Then, connect the voltmeter probes and turn on the power. Each ballast is designed to operate a specific minimum and maximum length of lamps. For example, the ballast's label may indicate that that it operates three or four lamps with a total lamp length of 20-32 feet. Thus, the maximum capacity of this ballast is four 8-foot HO lamps. Never attempt to operate a longer or shorter lamp footage than the range specified on the ballast.
Ballasts are available that operate a wide range of lamp numbers and lengths.
Except in a pinch, however, it's not cost-effective to use a larger ballast than you need. Also, the larger ballast probably won't fit in place of the old ballast without making some alterations. Always bolt the ballast securely inside the sign cabinet with its bottom flush against the metal mounting surface. For proper heat dispersion, no air gap should exist between the bottom of the ballast and the mounting surface. Ballasts should not be installed on the bottoms of outdoor sign cabinets where water collects.