Two fluorescent lamps connected in series are operated with just one Ballast. Since each lamp only receives half the voltage, a 110 V starter must be used with each lamp. This type of circuit is only suitable for use with the following lamp-ballast combinations:
| Lamp | Length [mm] |
| T16 4W | 136 |
| T16 6W | 212 |
| T16 8W | 288 |
| T26 15W | 438 |
| T26 18W | 590 |
| T38 20W | 590 |
| TC-L 18 W | 225 |
The circuit is not compensated. If ignition time for luminaires on a tandem
circuit is too long, simply reverse the polarity of one of the starters (turn
the starter through 180° and replace).
