What ampacity circuit breaker would I need for an 18 amp electric heater?
RTFM

After reading the user manual for this heater, as per National Electrical Code.


  National Electrical Code 2011
  
  ARTICLE 110 Requirements for Electrical Installations
  
  I. General
  
  110.3 Examination, Identification, Installation, and Use of Equipment.
  (B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling. 


It appears that it has subdivided heating elements, which requires 2 separate 240 volt 9 ampere branch circuits. It also states that "the electrical connection must be performed by a qualified tradesperson.".


  Connect each lamp circuit to a 230 - 240 Vac, 60Hz, 9 Amp copper wire circuit that is properly grounded. Electrical connection must be performed by a qualified electrical tradesperson.   
  
  Installation must conform with the latest edition Electrical Code ANSI/NFPA N0 70 in the 
  U.S.A. and PART 1 CSA C22.1 in Canada. 
  
  


Select the Conductors

Since you're dealing with space heating equipment, you'll want to check article 424 of the NEC. In article 424, you'll find that the conductors have to be sized to 125% of the load served.


  National Electrical Code 2011
  
  ARTICLE 424 Fixed Electric Space-Heating Equipment
  
  III. Control and Protection of Fixed Electric Space-Heating Equipment
  
  424.22 Overcurrent Protection.
  (E) Conductors for Subdivided Loads. Field-wired conductors between the heater and the supplementary overcurrent protective devices shall be sized at not less than 125 percent of the load served. The supplementary overcurrent protective devices specified in 424.22(C) shall protect these conductors in accordance with 240.4.  


So you'll have to do a bit of math to figure out the minimum ampacity of the conductors.

9 amperes * 125% = 11.25 amperes

Now that you know the minimum ampacity required, you'll want to check article 310.15 to figure out what size wire you need. A quick look at table 310.15(B)(16), shows that 14 AWG wire is good for 15 amperes at 60°C.  

Select the Overcurrent Protection Device

Now that you've selected the wire size, you can choose your overcurrent protection device.  In this case, you'll want 2 15 ampere dual pole circuit breakers.



Wiring the Circuits

When it comes to wiring the circuit, you have a couple choices.  You can pull two 14/2 with ground cables from the panel to the outlet, or you can pull a single 14/2/2 with ground cable.



Two 14/2 with Ground

Pull two 14/2 with ground cables from the panel to the outlet, and connect the heater as follows:



Don't forget to reidentify the white conductors, by marking them in an approved manner.

One 14/2/2 with Ground

A 14/2/2 with ground cable consists of a black, red, white, white with red stripe, and a bare conductor.  The only drawback here, would be that since there are more than 3 current carrying conductors, the conductors have to be derated by 80%.  Even with the derating, the conductors are still rated for 12 amperes at 60°C. Which is still above the 11.25 amperes required by the circuit, so there should be no problem using this cable (as long as the run is not overly long).



Again, don't forget to reidentify the white conductors.