Hi!

I was just wondering this, I think most high wattage T5's I've seen here are european. I know these are also being used in NA but I have had an impression those are not really popular there like in here Europe.( well, at least I have noticed that some years ago before LED installation madness, commercial T8 installation became quite rare and they mostly installed T5 fixtures)

Is there some specifict reason for unpopularity in NA or is my impression wrong?

Yes, in Canada as far as I can tell many supermarkets have F54T5 HO based systems or similar. But other than supermarkets only modern office buildings have them for bigger rooms like conference & cafeterias.

In NA, their T8's are what are high wattage T5 here, as their T8 requires dedicated ballasts like our T5, and most of the ballast for american T8, are cheap two wires electronic.

T5 lighting is popular in big warehouses here.

T5 hasn't garnered the same popularity here in the US at it has in Europe, I would say there is about one T5 installation for every twenty T8 installations here. There are a few national retail chains like Home Depot that use T5HO, but thats about it. There are certainly no local and regional businesses that use T5 around here. Its been well over two years since ive done a T5 relamping job, my company doesnt even keep T5 in inventory. The F32T8 has been the workhorse of commercial lighting here. I think the main issue with T5 is the lamp length is different than our T8 and T12 here, coupled with the mini bi-pin base, thereby making retrofits out of the question

The reason fr which T8 is replaced to T5 here is the supposed energy savings from converting from Magnetic to Electronic ballasts (as the T8 work on the same ballasts as T12 were). In the US the T8 allready use Electronic ballasts, so there is little incentive to look towards T5 (which is same efficacy as T8 of the same phosphor type)

I guess it comes from the time, when a more efficient alternative was desired instead of the T12.
At that time, the most common ballasts were series choke glow bottle preheat in 2xxV countries, but  RS in the 120V countries. The preheat has the ability to generate rather high voltage ignition kicks, so narrowing the tubes for the more efficient T8 was quite easy, the higher ignition voltage was of no problem for most installations. So the lamps were designed to really fit into the existing installations, both mechanically (same length,...) as well as electrically (same rated current), while provide a benefit of lower power and more light, so the F36T8 and F18T8 were born.
On the other hand in the US the common RS ballasts were not able to strike these, so there was a need for a special ballast. And when a special ballast was needed anyway, it became of not much use to keep the arc current the same as T12, so the lamps were designed with a highest efficacy for the given size and light output, hence the F32T8 or so. And this became even more efficient than the European F36T8, assuming all the phosphor and design knowledge the same - the reason being just less of the deign restrictions.
Now when the more efficient T5/HE and /HO came, they bring rather significant improvement over the European T8, but not that big difference towards the US T8's. So the main user of these was the Europe.
Now beside the former F40 size, the common sizes became way different between the US and Europe, so the T5's were developed mainly to fit into fixture sizes (the dropped ceiling panel module,...) based on the lamp sizes common there. And that does not match that well the sizes common in the US.
Of course, the companies have tried to sell their products worldwide, but for the US area it means more unusual fixture sizes, what reduces the popularity of these lamps...

The T5HE and European T8 have very similar efficacy on HF (about 101 Lm/W 36W/840 vs 104 Lm/W 28W/840). This would never pay back the cost of T5 conversion. The only thing pushing to convert is the fact that most T8 is using Magnetic ballasts, at about 42W input for 36W lamp

Well, the IEC efficacy figure look indeed the same, but it is just because the IEC standard requires the measurement to be done at 25degC ambient air. But the thing is, in real life it is very rare for an indoor fixture to have such low operating temperature, usually it is higher. And there the difference comes:
The T8 has maximum efficacy at the 25degC, so when the fixture operates 10degC above that, the efficacy starts to go down, usually by about 5%.
On the other hand the T5's have the maximum efficacy at the 35degC, so within a fixture the efficacy uses to be higher, usually by at least 10% compare to the official 25degC rating (with HE it is even more).
So the lamps having the same IEC efficacy of 100lm/W, at 35degC the T5 will have 110lm/W and the T8 just 95lm/W, so a 28W T5HE becomes nearly an equivalent of the 36W T8. And that is quite some difference (this includes the abut 10% gain in fixture optic efficiency due to the smaller lamp diameter in about the same size reflector, so blocking less of the reflected light).

Plus the 100lm/W rating is the case for T8's only for the long tubes, so 36W (3600lm) and above. But the T5 maintain this efficacy to way lower power packages, the F14T5HE emits about 1600lm (at 35degC, not the IEC rating), while the "original" T8 of that fixture size is 18W for a lamp and just 1300lm output. Plus you have to add the extra 10% of the higher fixture efficiency (assume generic in-ceiling fixtures). There the difference is bigger.

The 104 Lm/W is allready the 35C rating, not the IEC

Luminaire optics vary on a case by case basis, and for the most part, are not better with T5 :

 - If we take for example parabolic 4x14 T5 "squares" for drop ceilings, the manufacturers make the optics for them smaller than used to be with T8, so the optic size/lamp size proportion stays the same

 - If we take bare tube T8 luminaires, then the T5 cannot be bare tube (would be too much glare), so they have some more advanced optics, but not necessarily more efficient ones (for example a PMMA shade, some of them are quite lossy). In addition, a bare T8 tube runs colder, so maybe not too much higher than 25C (while the T5 may be way hotter than 35C too, when enclosed in an optic system)

Of course, there are uses where the 25degC is realistic, but these are not the workhorse installations.
With the T5 it is not that difficult to keep the 35degC, as it matters just at the cold spot and there it is an area behind the electrode on the tube end with the etch. So a place, where is no power dissipation at all, so there is quite easy headroom to tune the air movement right even within an enclosed fixture, so not that difficult for a fixture designer to get the temperature right there.
On the T8's, the most important cold spot is in the middle of the tube, so quite heated by the arc/phosphor losses, so very sensitive to what the air will do around the lamp. Well, not much headroom to optimize the fixture...

Anyway, that are the assumptions taking into account for the T5HE concept design, they are supposed to be representative for a majority of the fixture styles, definitely not all. Therefore there is still a good application base for the older formats as well...

T5s only make about 10% of sales in the USA. T12s still outsell T5s, and T8s make the most sales.

T5s started becoming popular here in the USA about 10-15 years ago. The majority of em are commercial use vs residential. at least for certain applications.  However, I'm seeing many of them being phased out already for LED strips. 

From what I've noticed at least here in Ontario Canada, I do see some T5 installations around, but they are not very popular.  T8 seems to rule the roost here.  I think the fact that the diminishing returns you get from switching from T8 to T5 doesn't make it worthwhile, especially since the vast majority of retrofits that were already done from T12 to T8 involved replacing a magnetic with an electronic ballast.  So the only savings seen going from T8 to T5 is the lamp wattage savings, not the system (lamp-ballast combo).  Add to that fact that the lamp lengths are different and you have to replace the whole fixture, so in the end it becomes a losing proposition.  I see more LED fixtures than T5 around now so I expect T5s to die off.