We have 9 dimmers 2 - 20 amp circuits, each dimmers is les than 400 watts on each, voltage to 119v om each circuit, 4.5 on 1 circuit and 7.9 on the other, 1 neutral for both circuits, Lights dim every 12 to 25 seconds
Electrical Supplies - Lutron - S-600-wh Skylark Incandescent Dimmer
Answers & Comments
The best thing you can do is call Lutron's tech support. It is a 24-hour, 7-day/week service and its free.
Having said that,
If the lights dim every 12-to-25 seconds, it tells me that something's effecting the line-voltage of the circuit. Sharing neutrals between two-lighting circuits is a common culprit, but I'm also wondering if there are other devices running off of those circuits besides the lighting. So what I'd recommend:
1) Try running both circuits off of the same phase in the breaker panel. Actually, at 7.9-Amps & 4.5-Amps (I'm assuming that's what you meant), you should be able to run them off of the same breaker. If that fixes the problem, your issue was most likely common neutral interaction. Solve by running a separate neutral... or... if you can swing it (assuming there are no other loads) leave it on that breaker.
2) Upgrade to a spec-grade dimmer like the NT-600-WH. The spec-grade products will compensate for line-voltage fluctuations that may occur from other devices that cause the line sag or un-sag.
Again, those are two base-level suggestions, but use tech support to take it a step further.