I am making spaghetti dinner for 200 people. I am not sure how to make 20 pounds of pasta ready for a three hour dinner. I thought about a steam table and cooking all the pasta and putting it in a steam table. Would that work, or is there a better way?
The common method would be to cook the pasta in advance, then shock it in cold water to stop the cooking before draining and refrigerating it. You would then heat it up in boiling water for about 20 seconds just to heat it through.
You would just need a portable burner to keep a pot of water boiling for service.
On the other hand, 20 pounds of pasta for 200 people is a portion size of only about 1.5 ounces per person. That is a very, very small portion, less than side-dish sized.
For side dishes, 2 ounces of pasta is more typical. If you are serving it as a main dish, 4-5 ounces would be more typical.
(Weights are for dry pasta.)
As a variation to SAJ14SAJ's suggestion:
Cook and shock it as suggested, but instead of heating it back up in water, heat it back up in whatever sauce you're using. You'll want to pull the pasta early (a little before al dente), and should save some of the pasta water so that you can thin back out the sauce if the pasta starts absorbing too much. (the pasta water, having starch in it, will combine better with the sauce than just plain water).
The main advantage to this is that you can have large trays of pasta in the oven, freeing up your stovetop.
Another important thing when cooking that much pasta is to not pour the hot water out when draining; you'll want a pasta insert for your largest cooking pot, so that you can pull and drain the pasta while keeping the water in the pot. You can then quickly add your next batch of pasta without needing to wait for a fresh batch of water to heat up.
If you use that trick, you cook more batches of pasta during the dinner, rather than requiring it all be ready ahead of time. (it's also rather important if you're trying to bring it back up to temperature quickly)
Update: I should have mentioned; you want to put the pasta into sauce that's already hot. This will reduce the problems that SAJ14SAJ mentioned. If you attempt to heat the pasta and sauce together you'll likely overcook the pasta.
I would add that you will find the pasta sticks together when resting, so work a few tablespoons of neutral tasting oil through the cooked cooled pasta with gloved hands to separate the strands. In chain Italian restaurants, they cook off 5 kg at a time, cool it immediately in ice water, drain, refrigerate and oil. This keeps for 36 hrs. And can be portioned and finished per order in whatever sauces. But one 5 kg deep 1/2 pan of cooked pasta yields only 30-35 portions. With each between 150-180grams.