Can F40 and F34 lamps be used with the same ballast?

Electrical Question:

What are the consequents of putting
two 4ft f34 watt bulbs
in with ballast that has 40 watt on it?

Some ballasts will work fine with both lamps, it really depends what the label says on the ballast. From experience if the lamps are T12 F40 or F34 then they are typically ok, however the new T8 slimmer 32 and 34 watt lamps will flicker when installed in an older style magnetic non-electronic ballast and will eventually burn out soon. Richard, check with your local utility company.
Often times they promote energy rebate programs where they refund up to 50% for changing out old F40 Lamps and Magnetic Ballasts and replace them with Electronic Ballasts and F32 Lamps.

A conversion like this will even cut down on air conditioning loads as well because the electronic ballasts run cooler than the older magnetic ballasts.
It's a great service that your mission is providing. Please let me know if there are other questions I can help you with.

Be Careful and Be Safe - Never Work on Energized Circuits!
Dave Rongey