To answer this question I guess we need to know the circuit construction details of commercial electronic fluorescent ballasts. Is there a downside (either power efficiency or ballast longevity) when connecting only 2 lamps to a 4-lamp electronic ballast?

Bonus question: The ballast is spec'd for T-8 lamps, but I intend to use T-12 lamps, so what is the possible downside of using T-12 lamps in a T-8 spec'd ballast? (I've been running a 4-lamp fixture in this mode for more than a year with no negative effects seen).

Maybe a more general question is - what exactly changes in terms of electrical excitation / voltage / current draw when a fluorescent tube lamp changes in diameter from 1 inch to 1.5 inches?

  • \$\begingroup\$ Good question , but FL’s come in all sorts of power ranges as well as ballasts. Exceed the power causes aging. Report all details you have. New ballasts and FL’s no longer need heaters or dependent on each tube. Longer tubes need a higher trigger voltage. Thicker ones can draw more power \$\endgroup\$ Jan 21, 2021 at 13:54
  • \$\begingroup\$ Some ballasts are universal T8 T12 \$\endgroup\$ Jan 21, 2021 at 13:59
  • \$\begingroup\$ The vast majority of 4-ft lamps sold at retail (either T-8 or T-12) have rated wattage of 32/34/36 watts, and if it matters for the question I would be using a T-12 with similar wattage rating as the spec'd T-8. \$\endgroup\$ Jan 21, 2021 at 14:00
  • \$\begingroup\$ Then it depends on ballast p/n. \$\endgroup\$ Jan 21, 2021 at 14:00
  • \$\begingroup\$ I'm sure there is a wide variety of ballasts available in industrial supply chains but when it comes to consumer / retail availability, ballasts spec'd as T-12 are either very rare or twice the price as T-8. Also I can find no 2-lamp T-8 ballasts at retail, and have never seen 4-lamp T-12 ballasts. Everything I'm talking about here are for 4-ft length lamps. \$\endgroup\$ Jan 21, 2021 at 14:02

1 Answer 1


Fluorescent tubes consume current generally according to the diameter of the tube and length while longer tubes require a higher trigger voltage after which they function Lower voltage from negative resistance.

Modern ballasts did not require a pre-heater for the tubes and both pins are shorted together. Universal ballasts are capable of triggering both T8’s and T12’s and are independent of the number of tubes energized.

Although not as efficient as LEDs with 125 to 150 lm per watt modern fluorescent tubes that are true daylight have triple phosphors to convert ultraviolet into three colour bands that gives a more pleasant colour than LEDs‘(IMHO). CRI is not the best indicator for FL vs LED’s as they have a broader spectrum.

These come in a variety of colour temperatures 5000° K for true daylight, 4100 ° K, Instead of the dual phosphor warm or cold. 89 l/w 20kh rated yet 50kh if used all the time, meaning starts reduce MTBF hrs.

F32T8/TL941 Philips are Tri-phosphor Lamps 4100° K 32W. There also exist higher efficiency types of 30, 28W but only on custom order per box of 10. TL950 would be 5000° K.

I have made my own fixtures just using the “tombstone” connectors in recessed lighting with one wire to each end (yellow being common).

  • \$\begingroup\$ So when I do not use one of the sets of wires coming from say a 4-lamp ballast when I only connect 2 lamps then it's not like there's a driver or output stage that's trying to drive an open set of leads (ie no load). Actually now that I think about it, the 4-lamp T-8 ballast has 2 red, 2 blue and 2 yellow wires, I connected them all to 2 T-12 lamps (yellow on one end, red+blue on the other end). A bit confusing this is... \$\endgroup\$ Jan 21, 2021 at 23:36
  • \$\begingroup\$ those are the obsolete tubes and ballasts for tubes with heaters. They are all "rapid start now" \$\endgroup\$ Jan 22, 2021 at 0:03
  • \$\begingroup\$ Some have different wire colours with blue may be rapid start. . Verify. If it’s 1 wire per end, it’s rapid start (HV pulse) \$\endgroup\$ Jan 22, 2021 at 0:27

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