Hi, can someone tell me the differences between T12 and T8 electrical specs?
Can an T12 ballast be used for an T8 tube?

T12 have 12" tube diameter and have argon gasfilling, and T8 have 8" tube diameter and have a krypton gasfilling and have a higher starting voltage, so they can only be operated with a switch-start or an electronic balalst. T8 lamps have lower wattage ratings than T12 lamps.
There are three types of T8 tubes generally:
Non-retrofit 15W, 20W, 30W and 50W, which have argon gas filling.
The T8 lamps that were mentioned above, which are the european retrofits for T12 lamps, and are available in 18W, 36W ,58 and in the UK also 70W and replaces the 20W, 40W, 65W and 80W T12 respectively. They have usually 75% krypton in argon.
The american rapidstart T8 energy saving, (which come in varous wattage, but the most common is the F32T8 32W T8 tube) that have usually pure argon, so they can be operated on a rapidstart ballast, and for some unknown reasons to me, they are considerably brighter than T12 lamps unlike the european T8 ones, probably because of their higher operating voltage as a result of using argon instead of krypton.

The T description itself is the diameter in 1/8-inch. So T12 is 1.5 in diameter, T8 is 1 in, and T5 is 5/8 in

Then there is a difference in the fill gas of the lamps. And this difference is not only between T12 and T8, but the same T size lamp can be filled with different gases : The American 34W energy savers are T12 at the same length as 40W, but different power due to different fill. And there are vintage Euro T8 lamps which are not energy saving - they take the same power as the T12s they are equivalent to

The 1st property that defines lamp power is the arc voltage. It depends on the gs fill in the amp and the length of the lamp. For example the 36W T8 is 103V

Then there is the ballast. Most types of magnetic ballasts act as a constant AC voltage source with specified output impedance :

For simple inductors ("chokes") the constant AC voltage is the mains voltage. The ballast provides the impedance in series with the lamp

For autotransformer ballasts (mostly American) the constant AC voltage is the ballast's OCV (as generated by the autotransformer part), then there is impedance on the output (not a separate choke after the transformer, but close enough for the most basic understanding)

Lets use the example of the choke on 240V :

The choke is installed in series with the lamp. The lamp arc subtracts from the voltage that reaches the ballast - so for 240V mains and 103V lamp, the ballast sees 137V

The ballasts impedance defines the current. The 36W T8 ballast is designed to deliver about 430mA into a 103V lamp. This means that the choke's impedance is such that under 137V it provides 430mA. The impedance is about : 137V / 0.43A = 318 ohm. This impedance is provided mostly by the magnetic reactance of the ballasst, and the rest by the resistance of the wire from which the core is made

(So, magnetic ballasts can be made with any wanted efficiency : For more efficient ballast, you use a wire with lower resistance (such as : thicker gauge) for the coil, and instead make higher magnetic reactance so the overall impedance is the same)



A long while back, the 40W T12 lamp was designed to a specific arc voltage and current (i dont know the exact numbers, but its close to that of the T8 - as far as i know a bit higher arc voltage and lower current in the T12

The 36W energy saving lamp was designed to work on the same ballast, yet have different power

This can be achieved by making a lamp with different arc voltage. If we make a lamp with somewhat lower arc voltage, this means that the lamp subtracts less voltage from the circuit, and more is getting to the ballast. At the existing ballast impedance, higher voltage into the same impedance causes higher current

We make a lamp with different arc voltage by filling it with different gas, or at different pressure. The 40W T12 is filled with argon. The Euro 36W T8 is filled with krypton, perhaps under somewhat different pressure too. The 34W American T12's are filled with mixture of argon and kryton and perhaps something else too, so have diferent arc voltage

So we get a lamp with lower arc voltage, but higher current, or in short we can vary one but the other will go in the opposite direction. The maximum power is reached somewhere in the center of the range. A lamp with a voltage which is a bit lower, the current will  be a smaller bit higher, so the overall power of the lamp will vbe still lower. Or a lamp with significantly higher voltage, will cause the current to be significantly lower, so low that the overall power will be lower too

This also explains why the American 34W lamps damage 40W ballasts : The lamp is of lower arc voltage, which means there is "higher voltage"-like condition in the ballast. The current is higher too as result. First, the ballast gets higher voltage and higher currnt, so the power dissipated in its losses is the change in square (^2) and it heats more. Second, there is a limit to how much magnetic flux the core in the balast can handle. If this limit is exceeded (by overloading the ballast with a lamp that takes higher current), the primary coil will pass more current which is not usefull to the ballast (as result of the overloaded ballast not being able to stop this current), causing additional overload there besides what is directly caused by the different lamp



The lower arc voltage when the lamp is working is not related to the attitude when the lamp is starting. The Euro krypton 36W T8 is perhps with lower arc voltage when it is working, but it requires higher voltage to strike

For some ballasts, this is a problem : The 40W T12 is usually instant starting in cold cathode straight on 240V, and if preheated it does not need any higher voltage to start fully (thats why all the vintage fixtures with resistive ballast work, despite that resistive ballast cannot boost the voltage). Other ballasts are made to boost the voltage up to a certain value, on which the lamp can start - this is Rapid Start and SRS

For some energy saving lamps, such as the 36W T8, the starting voltage from such systems is not sufficient - there will not be any discharge at such voltage. And thats why the lamp does not work on it

The exception is ballasts which give inductive surges of high voltage to strike the lamp - specifically Switch Start and Perfect Start. The surge from the ballast is at least some 600V or higher. This does not have to be as high for the 40W T12 but it doe not matter, but for the 36W T8 this is sufficient for starting as well so the T8 does work on those ballasts

i live in EU, so i can safely retrofit T12 with T8.
What about 15WT12?

For 15 watts, T8 and T12 work fine either way. Just make sure there's clearance for the T12 in the fixture.