in the load of the two ungrounded conductors.
So if you would come across this would just make note of it as a concern or report as a deficiency and recommend the breakers be tied together.
I would have to see it to tell you exactly what I would say about it, but I would definitely say that the breakers need handle ties.
I would also be looking at the breaker types and their positions relative to each other to make sure that they are pulling from opposite phases, otherwise you can have an overloading condition on the shared grounded conductor.
That would depend on how old the installation is. The handle tie/multi-pole CB requirement is from a code change to the 2008 NEC. For installations prior to that the simultaneous disconnect rule for MWBC’s, as in the OP, didn’t apply.