Most CC will be monochloramine (NH2Cl). Monochloramine results when chlorine displaces a hydrogen in an ammonia molecule.
HOCl + NH3 --> H2O + NH2Cl
Ammonia is broken down when chlorine oxidizes the nitrogen in the ammonia from the -3 state to the 0 state, where the nitrogen combines with another nitrogen to become nitrogen gas N2.
It takes 1.5 chlorine to oxidize 1 nitrogen. Since monochloramine already has 1 chlorine, it only takes another 0.5 chlorine to complete the reaction.
HOCl + 2NH2Cl --> N2(g) + H2O + 3H+ + 3Cl
For example, if you had 7 FC and 3 CC, you have a total of 10 TC. 3 CC takes 4.5 total chlorine to oxidize the nitrogen to nitrogen gas. Therefore, you would be left with 5.5 FC after the ammonia is oxidized.
There will be some dichloramine and trichloramine formed in the process, but the basic idea is that it takes 1.5 times the CC to fully oxidize the ammonia (assuming that all of the ammonia is in the form of CC). Typically, you need to go to shock level to get the process to happen at a reasonable rate.
Note 1) Most ammonia will be in the form of the ammonium ion. The net reaction can be shown as:
2NH4+ + 3HOCl --> N2(g) + 5H+ +3Cl- + 3H2O
Note 2) Some of the nitrogen is oxidized to nitrate, and can increase the overall amount of chlorine needed to oxidize all of the ammonia.
Note 3) It's not all ammonia. There are many contaminants in swimming pool water. Swimmers contribute many contaminants, such as hair, lotion, saliva, skin, cosmetics, hair care products, sweat, urine etc. Disinfection byproducts other than chloramine, such as trihalomethane, can occur
The process is speeded up when UV light hits the monochloramine.
3NH2Cl + UV --> N2 + NH4+ + 2H+ + 3Cl
Dichloramine and trichloramine are also degraded by UV light. This is why indoor pools can benefit from UV systems. UV can also help provide protection from chlorine resistant organisms, such as Cryptosporidium.