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

NEC 2008
210.4 (B) Disconnecting Means Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

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.

Ungrounded conductor 1 = 13A
Ungrounded conductor 2 = 6A
Grounded conductor = 7A

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

• Install a 20A double pole GFCI breaker in the service panel.
• Connect the bare (equipment ground) wire (from the circuit) to the ground buss bar in the service panel.
• Connect the white grounded (neutral) wire (from the circuit) to the neutral terminal of the GFCI breaker.
• Connect the white grounded (neutral) wire (from the GFCI breaker) to the neutral buss bar.
• Connect the black ungrounded (hot) wire to one terminal of the double pole GFCI breaker.
• Connect the red ungrounded (hot) wire to the other terminal of the double pole GFCI breaker.

At the first junction

• Connect the black or red ungrounded (hot) wire to the brass screw on the receptacle.
• Connect the white grounded (neutral) wire from the 12/3 cable to the white ungrounded (neutral) wire leading to the next junction point, and to a short length of scrap wire (pigtail) using a twist-on wire cap (or other approved method).
• Connect the pigtail from the previous step to the silver screw on the receptacle.
• Connect a short piece of scrap ground wire to the green screw on the receptacle.
• Connect all equipment ground conductors together, and to the junction box if a metal box is used.
• Connect the red or black ungrounded (hot) wire to the black ungrounded (hot) wire leading to the next junction point.

At the next junction

• Connect the red or black ungrounded (hot) wire from the previous junction to the brass screw on the receptacle.
• Connect the white grounded (neutral) wire to the silver screw on the receptacle.
• Connect the equipment ground wire to the green screw on the receptacle, and to the junction box if a metal box is used.

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

• You'll need a 20A double pole GFCI breaker. Two single pole GFCI breakers will not work.
• If you use a 20A overcurrent device, you must have more than one 15A receptacle on that leg of the circuit. Otherwise, you'll have to use a 20A receptacle.

As always, if you don't feel comfortable with your knowledge or skill level, don't be afraid to contact a qualified Electrician.

Column C

First off you'll notice the text "Column C to be used in all cases except as otherwise permitted in Note 3.)", in the title of table 220.55. This makes it simple. You have 2 appliances, so follow that over in the table, and you'll see 11 in Column C. So there you go, you can just use 11 kW. Done.

11,000 W / 240 V = 45.8333 A

So you'll need a 50 ampere breaker, and wire sized appropriately for the load.

Note 3

Note 3 says:

3. Over 1 3⁄4 kW through 8 3⁄4 kW. In lieu of the method provided in Column C, it shall be permissible to add the nameplate ratings of all household cooking appliances rated more than 1 3⁄4 kW but not more than 8 3⁄4 kW and multiply the sum by the demand factors specified in Column A or Column B for the given number of appliances. Where the rating of cooking appliances falls under both Column A and Column B, the demand factors for each column shall be applied to the appliances for that column, and the results added together.

Perfect, so instead of just using the value from column C you can do math. Let's step through it.

...it shall be permissible to add the nameplate ratings of all household cooking appliances rated more than 1 3⁄4 kW but not more than 8 3⁄4 kW...

8.4 kW + 5.0 kW = 13.4 kW

...and multiply the sum by the demand factors specified in Column A or Column B for the given number of appliances...

Let's check the table again... You have 2 appliances, both between 3 1/2 and 8 3/4 kW. So You'll look at column B, and find 65%.

13.4 kW * 65% = 8.71 kW

8710 W / 240 V = 36.2916 A

So using this method you can use a 40 ampere breaker, and appropriately sized wire. However, keep in mind that if you change the equipment, you'll have to do the calculation again. So while you can use this value, you may have to upgrade the circuit later if you change equipment.

Note 4

I'm not exactly sure how note 4 comes into play, but I think it can be used if this is the only equipment on the branch circuit. Just for fun, let's run through that one too.

4. Branch-Circuit Load. It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch-circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance. The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range.

You're only concerned with the second half of this note, since you have one counter-mounted cooking unit, and one wall-mounted oven, all supplied by a single branch-circuit, and located in the same room. So you can add the nameplate values, and treat it as a single range.

8.4 kW + 5.0 kW = 13.4 kW

So you can treat the units as a single 13.4 kW range. Check the column C again, this time for a single range. You'll find a value of 8 kW. But wait... The column header says "(Not over 12 kW Rating)". Your range is 13.4 kW. That's bigger than 12 kW. Now you'll have to check note 1

1. Over 12 kW through 27 kW ranges all of same rating. For ranges individually rated more than 12 kW but not more than 27 kW, the maximum demand in Column C shall be increased 5 percent for each additional kilowatt of rating or major fraction thereof by which the rating of individual ranges exceeds 12 kW.

That's easy enough.

13.4 kW - 8 kW = 5.4 kW

Since .4 is not a "major fraction", you can just use 5 kW. So you'll have to add 5% 5 times.

5% * 5 = 0.25
8000 W * 0.25 = 2000 W
8000 W + 2000 W = 10,000 W

That means you'll have to use 10 kW as your demand.

10,000 W / 240 V = 41.666 A

Which means you can use a 50 ampere breaker, and appropriately size wire.