I believe that , " branch circuits on different phases ( one from each phase ) can share the same neutrals ".
Looking in today at the forum... this is the kind of thread I cannot resist replying to even if I am on extended break....
....get ready for my usual long winded reply but hopefully it will help.
It would be better to say
two 120 volt branch circuits on opposite hot legs in a single phase residential 3 wire 120/240 volt system can share the same neutral..... not neutrals. Notice Househelper and I said
single phase meaning there are not 'two phases' or opposite phases. Now having said that I think everyone knows what is intended when someone says different phases in a single phase system. But for clarification what you really have is a single phase that has two hot wires one of which has been offset by 180 degrees from the other. See scuba daves graphic.
It's also good to have the correct terminology. If I have a NM-b (romex) 2 wire with ground 120 volt branch circuit I have one hot wire (black), one grounded wire (white) and one equipment ground (bare). Notice I said 'grounded' wire for the white. You will also hear this wire called the grounded leg. This wire is not technically a 'neutral' because it carries
all the current of the branch circuit. Which makes sense ... it serves only one hot wire.
This is a grounded leg..... your typical branch circuit ... even though it is common to call it a neutral..
A
neutral wire in the correct sense of the word serves
two or more hot wires and carries the
difference in current between those hot wires. And those hot wires cannot originate from the same point of the transformer winding ...ie... what we have been calling the 'same phase' in this thread.
Again though we all know what is intended using the term 'neutral' in a 2 wire with ground single phase branch circuit. Just remember it is really just a grounded leg intentionally connected to the mid point of the transformer by bonding it with the service neutral on the neutral bus of the main panel or service equipment. It is grounded by the fact there is a connection to earth both at the main panel and the transformer midpoint tap where the service neutral connects.
But I think it is important to understand the difference between a grounded leg and a neutral if we are to understand why
we cannot share a grounded leg and
we can share a neutral.
So I think we should start with what creates a neutral wire. Best example is your service neutral in a residential system. It is a single shared neutral that serves
both hot wires of your service. These hot wires terminate at the service equipment on different buses and different points on the transformer winding. You will also see that all the grounded legs (or neutrals if you prefer) terminate on the same neutral bus or buses that join with the service neutral. The current from all these branch circuits has only one path to the service transformer and that is over the service neutral. So both hot legs (phases if you must) coming from the transformer use a shared wire called the service neutral to have a completed circuit for
120 volt loads with the transformer.
At first thought you might expect that
all the current from both hot wires is returning to the transformer over that service neutral. However this is not the case. The physics of offsetting the single phase by 180 degress creates a cancelling effect between the neutral current generated by the two hot wires. Meaning if one transformer hot wire is supplying 50 amps to the house and the other is supplying 30 amps then the service neutral will only be carrying 50 - 30 = 20 amps. So it is a true neutral in that it
serves two hot wires and carries the
difference in current between those two hot wires.
This can only happen if the shared neutral is serving these two hot wires originating at opposite ends of the transformer winding. So it makes sense that if I take away one hot wires load the canceling effect ends and the service neutral will carry
all the load of the single hot wire. In the above example if I take away the load on the hot wire supplying 30 amps to the house the service neutral will now start carrying 50 amps not 20 .. or .. 50 - 0 = 50 amps.
Looking at it in terms of branch circuits. If I take the hot wires of two branch circuits and connect them to two 15 amp circuit breakers connected to the same bus in the panel and therefore the same service hot wire of the transformer, there is no canceling effect if I share a grounded leg (neutral) between those hot wires. If one of those hot wires is serving a space heater that uses 12 amps and the other is supplying a hair dryer that needs 12 amps and both are operating at the same time then there will be 24 amps on the shared grounded leg. Neither circuit breaker will trip because they only see 12 amps flowing thru them but the grounded leg (neutral) is overloaded and overtime will get hot and hotter yet as you add loads to either branch circuit. If I add 5 amps to either branch circuit the neutral is now carrying 29 amps. If this load is carried long enough the eventual result will be the grounded wire (neutral if you must) being 14 awg getting so hot that it will cause a fire and neither breaker will trip. Circuit breakers do not monitor the grounded leg or neutral.
EDIT: One thing I should add is that we never
combine the grounded legs of different branch circuits once they leave the panel. We either share a common neutral as when running a multiwire branch circuit or we keep the grounded legs isolated to the branch circuit and loads they serve. Never tie the grounded leg of one branch circuit with another this will create a parallel path for current and may overload any grounded leg or neutral you try to share.
Here is a couple drawings to help you visualize what is being said on this thread.