You might find more general information under the more comprehensive reference as constant voltage systems:
http://www.crownaudio.com/pdf/amps/138905-1_10-05_constant_voltage.pdfhttp://www.atlassound.com/Support/Default.aspx (Look for "70-V Class" toward the lower right)
http://www.rane.com/note136.htmlhttp://www.prosoundweb.com/article/understanding/http://www.parts-express.com/resources/70-volt-system.cfmThe three things I usually first look at for such applications are 1) what areas does the system have to cover, 2) where can the speakers be located and 3) is this announce only, announce and background music or announce with higher quality music? In some cases other considerations may be if the system is permanently installed or portable and if there are nearby areas where they don't want the sound going.
Is this an all new system or are there existing components such as microphones, mixer, equalizer, amplifier, etc. that can potentially be reused? That could definitely affect the budget and possibly what is practical or possible, for example if you want to use an existing 50W amp then it makes no sense to have two 30W speakers.
Would the speakers be mounted to a press box or concessions building where the amplifier is also located or would they be on poles or otherwise remote from the amplifier? If the speaker cable does have to run outdoors and perhaps through underground conduit then you may want to look at something like the unshielded, twisted pair wire in this product series,
http://www.westpenn-wpw.com/pdfs/aquasealcat.pdf.
Determining the power starts off no different than for any other system with figuring out what leavels you want at the listeners and then using the distance from the listeners to the speakers to derive the level required from of the speaker. The speaker sensitivity then lets you assess how much power would be needed, which you can then coordinate with the available power rating taps. There are details that can be considered such as line losses, transformer losses and whether a tap of X Watts represents what is delivered to the speaker or what is drawn by the transformer, but for this type of application you would rarely go to that level of detail and would not be doing something like calculating that with losses you need 74.3W and then trying to find a 74.3W amp. You might just figure out that you need 22W to get the desired and the speaker taps are 15W and 30W so you select 30W. Then maybe that since you have two of the 30W speakers you might want somewhere around a 75W to 100W amp.
Coverage and pattern are generally no different either. You want to cover the audience but unless you have buildings nearby or areas you want to avoid covering, such as neigboring residential areas, then you don't really have to worry too much about having too much coverage.
