electrical
I think I know the answer to this question however I have been known to be wrong before!
I am using GFI breakers to protect multi outlet circuits. One is 15 amp and the other is 20 amp. I realized that while I am running 12/3 using the black for the 15 amp circuit and the red for the 20 amp circuit will work I only have one white.
Question is: Can you use the neutral for both GFI's or do you have to provide a separate neutral for both circuits?
From Wikipedia
Split phase shared neutral (North American Wiring)
In split phase house wiring, for example, a duplex receptacle in a kitchen is typically connected with a cable that has three conductors, in addition to ground. The three conductors are usually colored red, black, and white. The white serves as a common neutral, while the red and black each feed, separately, the top and bottom hot sides of the receptacle. Typically such receptacles are supplied from a ganged breaker, i.e. a breaker in which the handles are tied together for a common trip, so that if one kitchen appliance malfunctions and pops the breaker, the other side of the duplex receptacle will be shut off as well. This is called a multiwire circuit.
Notes:
These types of circuits must use a double pole circuit breaker
While the NEC recognizes this configuration, there is open debate as to weather or not this is a safe practice. For more information see Understanding the Dangers of Multiwire Branch Circuits
EDIT
Switch legs, also called travelers must now be in a different color to be easily identified. A few years ago the white wire could be used as a traveler but code changed that to lessen the confusion of identifying the traveler. Now instead of running a piece of 14/2W/GRD or 12/2W/GRD for 3-way you have to use 14/3W/GRD or 12/3W/GRD. Same goes for 4 Way's also.
EDITED FOR CLARIFICATION:
If this is wired as you have drawn it, then it will not be a safety issue. The additional neutral will only share the current in both circuits. Because both circuits are fused for 15A, each neutral will only see a maximum of 15A. This is providing that both neutrals are solidly connected! If one were to become loose or disconnected then the other can potential see the full load, 15A*2=30A.
If anything else is fed from either of those breakers, it becomes a whole other issue! I recommend you wire it the right way. However to answer your question, I don't see it as a safety issue if that is the only circuit on those breakers.
With either breaker off, that circuit will be isolated from the energized circuit. The only common path between the two is the shared neutral. If the energized circuit were to draw the breaker maximum of 15A, the de-energized neutral potential would be at maximum only a few hundred millivolts, not posing a safety risk.
Also it is not against code to have more than one feed in a jbox or switch box.
Best Answer
What you're explaining is called a multiwire branch circuit (and has been discussed here many times before), which is where 2 ungrounded (hot) conductors will share a single grounded (neutral) conductor. There are special requirements for this type of circuit, which must be followed to insure safety. This type of installation is slightly more complicated when dealing with ground fault protection, because of the way ground fault circuit interrupters (GFCI) work.
Multiwire Branch Circuits
Disconnecting Means
This means that the breaker handles must be tied together in the service panel, so that if one breaker trips it will cut power to both circuits. This can be accomplished using handle ties, or a double pole breaker.
Legs
In multiwire branch circuits, each ungrounded conductor must terminate to different legs of the system. If this is not done the currents on the grounded (neutral) conductor will add instead of cancelling out, possibly overloading the grounded (neutral) conductor leading to a fire. Again, this can be easily accomplished using a double pole breaker.
GFCI
Ground fault circuit interrupting devices work by measuring the current on the ungrounded (hot) conductor, and the grounded (neutral) conductor. If the currents on these conductors differs by more than a specific amount, the circuit is opened preventing current from flowing. This can present a problem when using two separate GFCI breakers, because the current on the grounded (neutral) conductor will be the difference between the two ungrounded conductors.
In this situation, the breakers will detect a potential (false) ground fault and trip. This can easily be avoided by using a double pole GFCI breaker. This is because a double pole GFCI breaker monitors both ungrounded (hot) conductors, and a single shared grounded (neutral) conductor.
Wire Size
The conductors in a multiwire branch circuit (like in all branch circuits), must be appropriately sized based on the load to be served and the overcurrent device protecting the circuit.
Receptacle Rating
NEC article 210.21(B)(3), specifies the receptacle rating based on the circuit rating.
Proper Installation
Now that you know more than you've ever wanted to know, here is what you need to know.
In the service panel
At the first junction
At the next junction
Receptacle Ratings
If you used a 20A GFCI breaker in the panel, you'll want to use receptacles rated for 20A. However, if you have more than one receptacle on a single circuit, you can use receptacles rated at 15A. In a multiwire branch circuit situation, that means you'll need two receptacles fed by one ungrounded (hot) conductor to be able to use 15A receptacles. It's also important to be aware, that NEC considers a duplex receptacle as two receptacles. This means you can use a single 15A duplex receptacle, and be code compliant. Though Because the overcurrent device is rated at 20A, it may be appropriate to use 20A receptacles throughout the circuit.
Daisy Chaining
If you are going to be feeding multiple receptacles off either leg of the multiwire branch circuit, you have to make sure that the grounded (neutral) conductor is unbroken throughout the entire circuit even when devices are disconnected. This means you won't be able to use the "load" side terminal on a receptacle, to feed the grounded (neutral) wire through to another receptacle.
TL;DR
As always, if you don't feel comfortable with your knowledge or skill level, don't be afraid to contact a qualified Electrician.