too few bulbs on a ice cap?

[miesenjr]

If I have an Ice Cap 660 ballast do I have to run 4 bulbs? Or can I run 2 48" T5. I have a second ice Cap 400 series ballast and was thinking of putting two bulbs on it as well. This way I can have two on the 660 and two on the 400 allow for dawn/dusk lighting control. my hood is large enough for all 8 bulbs but in a tank with just fish and rock it seems to be overkill and a heat problem. The tank is 180 gallons and 30" deep.